Catalytic Oxidation Reagents

By Philip L. Fuchs

The Handbook is part of the Handbook of Reagents for Organic Chemistry series, aiming at collecting articles on a particular theme that individual researchers in academia or industry can use on a daily basis.

The Handbook starts with a section discussing the most important aspects of heteroarene functionalization. The introduction is followed by the alphabetical listing of the most relevant reagents drawn from the EROS database. The Editor, André Charette from the University of Montreal, has selected 120 reagent descriptions, many of them updated with heteroarene-specific reactions for this Handbook. Following the standard format for EROS, each article contains an overview of the synthesis and physical properties of the reagents or catalyst, conditions for its storage, and purification methods.

Given the importance of heteroarenes in biology and especially in medicinal chemistry, a Handbook that focuses exclusively on heteroarene functionalization has been long overdue. This Handbook will have a broad appeal to many individuals engaged in the area of medicinal chemistry, fine chemical synthesis and industrial-scale chemistry.

Key features:

• Builds on the success of the previously published Handbooks of Reagents for Organic Synthesis
• Compares the numerous new C-H functionalization reactions that have been developed in the past decade
• Heteroarene functionalization is widely used in the development of pharmaceuticals and other bioactive compounds
• Contains listings of secondary reagents for which more information is available in the online edition

• Language: English
• Publisher: Wiley
• Release date: Jul 29, 2013
• ISBN: 9781118704844

Book preview

Contents

Cover

Other Titles in this Collection

Title Page

Copyright

e-EROS Editorial Board

Dedication

Preface

Introduction

Terminal Oxidants Finder

Oxidation Catalyst Finder

Recent Review Articles and Monographs

Short Note on InChIs and InChIKeys

A: 4-Acetamido-2,2,6,6-tetramethyl-1-piperidinyloxyl¹

4-Acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammonium Tetrafluoroborate*

B: Bathocuproine

Bathophenanthroline

Benzenecarboperoxoic acid, 1, 1-dimethylethyl Ester

1,2-Benziodoxol-3(1H)-one, 1-Hydroxy, 1-Oxide, IBX

1,2-Benziodoxol-3(1H)-one, 1-Hydroxy, 1-Oxide, stabilized (stabilized IBX)

1,4-Benzoquinone

cis-4-Benzyloxy-α,α-bis-(3,5-dimethylphenyl)-L-prolinol (2S,4R)-

2,2′-Bipyrrolidines, (2S,2′S) and (2R,2′R)

2-[Bis-[3,5-bis(trifluoromethyl)Phenyl] [(trimethylsilyl)oxy]Methyl]Pyrrolidine

2,2-Bis[2-[4(S)-tert-butyl-1,3-oxazolinyl]]propane

Bis[[(1 R,1 ′′R)-3,3′′-[(1 R,2 R)-1,2-cyclohexanediylbis[(nitrilo-κN)-methylidyne]]bis[2′ -phenyl[1,1′-binaphthalen]-2-olato-κO ]](2-)]di-μ-oxodi-titanium and Bis[[(1 S,1 ′′S)-3,3′′- [(1 S,2 S)-1,2-cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[2′-phenyl[1,1′-binaphthalen]-2-olato-κO ]](2-)]di-μ-oxodi-titanium

Bis(2,2-dimethylpropanoato -κ-O)phenyl-iodine

Bis(1-methyl-1-phenylethyl) Peroxide

1,2-Bis(phenylsulfinyl)ethane

N,N′-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene Palladium(I) Allyl Chloride

Bis(trimethylsilyl) Peroxide

Bromamine-T

N-tert-Butyl-N-chlorocyanamide

tert-Butyl Hydroperoxide¹–³

tert-Butyl Hypochlorite¹

tert-Butyl Peracetate

C: Calcium Hypochlorite¹

Chloramine-T¹

Chlorobenzene

Chloro[N,N′-ethylenebis(salicylideneaminato)] manganese

Chromium(VI) Oxide¹

Chromyl Acetate

Cobalt, [5,10,15,20-tetraphenyl-21H,23H -porphinato(2−)-κN 21,κN 22,κN 23,κN 24]-

Cumyl Hydroperoxide

N,N ′ -(1R,2R)-1,2-Cyclohexanediylbis-[N -hydroxy-α -phenybenzeneacetamide]

D: 1,5-Diaza-cis-decalin

N,N-Dibromobenzenesulfonamide and N,N-Dibromo-p-toluenesulfonamide

Di-μ-chlorobis(1,5-cyclooctadiene)-diiridium(I)¹

Di-μ-chlorodichlorobis[(1,2,3,4,5-η)-1,2,-3,4,5-pentamethyl-2,4-cyclopentadien-1-yl]diiridium

(R)-2,10-Dichloro-5H-dinaphtho[2,1-g: 1,2-i][1,5]dioxacycloundecin-3,6,9(7h)-trione

Dichloro(ethoxy)oxovanadium(V)

1,1′-Difluoro-2,2′-bipyridinium Bis-(tetrafluoroborate)

1,2:4,5-Di-O-isopropylidene-β-d-erythro-hexo-2,3-diulo-2,6-pyranose¹

Di-μ-methoxobis(1,5-cyclooctadiene)-diiridium(I)

Dimethyldioxirane¹

(S,S)-2,2′-(Dimethylmethylene)bis(4-tert-butyl-2-oxazoline) and (R,R)-2,2′-(Dimethylmethylene)bis(4-tert-butyl-2-oxazoline)*¹

Dispiro[2H-pyran-2,4′-[4H-5,6,8b]triazaacenaphthylene-7′(5′H),2′′-[2H]pyran], 1′,2′,2′a,3,3′,3′′,4,4′′,5,5′′, 6,6′′,8′,8′a-tetradecahydro-1′,2′-dimethoxy-6,6′′-dimethyl-, monohydrochloride, (1′S,2R,2′S,2′′R, 2′aS,6S,6′′S,8′aS)-

F: N-fluoro-N-(phenylsulfonyl)-benzenesulfonamide

N-Fluoropyridinium Tetrafluoroborate

N-Fluoropyridinium Triflate

1-Fluoro-2,4,6-trimethylpyridinium Tetrafluoroborate

1-Fluoro-2,4,6-trimethylpyridinium Trifluoromethanesulfonate

H: Hydrogen Peroxide

Hydrogen Peroxide–Urea¹

N-Hydroxyphthalimide

I: Iodine

2-Iodobenzenesulfonic Acid

Iodosylbenzene¹

Iridium, [N -[(1R,2R)-2-(amino-κN)-cyclohexyl]-4-methylbenzenesulfonamidato-κN ]chloro[(1,2,3,4,5-η)-1,2,3,4,5-pentamethyl-2,4,-cyclopentadien-1-yl]-; Iridium, [N -[(1R,2R)-2-(amino-κN)-cyclohexyl]-4-methylbenzenesulfonamidato-κN ]chloro[(1,2,3,4,5-η)-1,2,3,4,5-pentamethyl-2,4,-cyclopentadien-1-yl]-; Iridium, [N -[(1S,2S)-2-(amino-κN)-cyclohexyl]-4-methylbenzenesulfonamidato-κN ]-chloro[(1,2,3,4,5-η)-1,2,3,4,5-pentamethyl-2,4,-cyclopentadien-1-yl]-

Iridium, Dichlorodi- μ-hydrobis[(1,2,3,4,5- η)-1,2,3,4,5-pentamethyl-2,4-cyclopentadien-1-yl]di-

Iron, Bis(pyridine)bis(2-pyridinecarboxylato-N1,O2)

L: Lithium Hydroperoxide

M: Manganese(III) Acetate

Manganese(II), bis(octahydro-1,4,7-trimethyl-1H-1,4,7-triazonine-κN¹,κN ⁴, κN ⁷)tri-μ-oxodi-, hexafluorophosphate

