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Synthetic Organic Photochemistry Synthetic Organic Photochemistry

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Synthetic Organic Photochemistry Synthetic Organic Photochemistry 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 © 1984 Pl enum Press, New York Soflcovcr reprint of the hardcover I sl edition 1984 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N. Y. lOOi3 All rights reserved 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<?pments remain a definite possibility, especially for those numerous photoreactions that proceed readily under the influence of solar radiation, the main future applications of synthetic photochemistry probably lie in the production of relatively small quantities of products having high intrinsic value, for example pharmaceutical intermediates. The situation could, how­ ever, be transformed by the introduction of improved radiation sources and, as noted above, use of the freely available quanta in solar radiation, especially in desert areas of the world unsuitable for agriculture. This latter approach was pioneered by Schonberg and Mustafa, but has not yet taken hold. For example, some twenty years ago a major chemical company seriously con­ sidered manufacturing the benzene/maleic anhydride photoadduct (a poten­ tially valuable polymer intermediate which is also interesting in having bio­ logical activity as an analgesic) by a sensitized solar irradiation method in a desert area; but the idea seems to have been abandoned as involving a too unorthodox type of technology. Despite the substantial failure of industry to capitalize on the exciting laboratory developments in organic photochemistry (often for good reasons, it must be said), the potential for successful exploitation is still undoubtedly very great. The worker in synthetic organic photochemistry soon finds himself or herself in a whole new world of chemistry where he must put aside most of his preconceptions about reactivity based upon the more conventional thermal chemistry, which still dominates most chemistry textbooks and teach­ ing courses. I strongly support the objective of the present book to provide for synthetic chemists, both academic and applied, an up-to-date and broadly based account of the exciting opportunities available to those prepared to forsake the bunsen burner for the ultraviolet lamp, or even natural sunlight. 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 ..............................
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