Synthetic Organic Photochemistry Synthetic Organic Photochemistry

Edited by William M. Horspool The University of Dundee Dundee, United Kingdon

Plenum Press • New York and London Library of Congress Cataloging in Publication Data Main entry under tille: Synthetic organic photochemistry. Includes bibliographical references and index. Contents: Photo-addition and -cydisalion processes of aromatic compounds / An• drew Gilbert-Enone photochemical cycloaddition and organic synthesis / Alan C. Weedon - Synthetic aspects of photochemical electron transfer reaction I Patric k S. Mariano and Jerome L. Stavinoha -(etc. ] I. Photochemistry. 2. Chemistry, Organic-Synthesis. I. HorspooJ, William M. QD71S.s96 1984 547' .2 84- 10480

ISBN-13: 978-1-46\2-9668-3 e-ISBN-] 3: 978- ]-4613-2681-6 DOl; 10.1007/978-1-4613-2681-6

No part of this book may be reproduced, stored in a retrieval system, or transmilted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written pennission from the Publisher To Una, Linda, and Andrew Contributors

Roger W. Binkley, Department of Chemistry, Cleveland State University, Cleveland, Ohio 44115

Howard A. J. Cariess, Department of Chemistry, Birkbeck College, University of London, London, WC1E 7HX, U.K. John D. Coyle, Department of Chemistry, The Open University, Milton Keynes, MK7 6AA, U.K. Thomas W. Flechtner, Department of Chemistry, Cleveland State University, Cleveland, Ohio 44115 Andrew Gilbert, Department of Chemistry, University of Reading, White• knights, Reading, Berkshire, RG6 2AD, U.K. William M. Horspool, Department of Chemistry, The University, Dundee DD1 4HN, U.K. Patrick S. Mariano, Department of Chemistry, University of Maryland, Col• lege Park, Maryland 20742 Douglas C. Neckers, Department of Chemistry, Bowling Green State Uni• versity, Bowling Green, Ohio 43403 Albert Padwa, Department of Chemistry, Emory University, Atlanta, Georgia 30322 Jerome L. Stavinoha, Texas Eastman Company, P.O. Box 7444, Longview, Texas 75607 Alois H. IA. Tinnemans, Department of Chemistry, Bowling Green State Uni· versity, Bowling Green, Ohio 43403 Alan C. Weedon, Department of Chemistry, The University of Western On• tario, London, Ontario, Canada N6A 5B7

vii Foreword

Of all major branches of organic chemistry, I think none has undergone such a rapid, even explosive, development during the past twenty-five years as organic photochemistry. Prior to about 1960, photochemistry was still widely regarded as a branch of physical chemistry which might perhaps have oc• casional applications in the generation of free radicals. Strangely enough, this attitude to the subject had developed despite such early signs of promise as the photodimerization of anthracene first observed by Fritzsche in 1866, and some strikingly original pioneering work by Ciamician and Silber in the early years of this century. These latter workers first reported such varied photo• reactions as the photoisomerization of carvenone to carvone camphor, the photodimerization of stilbene, and the photoisomerization of o-nitrobenzal• dehyde to o-nitrosobenzoic acid; yet organic chemists continued for another fifty years or so to rely almost wholly on thermal rather than photochemical methods of activation in organic synthesis-truly a dark age. When my colleagues and I first began in the 1950s to study the synthetic possibilities of photoexcitation in the chemistry of benzene and its derivatives, virtually all the prior reports had indicated that benzene was stable to ultraviolet radiation. Yet I think it fair to say that more different types of photoreactions than thermal reactions of the benzene ring are now known. Comparable growth of knowledge has occurred in other branches of organic photochemistry, and photochemical techniques have in particular made possible or simplified the synthesis of numerous highly strained organic molecules. Developments in organic photochemistry have been greatly stimulated by a growing understanding of the underlying mechanisms. In particular, the rules formulated by Woodward and Hoffmann have proved to have consid• erable predictive value despite the potentially complicating fact that most organic photoreactions are diabatic, i.e., the products are formed in their electronic ground states. The development in Japan of a photochemical process for the manufac- ix x Foreword

ture of caprolactam by photo-oximation of cyclohexane encouraged many workers to believe that further large-scale applications in organic synthesis lay just over the horizon. In fairness, it must be said that these expectations remain as yet unfulfilled, though there have been some useful applications within the pharmaceutical and insecticide industries. Although future larger scale devel