Manganese(II) tetraphenylporphyrin

9-Mesityl-10-methylacridinium Perchlorate

N-Methylmorpholine N-Oxide

(S)-(−)-4-(2-Methylpropyl)-2-(2-pyridyl)-2-oxazoline

Methyl(trifluoromethyl)dioxirane

Methyltrioxorhenium

Molybdenum Chloride Oxide¹

N: Noyori Oxidation (Sodium Tungstate Dihydrate, Hydrogen Peroxide, Methyltri-n-octylammonium Hydrogen Sulfate, Phosphonic Acid)

O: 3,3′,3α,3α′,4,4′,5,5′-Octahydro-3,3,3′,3′-tetraisopropyl-6,6′-spirobi-[6H -cyclopent[c]isoxazole]

4,4,4′,4′,5,5,5′,5′-Octamethyl-2,2′-bi-1,3,2-dioxaborolane

Osmium Tetroxide¹

μ-Oxo-bis[tetrakis(t -butyl)-phthalocyaninatoiron(III)]

Oxygen

P: Palladium (II) Sparteine Dichloride

Peracetic Acid¹

(8 α,9R)-(8′′ α,9′′R)-1,1′′-[1,3-Phenylenebis(methylene)]bis[9-hydroxycinchonanium] Dibromide and (9S)-(9′′S)-1,1′′-[1,3-Phenylenebis(methylene)]bis[9-hydroxycinchonanium] Dibromide

(−)(1S)-1-Phenylethyl Hydroperoxide and (+)(1R)-1-Phenylethyl Hydroperoxide

(8α,9R)-(8′′α,9′′R)-9,9′′-[1,4-Phthalazinediylbis(oxy)]bis[10,11-dihydro-6′-methoxycinchonan] & (9S)-(9′′S)-9,9′′-[1,4-Phthalazinediylbis(oxy)]bis[10,11-dihydro-6′-methoxycinchonan]

Picolinic Acid

Polyaniline

Potassium Bromate

Potassium Monoperoxysulfate

2-Pyrazinecarboxylic Acid

2,3-Pyrazinedicarboxylic Acid

Pyrrolidine, 3,4-bis(diphenylphosphino)-1-(phenylmethyl)-,(3R,4R) and Pyrrolidine, 3,4-bis(diphenylphosphino)-1-(phenylmethyl)-, (3S,4S)

2-Pyrrolidinemethanol-α, α-bis(3,5-dimethylphenyl)-(2S); and 2-Pyrrolidinemethanol-α, α-bis(3,5-dimethylphenyl)-(2R)∗

R: Rhenium(VII) Oxide

[RuCl2(p-cymene)2]2

Ruthenium(III) Chloride

Ruthenium Complex of N,N′,N′-Trimethyl-1,4,7-triazacyclononane and Ruthenium Complex of cis-Diaquabis (6,6′-Dichloro-2,2′-bipyridine)

Ruthenium Dodecacarbonyltri Triangulo

Ruthenium Hydroxide

S: Selenium(IV) Oxide

Sodium Bromate

Sodium Hypochlorite¹

Sodium Hypochlorite–N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) Chloride¹

Spiro[6H-1,3-dioxolo[4,5-c]pyran-6,5′-oxazolidine]-3′-carboxylic acid, tetrahydro-2,2-dimethyl-2′,7-dioxo-,2-methyl-2-propyl ester, (3aR, 5′S, 7aR)-

T: Tetrabutylammonium Bis(pyrazinecarboxylato)-dioxo-vanadium(V)

Tetrabutylammonium Dodecatungstophosphate

Tetrabutylammonium Peroxydisulfate

5,6,7,8-Tetrafluoro-1-hydroxy-1-oxobenziodoxol-3(1H)one (Tetrafluoro-IBX)

2,2,6,6-Tetramethylpiperidin-1-oxyl

Tetra-n-propylammonium Perruthenate

3,3′,5,5′-Tetra-tert-butyldiphenoquinone

(S)-2-(5-1H-Tetrazolyl)pyrrolidine and (R)-2-(5-1H-Tetrazolyl)pyrrolidine

4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine

Trichloroisocyanuric Acid¹

Trifluoroperacetic Acid¹

Trimethylamine N-Oxide

Triphenylmethyl Hydroperoxide¹

Tris(2-pyridylmethyl)amine

Tris[4-(trifluoromethyl)phenyl]phosphine

V: Vanadium,bis[N -[(1S)-1-(carboxy- κO)ethyl]-N -(hydroxy- κO)-l -alaninato(2-)- κN, κO ]-

Vanadyl Bis(acetylacetonate)¹

List of Contributors

Reagent Formula Index

Subject Index

General Abbreviations

Other Titles in this Collection

Reagents for Silicon-Mediated Organic Synthesis

Edited by Philip L. Fuchs

ISBN 978 0 470 71023 4

Sulfur-Containing Reagents

Edited by Leo A. Paquette

ISBN 978 0 470 74872 5

Reagents for Radical and Radical Ion Chemistry

Edited by David Crich

ISBN 978 0 470 06536 5

Catalyst Components for Coupling Reactions

Edited by Gary A. Molander

ISBN 978 0 470 51811 3

Fluorine-Containing Reagents

Edited by Leo A. Paquette

ISBN 978 0 470 02177 4

Reagents for Direct Functionalization of C–H Bonds

Edited by Philip L. Fuchs

ISBN 0 470 01022 3

Reagents for Glycoside, Nucleotide, and Peptide Synthesis

Edited by David Crich

ISBN 0 470 02304 X

Reagents for High-Throughput Solid-Phase and Solution-Phase Organic Synthesis

Edited by Peter Wipf

ISBN 0 470 86298 X

Chiral Reagents for Asymmetric Synthesis

Edited by Leo A. Paquette

ISBN 0 470 85625 4

Activating Agents and Protecting Groups

Edited by Anthony J. Pearson and William R. Roush

ISBN 0 471 97927 9

Acidic and Basic Reagents

Edited by Hans J. Reich and James H. Rigby

ISBN 0 471 97925 2

Oxidizing and Reducing Agents

Edited by Steven D. Burke and Rick L. Danheiser

ISBN 0 471 97926 0

Reagents, Auxiliaries, and Catalysts for C–C Bond Formation

Edited by Robert M. Coates and Scott E. Denmark

ISBN 0 471 97924 4

eROS

For access to information on all the reagents covered in the Handbooks of Reagents for Organic Synthesis, and many more, subscribe to e-EROS on the Wiley Online Library website.

A database is available with over 200 new entries and updates every year. It is fully searchable by structure, substructure and reaction type and allows sophisticated full text searches.

http://onlinelibrary.wiley.com/book/10.1002/047084289X

Title Page

This edition first published 2013

©2013 John Wiley & Sons Ltd

Registered office

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United Kingdom

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Library of Congress Cataloging-in-Publication Data

Handbook of reagents for organic synthesis.

  p.cm

 Includes bibliographical references.

 Contents: [1] Reagents, auxiliaries and catalysts for C–C bond formation / edited by Robert M. Coates and Scott E. Denmark [2] Oxidizing and reducing agents / edited by Steven D. Burke and Rick L. Danheiser [3] Acidic and basic reagents / edited by Hans J. Reich and James H. Rigby [4] Activating agents and protecting groups / edited by Anthony J. Pearson and William R. Roush [5] Chiral reagents for asymmetric synthesis / edited by Leo A. Paquette [6] Reagents for high-throughput solid-phase and solution-phase organic synthesis / edited by Peter Wipf [7] Reagents for glycoside, nucleotide and peptide synthesis / edited by David Crich [8] Reagents for direct functionalization of C–H bonds/edited by Philip L. Fuchs [9] Fluorine-Containing Reagents/edited by Leo A. Paquette [10] Catalyst Components for Coupling Reactions / edited by Gary A. Molander [11] Reagents for Radical and Radical Ion Chemistry/edited by David Crich [12] Sulfur-Containing Reagents / edited by Leo A. Paquette [13] Reagents for Silicon-Mediated Organic Synthesis / edited by Philip L. Fuchs [14] Catalytic Oxidation Reagents

A catalogue record for this book is available from the British Library.

e-EROS Editorial Board

Editor-in-Chief

David Crich

Wayne State University, Detroit, MI, USA

Executive Editors

André B. Charette

Université de Montréal, Montréal, Québec, Canada

Philip L. Fuchs

Purdue University, West Lafayette, IN, USA

Tomislav Rovis

Colorado State University, Fort Collins, CO, USA

Founding Editor

Leo A. Paquette

The Ohio State University, Columbus, OH, USA

I dedicate this handbook to our family's matriarch, my wife Diane, our two sons Devin and Sean, their wives Stefanie and Celeste, and their children, Dylan and Sadie, respectively.

Preface

The eight-volume Encyclopedia of Reagents for Organic Synthesis (EROS), authored and edited by experts in the field, and published in 1995, had the goal of providing an authoritative multivolume reference work describing the properties and reactions of approximately 3000 reagents. With the coming of the Internet age and the continued introduction of new reagents to the field as well as new uses for old reagents, the electronic sequel, e-EROS, was introduced in 2002 and now contains in excess of 4000 reagents, catalysts, and building blocks making it an extremely valuable reference work. At the request of the community, the second edition of the encyclopedia, EROS-II, was published in March 2009 and contains the entire collection of reagents at the time of publication in a 14-volume set.

While the comprehensive nature of EROS and EROS-II and the continually expanding e-EROS render them invaluable as reference works, their very size limits their practicability in a laboratory environment. For this reason, a series of inexpensive one-volume Handbooks of Reagents for Organic Synthesis (HROS), each focused on a specific subset of reagents, was introduced by the original editors of EROS in 1999:

Reagents, Auxiliaries, and Catalysts for C–C Bond Formation

Edited by Robert M. Coates and Scott E. Denmark

Oxidizing and Reducing Agents

Edited by Steven D. Burke and Rick L. Danheiser

Acidic and Basic Reagents

Edited by Hans J. Reich and James H. Rigby

Activating Agents and Protecting Groups

Edited by Anthony J. Pearson and William R. Roush

This series has continued over the last several years with the publication of a further series of HROS volumes, each edited by a current member of the e-EROS editorial board:

Chiral Reagents for Asymmetric Synthesis

Edited by Leo A. Paquette

Reagents for High-Throughput Solid-Phase and Solution-Phase Organic Synthesis

Edited by Peter Wipf

Reagents for Glycoside, Nucleotide, and Peptide Synthesis

Edited by David Crich

Reagents for Direct Functionalization of C–H Bonds

Edited by Philip L. Fuchs

Fluorine-Containing Reagents

Edited by Leo A. Paquette

Catalyst Components for Coupling Reactions

Edited by Gary A. Molander

Reagents for Radical and Radical Ion Chemistry

Edited by David Crich

Sulfur-Containing Reagents

Edited by Leo A. Paquette

Reagents for Silicon-Mediated Organic Synthesis

Edited by Philip L. Fuchs

This series now continues with the present volume entitled Catalytic Oxidation Reagents, edited by Philip Fuchs, long-standing member of the online e-EROS Editorial Board. This 14th volume in the HROS series, like its predecessors, is intended to be an affordable, practicable compilation of reagents arranged around a central theme that it is hoped will be found at arm's reach from synthetic chemists worldwide. The reagents have been selected to give broad relevance to the volume, within the limits defined by the subject matter. We have enjoyed putting this volume together and hope that our colleagues will find it just as enjoyable and useful to read and consult.

David Crich

Department of Chemistry

Wayne State University

Detroit, MI, USA

Introduction

The art/science of organic synthesis continues to evolve at an ever-increasing pace. In academia and especially in the pharmaceutical industry, chemists are paying close attention to issues related to atom economy,¹ the E factor (kg waste/kg product)² and process mass intensity (PMI = total mass in a process/mass of product).³ The American Chemical Society Green Chemistry Institute's Pharmaceutical Roundtable has adopted PMI as the current metric for evaluating and benchmarking manufacturing processes.

Superimposed upon these important macroscopic considerations, are strategy-level factors dealing with the design and execution of efficient synthetic operations. A 2007 handbook in this series featured the use of C–H functionalization reagents⁴ as a direct means of increasing structural intricacy.⁵ In the last several years, the Baran group at Scripps has advanced the logic of C–H oxidative functionalization to a whole new level.⁶

Asymmetric reactions employing enantiopure catalysts acting on prochiral substrates are especially prized, since one molecule of catalyst spawns a multitude of chiral progeny. The degree of intricacy (°I) of a molecule is defined as the sum of the individual integer values of five component features, where C* is the number of chiral centers, U is the number of prochiral unsaturations plus rings, AR is the number of aryl or heteroaryl nonhydrogen atoms, and X is the number of heteroatoms (eq 1).⁵ One of the most striking examples of increase in intricacy is the enzymatic oxidation of monosubstituted arenes exploited so effectively by Hudlicky et al.⁷ The intricacy of bromobenzene I is 7, which includes the ring unsaturation and the four unique olefinic prochiral centers. The enzymatic oxidation converts two of the prochiral centers to a pair of heteroatom-bearing stereocenters with concomitant enantiogenesis. Thus, diol II is an enantiopure substrate-bearing functionality appropriate for further manipulation at each of its resident carbon atoms.

(1)

Reactions having positive Δ°I indicate creation of new molecular complexity, while reactions with Δ°I equal to zero (protection /deprotection) or a negative Δ°I value, signal a status quo or worse, decrease of complexity.⁵ Consideration of organic transformations via the perspective of change in intricacy leads to the conclusion that oxidations are higher value-added processes than are reductions.⁸ A complementary conclusion was delivered by Burns et al. in their pivotal 2009 Angewante review discussing redox economy in organic synthesis.⁹

The past several years have seen exponential growth in both the number of papers and complexity of substrates used in C–H functionalization. The goal of increasing molecular intricacy is well served by reactions that effect functionalization of C–H bonds, since the conversion of a C–H bond to a new heteroatom-bearing chiral center is another notable Δ°I = 4 process that efficiently creates enantiopure IV from prochiral cyclohexene III (eq 2).¹⁰

(2)

The continued development of catalysts capable of efficiently undergoing oxidative insertion with aryl and heteroaryl chlorides is proud testament to the creativity of many synthetic organic chemists.¹¹ Based on the lessons learned in designing such catalysts, it is now possible to use chlorobenzene and other inexpensive aryl chlorides as terminal oxidants in palladium-catalyzed room temperature oxidations of unactivated secondary alcohols. The reader is alerted to the chlorobenzene EROS entry and the seminal 2011 paper by the Navarro group¹² employing chlorotoluene (co-product toluene rather than benzene) that details optimal conditions using the commercially available NCH catalyst VII (eq 3).

(3)

This EROS handbook includes a pair of reagent finder tables. This format provides visual access to the handbook's contents, which may stimulate reader creativity. The first table incorporates all reagents that serve as terminal oxidants arranged by functional class subdivided by increasing molecular weight. The second reagent finder table lists specific oxidation catalysts, also arranged by reagent class subdivided by increasing molecular weight.

1. Trost, B. M., Angew. Chem., Int. Ed. Engl. 1995, 34, 259–281.

2. (a) Sheldon, R. A., Green Chem. 2007, 9, 1273–1283. (b) Zhang, T. Y., Chem. Rev. 2006, 106, 2583–2595. (c) Dach, R.; Song, J. J.; Roschangar, F.; Samstag, W.; Senanayake, C. H., Org. Process Res. Dev. 2012, 16, 1697–1706.

3. (a) Concepcion, J.-G.; Ponder, C. S.; Broxterman, Q. B.; Manley, J. B., Org. Process Res. Dev. 2011, 15 (4), 912–917. (b) Concepcion, J.-G.; Constable, D. J. C.; Ponder, C. S., Chem. Soc. Rev. 2012, 41 (4), 1485–1498. (c) Jimenez-Gonzalez, Concepcion; Ollech, Caleb; Pyrz, William; Hughes, David; Broxterman, Quirinus B.; Bhathela, N., Org. Process Res. & Dev. 2013, 17(2), 239–246. (d) Kjell, Douglas P.; Watson, Ian A.; Wolfe, Chad N.; Spitler, Jeremy T., Org. Process Res. & Dev. 2013, 17(2), 169–174. (e) Busacca, C. A.; Fandrick, D. R.; Song, J. J.; Senanayake, C. H., Adv. Synth. Catal. 2011, 353 (11–12), 1825–1864.