D. Bryce-Smith Professor of Organic Chemistry Preface

Organic photochemistry has in the last 30 years reached the full maturity of a scientific discipline. From its early beginnings the subject has continued to grow rapidly and hardly a month passes without the scientific literature announcing a novel photo-reaction or a new natural product synthesis using a photo-reaction as a key step. The steady development of the subject has seen its application to the synthesis of many compounds which could not have been synthesized by conventional routes and where often the photo• chemical path can be superior to existing ground-state steps. There is, however, a reluctance for the synthetic chemist to embrace the technique wholeheartedly regardless of the synthetic usefulness and the detailed understanding of many of the photochemical processes. This textbook is aimed at the chemist who is looking for newer and superior methods. Sadly it is difficult to make a single volume totally com• prehensive. However the contents have been selected with an eye for areas where there is currently active interest and where there is undoubted synthetic potential. The topics covered are aromatic compounds, enones, electron• transfer reactions, phthalimides, heterocycles, azirines, photo-protecting groups, oxetan formation, and a brief outline of equipment and techniques. Where possible the authors have highlighted the synthetic value of the reactions under discussion. It is hoped, by this approach, that more inves• tigators in academic life and, more importantly, in industry will be attracted to this exciting and worthwhile area.

xi Contents

1. Photoaddition and Photocyclization Processes of Aromatic Compounds...... 1 Andrew Gilbert

I. Introduction ...... 1 2. Intermolecular Reactions ...... 2 2.1. Arenes and Ethenes ...... 2 2.2. Arenes and Acetylenes ...... 9 2.3. Arenes and 1,3-Dienes ...... 12 2.4. Acyclic Additions to Arenes and Reduction Processes ...... 14 2.5. Arene Photodimerizations ...... 18 3. Intramolecular Cyclization Processes ...... 21 3.1. Arene-Ethene and Arene-Ethyne Systems ...... 21 3.1.1. Hydrocarbon Systems...... 21 3.1.2. Aryl Enones ...... 25 3.1.3. Aryl Ethenyl Ethers, Thioethers, and N-Aryl Enamines. 27 3.1.4. Cyclizations of N-Arylanilides and N-Benzoylenamines ...... 29 3.2. Aryl Butadiene and Butenyne Systems ...... 33 3.3. Intramolecular Photocoupling of Arenes ...... 37 3.3.1. Stilbene-Phenanthrene-Type Cyclizations ...... 37 3.3.2. Photocoupling of Arenes with Intervening Units Other Than Ethylene...... 44 3.3.3. Coupling of Arenes as a Result of Halogen Acid Photoelimination 46 3.4. Photocyclizations of o-Substituted Arenes ...... 49 References ...... 50

2. Enone Photochemical Cycloaddition in Organic Synthesis...... 61 Alan C Weedon

I. Introduction ...... 61 2. The Reaction Mechanism ...... 62 3. Regiochemistry of Enone Cycloaddition ...... 70 4. Stereochemistry of Enone Cycloaddition ...... 91 xiii xi v Contents

5. Intramolecular Enone Cycloadditions...... 99 6. The de Mayo Reaction...... 105 6.1. Cyclic Diketones ...... 106 6.2. Acyclic 1,3-Diketones...... 110 6.3. Intramolecular Additions of Enolized Diketones ...... 114 7. Intermolecular Enone Cycloaddition ...... 118 8. Photochemical Cycloaddition between Enones and Allenes ...... 131 References ...... 134