4. Fuchs, P. L. Handbook of Reagents for Organic Synthesis, Reagents for Direct Functionalization of C–H Bonds; John Wiley & Sons, Ltd., 2007.

5. (a) Fuchs, P. L., Tetrahedron 2001, 57, 6855–6875. (b) Creation of enantiomerically enriched material was not part of the original formula, but an operation that generates a single enantiomer is a value-added process, consistent with the philosophy described in this article. Enantiogenesis was added to the degree of intricacy formula as described in Ref. 4.

6. (a) Will, R.; Baran, P. S., Chem. Soc. Rev. 2011, 40, 1976–1991. (b) Brückl, T.; Baxter, R. D., Ishihara, Y.; Baran, P. S., Acc. Chem. Res. 2012, 45, 826–839. (c) Newhouse, T.; Baran, P. S., Angew. Chem., Int. Ed. 2011, 50, 3362–3374. See also the 2009–2013 list of reviews.

7. Hudlicky, T.; Gonzalez, D.; Gibson, D. T., Aldrichim. Acta 1999, 32, 35–62.

8. (a) Caron, S.; Dugger, R. W.; Ruggeri, S. G.; Ragan, J. A.; Ripin, D. B., Chem. Rev. 2006, 106, 2943–2989. (b) Thomas, J. M.; Raja, R., Catal. Today, 2006, 117, 22–31. (c) Hermans, I.; Spier, E. S.; Neuenschwandeer, U.; Turra, N.; Baiker, A., Top. Catal. 2009, 1162–1174. (d) Cavani, F.; Teles, J. H., ChemSusChem, 2009, 2, 508–534.

9. (a) Burns, N. Z.; Baran, P. S.; Hoffmann, R. W., Angew. Chem., Int. Ed. 2009, 48, 2854–2867. (b) Renata, H.; Zhou, Q.; Baran, P. S., Science 2013, 339, 59–63. (c) Gaich, T.; Baran, P. S., J. Org. Chem. 2010, 75, 4657–4673.

10. Bisai, Bisai, and Singh have provided an outstanding update of the terminal oxidant PhCO3t-Bu. The specific example in the asymmetric Kharasch–Sosnovsky allylic oxidation reaction is from Boyd, D. R.; Sharma, N. D.; Murphy, D.; Malone, J. F.; James, S. L.; Allen, C. C. R.; Hamilton, J. T. G., Org. Biomol. Chem. 2010, 8, 1081.

11. (a) Xi, Z.; Liu, B.; Chen, W., J. Org. Chem. 2008, 73 (10), 3954–3957. (b) Tewari, A.; Hein, M.; Zapf, A.; Beller, M., Tetrahedron 2005, 61 (41), 9705–9709. (c) Alacid, E.; Najera, C., Adv. Synth. Catal. 2006, 348 (7–8), 945–952. (d) Diebolt, O.; Braunstein, P.; Nolan, S. P.; Cazin, C. S., J. Chem. Commun. 2008, 27, 3190–3192. (e) Ackermann, L., Pure Appl. Chem. 2010, 82 (7), 1403–1413. (f) Hartmann, C. E.; Nolan, S. P.; Cazin, C. S., J. Organomet. 2009, 28 (9), 2915–2919. (g) Gowrisankar, S.; Sergeev, A. G.; Anbarasan, P.; Spannenberg, A.; Neumann, H.; Beller, M., J. Am. Chem. Soc. 2010, 132 (33), 11592–11598. (h) Lee, D.-H.; Jin, M.-J., Org. Lett. 2011, 13 (2), 252–255. (i) Ruan, J.; Iggo, J. A.; Berry, N. G.; Xiao, J., J. Am. Chem. Soc. 2010, 132 (46), 16689–16699. (j) Sau, S. C.; Santra, S.; Sen, T. K.; Mandal, S. K.; Koley, D., Chem. Commun. 2012, 48 (4), 555–557. (k) Lee, D.-H.; Taher, A.; Ahn, W.-S.; Jin, M.-J., Chem. Commun. 2010, 46 (3), 478–480. (l) Chartoire, A.; Lesieur, M.; Slawin, A. M. Z.; Nolan, S. P.; Cazin, C. S., J. Organomet. 2011, 30 (16), 4432–4436.

12. Landers, B.; Berini, C.; Wang, C.; Navarro, O., J. Org. Chem. 2011, 76, 1390–1397.

Philip L. Fuchs

Department of Chemistry, Purdue University, West Lafayette, IN, USA

Terminal Oxidants Finder

Oxidation Catalyst Finder

Recent Review Articles and Monographs

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Dohi, Toshifumi; Ito, Motoki; Yamaoka, Nobutaka; Morimoto, Koji; Fujioka, Hiromichi; Kita, Yasuyuki. Hypervalent iodine(III): selective and efficient single-electron-transfer (SET) oxidizing agent. Tetrahedron 2009, 65(52), 10797–10815.

Dohi, Toshifumi; Kita, Yasuyuki. Hypervalent iodine reagents as a new entrance to organocatalysts. Chem. Commun. 2009, (16), 2073–2085.

Freudendahl, Diana M.; Santoro, Stefano; Shahzad, Sohail A.; Santi, Claudio; Wirth, Thomas. Green chemistry with selenium reagents: development of efficient catalytic reactions. Angew. Chem., Int. Ed. 2009, 48(45), 8409–8411.

Gligorich, Keith M.; Sigman, Matthew S. Recent advancements and challenges of palladium(II)-catalyzed oxidation reactions with molecular oxygen as the sole oxidant. Chem. Commun. 2009, (26), 3854–3867.

Podgorsek, Ajda; Zupan, Marko; Iskra, Jernej. Oxidative halogenation with green oxidants: oxygen and hydrogen peroxide. Angew. Chem., Int. Ed. 2009, 48(45), 8424–8450.

Russo, Alessio; Lattanzi, Alessandra. Asymmetric oxidations of electron-poor alkenes promoted by the β-amino alcohol/TBHP system. Synthesis 2009, (9), 1551–1556.

Uyanik, Muhammet; Ishihara, Kazuaki. Hypervalent iodine-mediated oxidation of alcohols. Chem. Commun. 2009, (16), 2086–2099.

Vedernikov, Andrei N. Ligand-enabled Pt(II)–C(sp3) bond functionalization with dioxygen as a direct oxidant. Chem. Commun. 2009, (32), 4781–4790.

Cavani, Fabrizio. Catalytic selective oxidation: the forefront in the challenge for a more sustainable chemical industry. Catal. Today 2010, 157(1–4), 8–15.

Gunay, Ahmet; Theopold, Klaus H. C–H bond activations by metal oxo compounds. Chem. Rev. 2010, 110(2), 1060–1081.

Sorokin, A.B.; Kudrik, E.V. Phthalocyanine metal complexes: versatile catalysts for selective oxidation and bleaching. Catal. Today 2010, 159(1), 37–46.

Sridharan, Vellaisamy; Menéndez, J.C. Cerium(IV) ammonium nitrate as a catalyst in organic synthesis. Chem. Rev. 2010, 110(6), 3805–3849.

Sun, Chang-Liang; Li, Bi-Jie; Shi, Zhang-Jie. Pd-catalyzed oxidative coupling with organometallic reagents via C–H activation. Chem. Commun. 2010, 46(5), 677–685.

Veser, Goetz. Multiscale process intensification for catalytic partial oxidation of methane: from nanostructured catalysts to integrated reactor concepts. Catal. Today 2010, 157(1–4), 24–32.

Cho, Seung H.; Kim, Ji Y.; Kwak, Jaesung; Chang, Sukbok. Recent advances in the transition metal-catalyzed twofold oxidative C–H bond activation strategy for C–C and C–N bond formation. Chem. Soc. Rev. 2011, 40(10), 5068–5083.