3. Synthetic Aspects of Photochemical Electron Transfer Reactions...... 145 Patrick S. Mariano and Jerome L. Stavinoha

I. Introduction...... 145 1.1. Synthetic Applications of Photochemical Reactions...... 145 1.2. Electron Transfer in Excited-State Chemistry...... 146 1.2.1. Exciplex Formation and Decay...... 146 1.2.2. Electron Transfer Efficiencies and Rates...... 148 1.3. Chemical Consequences of Electron Transfer...... 151 1.3.1. General Survey and Predictions...... 151 1.3.2. Overview of Coverage...... 156 2. Reaction Pathways Followed in Electron Transfer Photochemistry...... 156 2.1. Alkenes and Strained Ring Hydrocarbons...... 156 2.1.1. Alkenes as Electron Acceptors and Donors...... 156 2.1.2. Cyclopropene Electron Transfer Photochemistry...... 167 2.1.3. Cyclopropanes and Other Strained Ring Systems as Electron Donors...... 171 2.2. Amines...... 176 2.2. J. General Considerations...... 176 2.2.2. Amine-Olefin Electron Transfer Photochemistry...... 177 2.2.3. Amine-Ketone Electron Transfer Photochemistry...... 181 2.3. Ketones...... 186 2.3. J. General Considerations...... 186 2.3.2. Ketone Electron Transfer Photochemistry...... 186 2.3.3. Quinone Electron Transfer Photochemistry...... 189 2.4. Electron Transfer Photooxygenations...... 194 2.4.1. General Considerations...... 194 2.4.2. Class 1 Photooxygenations ...... 195 2.4.3. Class 2 Photooxygenations ...... 196 2.4.4. Class 3 Photooxygenations ...... 198 2.5. Aromatic Systems...... 202 2.5.1. Aromatic Photosubstitution Reactions...... 202 2.5.2. Photobenzylations by Electron Transfer Mechanisms...... 215 2.5.3. Birch and Related Photoreductions of Aromatic Compounds...... 219 2.6. Electron Transfer Photochemistry of Nitrogen Heteroaromatic Salts and Related Systems...... 222 2.6. J. General Considerations...... 222 Contents xv

2.6.2. Electron Transfer Photochemistry of Pyridinium and Related Salts 223 2.6.3. Iminium Salt Photochemistry...... 226 2.7. Other Electron Transfer Initiated Photoreactions...... 231 2.7.1. Olefin Cycloadditions and Retro Processes...... 231 2.7.2. Phthalimide Photochemistry...... 234 2.7.3. Photodecarboxylations...... 235 2.7.4. Metal Catalyzed Photoreactions ...... , ...... 237 References...... 239

4. Phthalimide and Its Derivatives...... 259 John D. Coyle

I. Introduction...... 259 2. Hydrogen Abstraction ...... 259 2.1. Photoreduction and Photoaddition ...... 259 2.2. Intramolecular Photocyclization ...... 263 3. Reaction with Alkenes ...... 272 3.1. Ring Expansion...... 272 3.2. Solvent Incorporation...... 274 3.3. Intramolecular Reaction...... 275 4. Cleavage Reactions...... 277 4.1. a-Cleavage...... 277 4.2. ~-Cleavage...... 277 5. Spectra and Excited States...... 279 6. Summary...... 279 References ...... 280

5. Photochemical Addition Reactions in the Benzo(b)Thiophene, Benzo(b)Furan, and Indole Series...... 285 Douglas C Neckers and Alois B.A. Tinnemans

I. Introduction...... 285 1.1. Absorption and Emission Characteristics ...... 287 2. Photochemistry...... 288 2.1. Oxetan Formation...... 289 2.2. Dimerization...... 289 2.3. Photochemical Reactions with Acetylene Esters...... 291 3. Benzo(b)thiophene...... 293 4. Indoles...... 296 5. Benzo(b)thiophenes...... 299 6. Benzo(b)furans...... 301 7. Thermal Reactions of the 2-Hetero-bicyclo(3.2.0)heptadienes...... 302 8. I.3-Dimethylindole...... 305 9. Photocycloadditions of Benzo(b)thiophene to Alkenes...... 307 References ...... 308 xvi Contents