Clot, Eric; Eisenstein, Odile; Jasim, Naseralla; MacGregor, Stuart A.; McGrady, John E.; Perutz, Robin N. C–F and C–H bond activation of fluorobenzenes and fluoropyridines at transition metal centers: how fluorine tips the scales. Acc. Chem. Res. 2011, 44(5), 333–348.

Dauth, Alexander; Love, Jennifer A. Reactivity by design-metallaoxetanes as centerpieces in reaction development. Chem. Rev. 2011, 111(3), 2010–2047.

De Faveri, Giorgio; Ilyashenko, Gennadiy; Watkinson, Michael. Recent advances in catalytic asymmetric epoxidation using the environmentally benign oxidant hydrogen peroxide and its derivatives. Chem. Soc. Rev. 2011, 40(3), 1722–1760.

Sun, Chang-Liang; Li, Bi-Jie; Shi, Zhang-Jie. Direct C–H transformation via iron catalysis. Chem. Rev. 2011, 111(3), 1293–1314.

Suzuki, Takeyuki. Organic synthesis involving iridium-catalyzed oxidation. Chem. Rev. 2011, 111(3), 1825–1845.

Wendlandt, Alison E.; Suess, Alison M.; Stahl, Shannon S. Copper-catalyzed aerobic oxidative C–H functionalizations: trends and mechanistic insights. Angew. Chem., Int. Ed. 2011, 50(47), 11062–11087.

Zhou, Meng; Crabtree, Robert H. C–H oxidation by platinum group metal oxo or peroxo species. Chem. Soc. Rev. 2011, 40(4), 1875–1884.

Boisvert, Luc; Goldberg, Karen I. Reactions of late transition metal complexes with molecular oxygen. Acc. Chem. Res. 2012, 45(6), 899–910.

Campbell, Alison N.; Stahl, Shannon S. Overcoming the oxidant problem: strategies to use O2 as the oxidant in organometallic C–H oxidation reactions catalyzed by Pd (and Cu). Acc. Chem. Res. 2012, 45(6), 851–863.

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Short Note on InChIs and InChIKeys

The IUPAC International Chemical Identifier (InChI™) and its compressed form, the InChIKey, are strings of letters representing organic chemical structures that allow for structure searching with a wide range of online search engines and databases such as Google and PubChem. While they are obviously an important development for online reference works, such as Encyclopedia of Reagents for Organic Synthesis (e-EROS), readers of this volume may be surprised to find printed InChI and InChIKey information for each of the reagents.

We introduced InChI and InChIKey to e-EROS in autumn 2009, including the strings in all HTML and PDF files. While we wanted to make sure that all users of e-EROS, the second print edition

of EROS, and all derivative handbooks would find the same information, we appreciate that the strings will be of little use to the readers of the print editions, unless they treat them simply as reminders that e-EROS now offers the convenience of InChIs and InChIKeys, allowing the online users to make best use of their browsers and perform searches in a wide range of media.

If you would like to know more about InChIs and InChIKeys, please go to the e-EROS website: http://onlinelibrary.wiley.com/book/10.1002/047084289X and click on the InChI and InChIKey link.

Short Note on InChIs and InChIKeys

The IUPAC International Chemical Identifier (InChI™) and its compressed form, the InChIKey, are strings of letters representing organic chemical structures that allow for structure searching with a wide range of online search engines and databases such as Google and PubChem. While they are obviously an important development for online reference works, such as Encyclopedia of Reagents for Organic Synthesis (e-EROS), readers of this volume may be surprised to find printed InChI and InChIKey information for each of the reagents.

We introduced InChI and InChIKey to e-EROS in autumn 2009, including the strings in all HTML and PDF files. While we wanted to make sure that all users of e-EROS, the second print edition of EROS, and all derivative handbooks would find the same information, we appreciate that the strings will be of little use to the readers of the print editions, unless they treat them simply as reminders that e-EROS now offers the convenience of InChIs and InChIKeys, allowing the online users to make best use of their browsers and perform searches in a wide range of media.

If you would like to know more about InChIs and InChIKeys, please go to the e-EROS website: http://onlinelibrary.wiley.com/book/10.1002/047084289X and click on the InChI and InChIKey link.

A

4-Acetamido-2,2,6,6-tetramethyl-1-piperidinyloxyl¹

(reagent and catalyst for selective oxidation of alcohols²)

Alternate Names: 4-acetamido-TEMPO; 4-acetylamino-TEMPO.

Physical Data: mp 146–147 °C,² 147.5 °C.³

Solubility: nearly insol hexane, ether; sol ethanol, acetone, acetonitrile, methylene chloride; slightly sol water, from which it can be recrystallized.

Form Supplied in: red or pink solid, commercially available.

Preparative Method: can be easily prepared in high yield from 4-amino-2,2,6,6-tetramethylpiperidine.²

Handling, Storage, and Precautions: completely stable and nonhygroscopic.

Oxidation of Alcohols

4-Acetamido-TEMPO is a representative of the nitroxide radicals (1) which have been used as reagents or catalysts for the oxidation of organic compounds.¹ Several nitroxide radicals have been converted to the corresponding oxoammonium salts (2) and used as stoichiometric oxidizing reagents.¹ Nitroxide radicals have also been used as catalysts in the the presence of a secondary, stoichiometric oxidant (see 2,2,6,6-Tetramethylpiperidin-1-oxyl).¹

4-Acetamido-TEMPO (3) is not, itself, a reagent for the oxidation of alcohols. Rather, it serves as a convenient precursor for the in situ preparation of an oxoammonium salt (4), which is the true oxidant. The oxoammonium salt (4) is the product of the acid-catalyzed disproportionation of (3) to (4) and (5) in the presence of p -Toluenesulfonic Acid (eq 1) and is a highly selective reagent for alcohol oxidation (eq 2).¹ The reactions are carried out in methylene chloride in which TsOH·H2O is essentially insoluble. The only products of the reaction are the desired carbonyl compound product (7) and the hydroxylamine salt (5), which is completely insoluble in methylene chloride. Product isolation simply involves the filtration of (5) and the evaporation of methylene chloride. Compound (5) can be converted back to the nitroxide radical (3) in quantitative yield.²

(1)

(2)

The reaction has been used for the oxidation of a variety of alcohols with excellent yields of isolated products.² Primary alcohols are converted to the corresponding aldehydes, and no over-oxidation is observed. Secondary alcohols react as well as primary alcohols and provide the corresponding ketones. The mildness of this reaction has been demonstrated by the oxidations of nerol (8) and geraniol (9) to the corresponding cis-citral (10) and trans-citral (11).

(3)

Oxidation with this reagent does not take place when there is an oxygen or nitrogen in the β-position to the alcohol being oxidized and does not take place with 1,2-diols or sugars. Amines,¹,⁴ thiols,⁵ phenols,⁶ indoles,⁷ benzyl ethers,¹ and ketones (very slow)¹ react with oxoammonium salts and may interfere with alcohol oxidation. However, sulfides, most ethers, amides, esters, and double bonds do not react with oxoammonium salts and should not interfere.¹

A number of other oxoammonium salts have been described in the literature.¹ In addition, TEMPO-type nitroxide radicals have been used as specific catalysts for the oxidation of alcohols using one or several secondary, stoichiometric oxidants.¹,⁸ Neither 4-acetamido-TEMPO nor any of its derivatives have been used in this manner, but they should function satisfactorily.

1. Bobbitt, J. M.; Flores, M. C. L., Heterocycles 1988, 27, 509 (general review on oxoammonium salts, but they are called nitrosonium salts in the title).

2. Ma, Z.; Bobbitt, J. M., J. Org. Chem. 1991, 56, 6110.

3. Rozantsev, E. G.; Kokhanov, Y. V., Bull. Acad. Sci. USSR, Div. Chem. Sci. 1966, 8, 1422.

4. Hunter, D. H.; Racok, J. S.; Rey, A. W.; Ponce, Y. Z., J. Org. Chem. 1988, 53, 1278.

5. Kashiwagi, Y.; Ohsawa, A.; Osa, T.; Ma, Z.; Bobbitt, J. M., Chem. Lett. 1991, 581.

6. Bobbitt, J. M.; Ma, Z., Heterocycles 1992, 33, 641.

7. Bobbitt, J. M.; Guttermuth, M. C. F.; Ma, Z.; Tang, H., Heterocycles 1990, 30, 1131.

8. (a) Anelli, P. L.; Montanari, F.; Quici, S., Org. Synth. 1990, 69, 212. (b) Inokuchi, T.; Matsumoto, S.; Torii, S., J. Org. Chem. 1991, 56, 2416. (c) Leanna, M. R.; Sowin, T. J.; Morton, H. E., Tetrahedron 1992, 33, 5029.

James M. Bobbitt & Zhenkun Ma

University of Connecticut, Storrs, CT, USA

4-Acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammonium Tetrafluoroborate*

(reagent used to oxidize alcohols to aldehydes or ketones. Of special interest, allyl alcohols can be oxidized without isomerization and phenolic benzyl alcohols can be oxidized without blocking the phenol group; all of these reagents have been used for the oxidation of primary and secondary alcohols to aldehydes or ketones, respectively. They also have a certain number of additional minor oxidation reactions. Many other salts are known¹)

Physical Data: mp 195–196 °C dec (Kofler). 1a, mp 193–194 °C (dec.); 1b, mp 177–178 °C (dec.); 10a, mp 162–163 °C; 10b, mp 157–158 °C; 10c, mp 118–119 °C; 10d, mp 76–78 °C; 11a, mp 162–162.5 °C; 11b, mp 121–123 °C; 11c, mp 206–207 °C.

Solubility: partially soluble in water (6 g/100 mL at 0 °C, 8 g/ 100 mL at 20 °C, and about 100 g/100 mL at 100 °C; reacts very slowly with hot water), slightly soluble in methylene chloride (0.04 g/100 mL at 20 °C). 1a and 1b are slightly soluble in water and methylene chloride, while insoluble in ether; 10a–10d are insoluble in ether; 11a–11c are insoluble in carbon tetrachloride. Otherwise, solubilities are variable. Most reactions are carried out in methylene chloride, less often in acetonitrile.

Form Supplied in: all of these salts are prepared from the corresponding parent nitroxides, 5, 12, and 13, which are commercially available. None of the salts are commercially available currently.

Preparative Methods: in a slightly modified procedure,³ 100 g of 4-acetylamino-2,2,6,6-tetramethyl-1-piperidineoxyl (0.469 mol) is stirred with 50 mL of H2O, and 90 g of 48% aqueous HBF4 (0.492 mol) is added over about 1 h. The mixture is placed in an ice bath, and 332.2 g of commercial bleach (5.25% NaOCl, 0.235 mol) is added over about 2 h. The mixture is allowed to stir in ice for about 3 h and then filtered. The solid bright yellow salt is pressed as dry as possible and washed with two 50-mL portions of ice water and two 100-mL portions of CH2Cl2. The salt is dried at room temperature and in air to constant weight to give 90–95 g (about 65%) of the reagent, mp 195–196 °C. Further drying in a vacuum desiccator reduced the weight only slightly. For recovery of the remainder of the salt, the combined filtrates (water and CH2Cl2) are treated with 20 g of NaHCO3 and 20 mL of 95% ethanol to reduce the oxoammonium salt to the starting nitroxide. The layers are separated and the aqueous phase is extracted with three 50-mL portions of CH2Cl2. The extracts are combined, dried over MgSO4, and evaporated to give 25–30 g of nitroxide, mp 145–147 °C, suitably pure for the preparation of more oxoammonium salt. Oxoammonium salts are prepared from commercially available, nitroxide free radicals, 5, 12, and 13. There are essentially two methods for the preparation of the oxoammonium salts from nitroxides. The first is based on the acid-catalyzed disproportionation of nitroxide in strong acid as shown for 1a in eq 1.¹⁴,²⁰ The method has been used both in water and dry ether for the preparation of many oxoammonium salts. The second is based on the halogens as shown for 11b in eq 2.¹⁶,¹⁹ In the halide reactions, the elemental halogens act both as oxidizing agents and the source of the anions.

(1)

(2)

The preparation of 1a and its applications are described in a tested procedure in Organic Syntheses.²⁰ The procedure involves the use of a second oxidant, bleach, to increase the yield of salt.

Purity: recrystallized, if necessary, from an equal weight of water. Solution heated and cooled as rapidly as possible. The purity of the salt can be assayed using 1-decanol as substrate. It is normally close to 100%.

Handling, Storage, and Precautions: the salt (tetrafluoroborate) is non-hygroscopic and indefinitely stable. No hazards are known. For anhydrous reactions, the previously dried salt can be dried in a vacuum over P2O5 at 35 °C, although the reduction in weight is slight. The perchlorate salts 1b and 10b have been used extensively,³ but since 1b detonated on drying under vacuum,¹³ they should both be avoided. Compound 1a appears to be nonhygroscopic, stable indefinitely, and to have ideal solubility properties in methylene chloride. For these reasons, it will be the main theme of this article. The other salts have various degrees of instabilities and some are quite hygroscopic. It should be understood that the other oxoammonium salts have similar properties. Nothing is known about their toxicity.

Original Commentary

James M. Bobbitt & Nabyl Merbouh

University of Connecticut, Storrs, CT, USA

Introduction

The title reagent 1a has some remarkable oxidizing properties, and is commercially available from several sources. It can also be prepared from 4-amino-2,2,6,6-tetramethylpiperidine.³ In this preparation, one caution applies. When the nitroxide was prepared, potassium carbonate was used as a base.³ This should be replaced by sodium carbonate since potassium tetrafluoroborate is quite insoluble in water and causes impurity problems with the oxoammonium salt. The nitroxide has interesting solubility properties. It is only slightly soluble in ether and cold water. It can be extracted from an aqueous layer with several portions of CH2Cl2, or from an ether layer with several portions of water. If necessary, the nitroxide can be recrystallized from ethyl acetate.

Reference 3 describes the synthesis and reactions of two oxoammonium salts, the perchlorate and the tetrafluoroborate, but all of the oxidations described were carried out with perchlorate. Unfortunately, the perchlorate proved to be unsafe, and the tetrafluoroborate is now the salt of choice. The oxidizing properties and solubilities of the two salts are essentially identical.

Oxidations in CH2Cl2 with Silica Gel

The reaction shown below is actually carried out in a neutral to slightly acidic medium. Silica gel is a potent catalyst, but is not needed for the oxidation of allyl or benzyl alcohols. The oxoammonium salt 1 is bright yellow and its reduction product, the hydroxyamine salt 2, is white in color. The oxidant is sufficiently soluble in the solvent to allow the reaction to occur, but 2 is almost completely insoluble. Thus, a yellow slurry of the reagent 1 (about 5–10% excess) and the appropriate alcohol is stirred in CH2Cl2 (1–2 mmol in 10 mL), with or without silica gel, until the slurry becomes white (eq 3). Filtration through a ca. 0.5-cm pad of silica gel removes the salt 2 and a small trace of 1. The aldehydes or ketones formed are quite pure and are isolated in good yields.³

The hydroxyamine salt 2 can be converted back to nitroxide in the following manner. The precipitated reagent and silica gel are stirred with a small portion of warm water and filtered to remove silica gel. The filtrate is basified with NaHCO3 and treated with an excess of 30% H2O2. The nitroxide forms in about 24 h and can be extracted with CH2Cl2.

This oxidation method has some advantages as well as disadvantages. Advantages are that the reaction is colorimetric (yellow to white), essentially quantitative, and there is no further oxidation to an acid. If there is a hydroxy group in the product which can form a stable hemiacetal, the product will be a lactone. Since 2 is almost insoluble in CH2Cl2, the reaction work-up requires only a filtration. The reaction is ideal for preparing unstable or volatile aldehydes (or ketones) on an ‘as needed’ basis. Since the products are essentially pure as formed, the oxidation can be combined with various other reactions, such as Wittig or Grignard reactions. Wittig reactions work especially well when the Wittig reagent is added to the solution of aldehyde or ketone, either in CH2Cl2 or in diethyl ether. Isolated yields are as high as 90% for the combined reactions.