6. Azirine Photolysis and Cycloaddition Reactions...... 313 Albert Padwa

1. Introduction...... 313 1.1. UV Spectra of Arylazirines ...... 314 1.1.1. Evidence for Nitrile Ylide Formation...... 315 2. Cycloaddition Reactions...... 316 2.1. Addition with Electron-Deficient Olefins...... 316 2.1.1. Mechanism of Cycloaddition to Olefines ...... 316 2.2. Addition to Aldehydes and Ketones...... 322 2.3. Addition to Esters...... 324 2.4. Additions to Acid Chloride and Anhydrides...... 326 2.5. Additions to Carbon Dioxide and Carbon Disulfide...... 328 2.6. Additions to Ketenes. Carbodiimides. Isocyanates. and Isothiocyanates. . . . 328 3. Photodimerization of Azirines ...... 329 4. Intramolecular Cycloaddition Reactions ...... 334 4.1. Rearrangement of 2-Vinylazirines ...... 334 4.2. Rearrangements of Isoxazoles and Oxazoles ...... 337 5. Mechanisms for Cycloaddition Reactions of Nitrile Ylides...... 340 5.1. The l,l-Cycloaddition Process...... 345 5.2. Summary of 1.1-Cycloaddition Mode...... 350 5.3. Other Examples of 1.1-Cycloaddition...... 351 5.4. 1.1- and 1.3-Cycloaddition in Competition...... 353 6. Fragmentation Reactions...... 360 6.1. Competitive Involvement of Ylide Formation...... 364 7. Group Migration Reactions...... 365 References ...... 367

7. Photoremovable Protecting Groups... .•.•.••....•. .•.•. .•... .•..• 375 Roger W. Binkley and Thomas W. Flechtner

1. Introduction...... 375 2. Alcohols...... 377 2.1. Sulfonic Acid Esters...... 377 2.2. o-Nitrobenzyl Ethers...... 382 2.3. Nitrates. Nitrites. Dimethylthiocarbamates. and Aryl Azides...... 385 3. Diols ...... 386 4. Phenols...... 392 5. Aldehydes and Ketones...... 394 6. Carboxylic Acids and Amides...... 397 6.1. o-Nitrobenzyl Esters...... 398 6.2. o-Nitrophenylamino Derivatives...... 400 6.3. Phenacyl Esters...... 403 6.4. 2.4-Dinitrophenylthio Esters. Benzoin Esters. and Aryl Azides ...... 404 7. Amines...... 407 7.1. Benzyloxycarbonyl Compounds...... 407 7.2. Sulfonamides...... 412 7.3. Formamides. N-Oxides. and 4.5-Diaryl-4-oxazoline-2-ones ...... 413 Contents xvii

8. Phosphates ...... 414 9. Conclusion ...... 416 References ...... 417

8. Photochemical Synthesis of Oxetans ...... 425 Howard A. J. Carless

I. Introduction ...... 425 2. Reaction Mechanism ...... 427 2.1. Carbonyl Excited States ...... 430 2.2. Triplet Energies ...... 431 2.3. Competing Reactions...... 432 2.4. Hydrogen Abstraction ...... 434 3. Alkene Addends ...... 434 3.1. Simply Substituted Alkenes ...... 435 3.2. Dienes and Trienes ...... 442 3.3. Allenes and Ketenimines ...... 447 3.4. Electron-Deficient Alkenes ...... 449 3.5. Vinyl Ethers...... 451 3.6. Heteroaromatics ...... 455 4. Carbonyl Addends ...... 457 4.1. a-Dicarbonyls ...... 457 4.2. Quinones...... 460 4.3. Miscellaneous Carbonyls .. 463 4.4. Enones and Ynones .. 466 5. Intramolecular Cycloadditions...... 468 6. Chemical Reactions of Oxetans ...... 475 References ...... 476

9. Equipment and Techniques...... 489 William M. Horspool

I. Mercury Vapor Lamps ...... 490 2. Lamps in Conjunction with Filters. 492 3. Photochemical Apparatus .. 495 4. Actinometry ...... 500 4.1. Solution Phase Systems .. . 500 4.2. Electronic Actinometers ...... 505 5. Purity of Solvents and Gases .... . 506 References ...... 508 Index ...... 511