(3)

The oxidation of allyl alcohols when there is a possibility of double bond isomerization is accomplished without isomerization,³ as in the case of 3.⁴ Benzyl alcohols containing one phenol group can be cleanly oxidized to aldehydes without phenol oxidation,³ as in the case of 4.⁵ Two phenol groups give decomposition. Yields of aldehydes or ketones are shown in parentheses.

There are also some disadvantages for this oxidation method. The oxidation does not take place when there is a β-oxygen or a nitrogen group attached to the alcohol being oxidized (but see below). Since the reaction conditions are slightly acidic, acetals are opened. In addition, there are two slow reactions that interfere with slow oxidations, such as those of primary alcohols. Benzyl groups are slowly oxidized and cleaved.⁶ This causes no problem with benzyl alcohols or allyl alcohols, which are oxidized rapidly. In the second reaction, activated double bonds (containing oxygen, nitrogen, or three alkyl groups) are slowly oxidized.³,⁷ Again, this is not a problem with allyl or benzyl alcohols.

One more reaction of certain oxoammonium salts is known and may be of major importance. This is the oxidation of ketones to α-diketones,⁸,⁹ and the oxidation of 1,3-diketones to 1,2,3-triketones.⁹

Oxidations Under Basic Conditions

Oxidations can also be carried out in a base, for example pyridine in CH2Cl2, but the reaction takes a different course as shown in eq 4.¹⁰ Equal amounts (2 equiv) of salt and the base must be used. The products are the orange-colored nitroxide (5) and the base salt of tetrafluoroboric acid.

(4)

Fortunately, compounds 5 and 6 can both be easily removed for product isolation. The orange-colored nitroxide is only slightly soluble in dry ether, but the pyridine salt is completely insoluble in CH2Cl2. Thus, the filtration of the reaction mixture removes 6, and evaporation of CH2Cl2 leaves a residue of product and nitroxide. This residue can be extracted with ether, leaving most, but not all of the nitroxide behind. The last traces of nitroxide can be removed on a short column of silica gel.

The reaction has been successfully applied to several sugar derivatives¹⁰ such as 7, 8, and 9. It is noteworthy that only oxidation of the anomeric carbon takes place, but the secondary alcohol groups in the sugar are not touched. The reaction has also been successfully applied to other acetals and β-oxygen alcohols, thus alleviating some of the objections of the simpler reaction in CH2Cl2 alone.

First Update

James M. Bobbitt

University of Connecticut, Storrs, CT, USA

Nabyl Merbouh

Simon Fraser University, Burnaby, Canada

Alcohol Oxidations

With 1a

A typical oxidation with 1a is shown eq 5, but it is probably specific for this reagent due to the desirable solubilities of the salt and its reduced product, 2.³ In essence, the substrate is dissolved in methylene chloride and treated with the bright yellow 1a, which is only slightly soluble. The salt is best mixed with an equal weight of chromatographic grade silica gel to give faster reactions. The yellow slurry is stirred until it becomes a white slurry and no starch iodide paper test is observed. The slurry is filtered to recover 2 (insoluble in methylene chloride), and the filtrate contains a high yield of essentially pure carbonyl compound. Compound 2 can be recycled back to nitroxide, 5.

(5)

There are some specific advantages to this reaction. First, the reaction is colorometric. Second, product isolation is trivial. Third, the reaction is stereospecific in that a double bond and chiral configurations are maintained even when the oxidation is on adjacent atoms. Finally, the reaction can be carried out in the presence of a phenol group.

There are also disadvantages. Alcohols having a beta oxygen or nitrogen function are not oxidized.³,²¹ Free amino groups react rapidly (see below). Aliphatic alcohol oxidations (which are slow) can be complicated sometimes by reactions with isolated, activated double bonds (see below) and, in some cases, by benzyl ethers (see below).⁶ Frequently, these secondary reactions are quite slow in methylene chloride and faster in acetonitrile.

Allylic and benzylic alcohols react in 2–3 h. Aliphatic secondary alcohols and acetylenic alcohols react slightly faster than primary alcohols. All of the reactions are faster in the presence of silica gel. The reaction is quite specific in that no carboxylic acids are formed from primary alcohols and carbon–carbon bond cleavage is almost unknown. The one exception is that for diols in which five-or six-membered hemiacetal rings can be formed, the products are lactones.²²

Oxidations with Other Salts

Oxidations with the other salts give comparable reactions, although some salts are not stable or are hygroscopic. The specific anions seem to be important in determining the rates of the various oxidations.¹⁸,¹⁹ In general, the halide salts react faster than the tetrafluoroborate or perchlorate mediated oxidations. The reaction conditions are similar to those given above, but the solubilities of the products may not be the same.

Oxidations in Base

Since amine groups react readily with oxoammonium salts (see below), only such organic bases as pyridine and its derivatives can be used to influence these reactions. Only one such reaction has been carefully studied, the oxidative dimerization of primary alcohols containing a beta oxygen and those give good yields of dimeric esters as shown in eq 6.²³

(6)

This reaction is complicated by the fact that the reduced product from the oxoammonium salt, 2, reacts with pyridine to give a free hydroxyamine base, which in turn reacts with more oxoammonium salt 1a to give nitroxide 5 as the final product.

Role of Oxoammonium Ions in Nitroxide Catalyzed Oxidations

Oxidations with stoichiometric amounts of oxoammonium salts, although convenient, have not been extensively used, primarily because the salts are not commercially available. The vast majority of oxoammonium oxidations have been catalytic reactions in which a nitroxide is used with a primary oxidant such as NaOCl (or many other similar reagents). In effect, the oxoammonium ion generated from nitroxide in such reactions provides a high degree of specificity for alcohol oxidation, with the convenience of cheaper and more available oxidants. Catalytic reactions have been extensively reviewed¹,²,¹¹,¹²,²⁴ and will not be considered in this paper.

Oxidations of Activated Methylene Groups to Carbonyl Groups

These reactions are not well known. Some reactions, such as those in eqs 7–9,⁹,²⁵–²⁷ probably take place through enol forms. Others, such as those in eq 10²⁸ do not, and are not easily explained.

(7)

(8)

(9)

(10)

Reactions with Double Bonds

Isolated Double Bonds

These reactions take place slowly in methylene chloride and faster in acetonitrile (eq 11). The alkene must contain three or four alkyl substituents²⁹ or two phenyl groups.⁷

(11)

Enol Ether and Enol Amine Reactions

These reactions are similar to the one discussed above for double bond reactions except that the intermediate is trapped as an ether (eq 12)⁷ or as a thymine derivative (eq 13).³⁰

(12)

(13)

Reactions with Enolate Ions and Grignard Reagents.³¹,³²

Equations 14 and 15.

(14)

(15)

Reactions with Amines.³³–³⁵

Reactions with amines are complicated by the fact that the starting amine reacts with any reduced oxidant (such as 2) to give the free hydroxylamine base, which reacts with oxoammonum salt to give nitroxide (reverse of eq 3). The reactions have not been well explored, and yields are low (eqs 16 and 17). The apparent reaction, however, is much like alcohol oxidation in that an imine is formed by the loss of two hydrogens. Amine oxidation has been more successfully explored in nitroxide catalyzed electrochemical oxidations in the presence of 2,6-lutidine.³⁶

(16)

(17)

Reactions with Phenols.³⁵,³⁷

Equation 18.

(18)

Reactions with Benzyl Ethers.⁶

These reactions only show up only when alcohol oxidation is very slow. For example, benzyl alcohols can be oxidized in the presence of benzyl ethers as in eq 19.³

(19)

Reactions with Sulfur Compounds.³⁸

Thiols are oxidized to disulfides (eq 20). Otherwise sulfur oxidations are controversial. Sulfides and sulfones are reported to be unreactive.³³ Salt 1a reacts almost violently with DMSO if one attempts to measure an NMR spectrum in that solvent, but the products are not known.³⁹

(20)

Second Update

Tsutomu Inokuchi & Li-Jian Ma

Okayama University, Okayama, Japan

Metal-free Aerobic Oxidation of Alcohols.⁴⁰,⁴¹

Environmentally benign aerobic alcohol oxidations have gained importance recently. TEMPO and its derivatives are used for aerobic oxidation of alcohols in various nonmetallic and transition metal-mediated reactions. A transition metal-free catalytic system consisting of catalytic sodium nitrite⁴²–⁴⁴ or PEG (polyethylene glycol)–NO2⁴⁵ with air as the terminal oxidant under acidic conditions was recently developed (eq 21).

(21)

In place of NaNO2, tert-butyl nitrite (TBN) was identified as an efficient NO equivalent for the activation of molecular oxygen.⁴⁶,⁴⁷ Various alcohols including benzylic alcohols substituted with sulfide were converted into their corresponding carbonyl compounds with TEMPO/HBr/TBN as catalyst, which is suitable for the oxidation of solid alcohols with high melting points and/or low solubility using a minimal amount of solvent to form a slurry (eq 22).

(22)

The cascade systems of O2–NaNO2–bromine or O2–Bu4NBr–TEMPO⁴⁸,⁴⁹ or O2–NaNO2–PhI(OAc)2⁵⁰ have also been developed (eqs 23 and 24).

A novel catalyst system that uses 1–4 mol % of TEMPO in combination with 4–6 mol % of aqueous hydroxylamine as a NO source is also effective (eq 25).⁵¹

Metal-catalyzed Aerobic Oxidation of Alcohols.⁵²–⁶¹

Aerobic metal-mediated TEMPO oxidations of alcohols have been examined. These processes have been mainly conducted with copper catalysts. This catalyst system is also useful for oxidation of benzylic alcohols and the reaction is explained by the mechanism below (eq 26).

Ionic liquid [bmpy]PF6 was used as a reaction medium,⁶⁰ and fluorinated bipyridine ligands were successfully used for aerobic oxidation of alcohols under the fluorous biphasic system (FBS).⁶² The sequential reaction including alcohol oxidation by TEMPO/Cu and the asymmetric aldol reaction by peptide catalysis was realized using resin-supported TEMPO catalysts.⁶³ Simultaneous irradiation with visible light improved the catalytic oxidation with Cu/TEMPO, especially for unprotected alkyl glycosides (eq 27).⁵⁵

(23)

Other effective metals in place of Cu include a vanadyl complex,⁶⁴ polymer incarcerated ruthenium (PIRu),⁶⁵ and combined bimetals such as Cu–Mn oxides,⁶⁶,⁶⁷ and Mn–Co.⁶⁸,⁶⁹

Cascade Aerobic Alcohol Oxidations

Efficient and clean aerobic oxidation using 1 atm of oxygen or air, producing aldehydes/ketones from the widest spectrum of alcohols, including allenols and propargylic alcohols, at room temperature within a few hours was developed by cascade reaction of O2–NO/NO2–Fe(III)–TEMPO systems (eqs 28 and 29).⁷⁰ These processes have been mainly conducted with 4-acetamido-TEMPO/ FeCl3/NaNO2 catalysts⁷¹ or TEMPO/cobaloxime/Co(NO3)2 catalyst.⁷²

(24)

(25)

(26)

(27)

(28)

(29)

A combination TiO2–dye cascade system was developed.⁷³ Upon irradiation with visible light, excited dye molecules (Alizarin Red) inject electrons into TiO2 and form dye radicals, which can oxidize TEMPO to TEMPO+, which selectively oxidizes alcohols in the presence of O2 to the corresponding aldehydes. The selectivity is improved when the irradiation exploits visible light.

Supported TEMPO or Immobilized TEMPO

A combination of IL-supported TEMPO (14) with BAIB (bis(acetoxy)-iodobenzene)⁷⁴ or IL-supported TEMPO with tetra-n-butyl-ammonium peroxymonosulfate (n-Bu4NHSO5), available from Oxone and n-Bu4NHSO4 in an ionic liquid medium, has been developed.⁷⁵

Organically modified silica-based SiliaCat TEMPOs (15, 16) are an oxidizing catalyst that can efficiently replace homogeneous stable nitroxyl radicals and effect the oxidation in either organic solvents or water, with or without KBr as cocatalyst.⁷⁶,⁷⁷

Oxidation–hydrocyanation of γ,δ-unsaturated alcohols using immobilized TEMPO/PhI(OAc)2 in combination with HbHNL (hydroxynitrile lyase from Hevea brasiliensis) proceeds smoothly, the resulting cyanohydrin derivatives being obtained in good overall yields and with high ees (eq 30).⁷⁸

Other immobilized TEMPOs shown below include phosphonium-supported TEMPO (17),⁷⁹ polyurethane-and polystyrene-supported TEMPO (18),⁸⁰ molecularly imprinted polymeric TEMPO (19),⁸¹ a highly active and easily recoverable TEMPO with the attachment of multiple triazole moieties and perfluoroalkyl chains (20),⁸² readily isolable fluorous-tagged TEMPOs,⁸³a and TEMPO-linked metalloporphyrins.⁸³b

(30)

(31)

(32)

Site-isolated encapsulated linear polymeric catalysts bound with TEMPO have 2.5 times faster reaction rates than the analogous resin-supported catalysts, in the oxidation of sec-phenethyl alcohol to acetophenone (eq 31).⁸⁴

TEMPO–iodobenzene hybrid catalyst was used in the double catalytic reaction of arylmethyl and alkylmethyl alcohols as a substrate in a 9% acetic acid solution of peracetic acid (PAA) as a cooxidant, giving the corresponding carboxylic acids in good to excellent yields (56~99%) (eq 32).⁸⁵

Industrial Applications

Sustainability of the TEMPO oxidation finds a variety of industrial applications as highly selective oxidation catalysts for the production of pharmaceuticals, flavors and fragrances, agrochemicals, and a variety of other specialty chemicals, and examples are reviewed.⁸⁶ In developing the manufacturing route for the cathepsin K inhibitor SB-462795, the TEMPO oxidation of a secondary alcohol for the final chemical stage has been exploited (eq 33).⁸⁷

(33)

Other examples are synthesis of betulinic acid by oxidation with 4-acetamido-TEMPO/NaClO2/NaOCl (86%)⁸⁸ and unprotected dialdo-glycosides with TEMPO–TCC (trichloroisocyanuric acid) (65~100%) (eq 34).⁸⁹

(34)

In the pilot plant synthesis of (2-chlorophenyl)[2-(phenylsulfonyl)pyridin-3-yl]methanone, reactivities of TEMPO derivatives, such as 4-MeO-TEMPO and 4-AcNH-TEMPO, were compared (eq 35).⁹⁰

(35)

Technical production of aldehydes by continuous bleach oxidation of alcohols was achieved in a tube reactor. This system is essential for the synthesis of activated α-substituted aldehydes and also recommended for nonactivated aldehydes (eq 36).⁹¹

(36)

Applications for Chiral Intermediates

An efficient tandem reaction system was developed in which primary alcohols were oxidized to the corresponding aldehydes followed by an asymmetric α-oxyamination with a resin-supported peptide catalyst (eq 37).⁹² A similar transformation has also been achieved using an anodic oxidation (eq 38).⁹³

(37)

(38)

A chiral cobalt-catalyzed reaction features enantioselective aerobic oxidative kinetic resolution of (+)-α-hydroxy esters, using molecular oxygen as the sole oxidant (eq 39).⁹⁴

(39)

A convenient oxidation for a variety of electron-rich benzylic alcohols using HIO3 and I2O5 as safer stoichiometric oxidants with catalytic amounts of TEMPO or 4-hydroxy-TEMPO in water was devised.⁹⁵

This article includes some reactions of 4-methoxy-2,2,-6,6-tetramethyl-1-piperidinyloxy (4-methoxy-TEMPO) [95407-69-5], 4-acetylamino-2,2,6,6-tetramethyl-1-piperidinyloxy (4-acetylamino-TEMPO) [14691-89-5], and derivatives of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-hydroxy-TEMPO) [2226-96-2] and 1-piperidinyloxy, 2,2,6,6-tetramethyl-4-(2-propyn-1-yloxy)-[147045-24-7].

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