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A Thesis Presented by in Partial Fulfilment of the Requirements for The i SYNTHETIC USES OF ORGANOSELENIUM REAGENTS a thesis presented by RAYMOND ALBERT HOONG FAI HUI in partial fulfilment of the requirements for the award of DOCTOR OF PHILOSOPHY Whiff en Laboratory Chemistry Department Imperial College January 1981 London SW7 To my -parents and my family. II ACKNOWLEDGEMENTS I thank Professor Sir D.E.R. Barton, F.R.S., for the opportunity to work with him and for his supervision and patient guidance during the course of this work. I also thank Dr. S.V. Ley for his help3 advice and many ideas throughout the course of this work. I am also grateful to Mrs. J. Lee and Mr. J. Bilton for excellent mass spectra and mass measurements3 Mr. K.I. Jones and his staff for microanalyses3 Mr. J. Burgess for XL-100 n.m.r. spectra3 the technical staff of Imperial College3 and Mrs. B. Day and Mrs. I. Hamblin for their friendship and help in the organic stores. Finally3 I thank Shell Sarawak Ltd. for a scholarship in my initial academic career but especially my parents and family for support and encouragement throughout. iv ABSTRACT A review of aspects of the recent organic chemistry of selenoxides, seleninic and selenenic acids and anhydrides, and of benzeneperoxy- seleninic acid is presented. The reactions of benzeneseleninic anhydride (BSA)(1) and benzeneseleninic acid (15) with various organic substrates were investigated, with a general aim of extending the synthetic applications of BSA (1) and (15). Some work with "benzeneselenenic anhydride" (17) generated in situ was also carried out. Ph-Se-O-Se-Ph (1) (15) (17) Benzylic alcohols were smoothly and rapidly oxidised by BSA to the corresponding benzaldehydes in very high to quantitative yields. Benzeneseleninic acid could also be used successfully in benzene at reflux but less efficiently in tetrahydrofuran. Steroidal and triterpenoid alcohols on reaction with BSA for brief periods usually gave good-yields of the corresponding conjugated unsaturated ketones. As examples, cholestan-33-ol (41) gave cholesta- 1,4-dien-3-one (42) (50%) while methyl oleanolate (52) gave the Ring A enone (53) (83%). The reaction was thought to proceed via the corresponding ketone. Products of overoxidation, e.g. A-nor diketones, were occasionally observed, especially with triterpenoid substrates. Lupeol (54) suffered additional oxidation by BSA at the allylic carbon to give the enone-enal (55) (60%). This side-chain oxidation was also observed with lupeol acetate (69). In both cases selenenylated products, e.g. (71) were also obtained. The reaction of (69) with "benzeneselenenic anhydride" II formed in situ was investigated in some detail Simpler allylic and steroidal homoallylic alcohols usually gave complex mixtures on reaction with BSA. However,rc-decanol gav e rc-decanal in moderate yield (72%). Carbocyclic methyl arenes could be oxidised by BSA to the aryl aldehydes but the process was not general. In addition, carboxylic acids were often formed via further oxidation by BSA, especially in extended reactions. Electron-rich methyl arenes tended to undergo benzene- selenenylation. However, useful yields of aryl aldehydes could be obtained from heterocycles with "active" methyl groups. Methylene arenes gave many carbonyl-containing and selenenylated compounds due to multiple oxidations and were generally not synthetically useful substrates for BSA. vi Esters were unsuccessfully oxidised by BSA but steroidal six-ring lactones gave the corresponding unsaturated lactones (161), (163) in good yields. Under certain conditions angular hydroxylation occurred with the D-homo- lactone to give (164) and (165). The structure and stereochemistry of (164) was proved by X-ray crystallography. 0 H (161) H (163) 0 AcO H H (164) (165) Finally, certain phenols on reaction with BSA gave selenenylated qui nones and phenols, while benzaldehydes could be obtained from benzyl bromides on treatment with BSA. vii CONTENTS Acknowledgements iii Abstract iv Preamble and Review Preamble 1 Review Selenoxides 2 Seleninic Acids and Anhydrides 21 Selenenic Acids and Anhydrides 50 Benzeneperoxyseleninic Acid 61 References to Review 69 Results and Discussion A. Oxidation of Alcohols A(i) Introduction 79 A(ii) Oxidation of Benzylic Alcohols 80 A(iii) Oxidation of Propargylic, Allylic, and Saturated Straight-chain Alcohols 93 A(iv) Oxidation of Steroidal and Triterpenoid Alcohols 97 B. Oxidation of Aromatic Hydrocarbon Side Chains B(i) Introduction 128 B(ii) Oxidation of Carbocyclic Methyl Arenes 130 B(iii) Oxidation of Heterocyclic Methyl Arenes 157 B(iv) Oxidation of Methylene Arenes and Related Aroyl Compounds 168 C. Oxidation/Dehydrogenation of Esters and Lactones C(i) Introduction 181 C(ii) Reaction of Esters with Benzeneseleninic Anhydride 183 C(iii) Reaction of Lactones with Benzeneseleninic Anhydride 187 D. Miscellaneous Oxidations D(i) Oxidation of Phenols 200 'D(ii) Oxidation of Bromides 205 Experimental 208 References 253 Appendix: Literature Publications PREAMBLE AND REVIEW The development of modern organoselenium chemistry began to gather momentum some two decades ago and in this short span of time has either acquired or been shown to have a wide ranging importance in many varied fields 1 '2 . However, selenium compounds are highly toxic and often malodorous although it is one of the quirks of nature that selenium in 1 3 trace amounts is essential in the sustaining of life ' . For the organic chemist, perhaps the most significant importance of selenium lies in its contributions to the methodology of organic synthesis and excellent reviews in this respect have recently been published 4 '5 *6 '7 , while some coverage of the more recent literature has also been given 8-13 by others " . The work in this thesis is concerned in the main with extension of the uses of some organic selenium-oxygen reagents and it is therefore appropriate to review here some of the recent chemistry of selenoxides, selenenic acids, selenenic anhydrides, seleninic acids and seleninic anhydrides. 2 REVIEW 0 SELENOXIDES Ri-Se-R2 In 1967 it was observed by Huguet 14 that oxidation of certain alkyl 15 selenides gave olefinic products, while Jones et al later reported the decomposition of steroidal selenoxides to steroidal alkenes. Since then, the formation of selenides from suitable precursors followed by their oxidation to the corresponding selenoxides and thermal decomposition to unsaturated compounds has become well established in synthetic methodology 79 16 (Scheme 1). The procedure 4-11 has been comprehensively reviewed and its scope, limitations, chemical and steric requirements and other features have been extensively defined and discussed 4-11 SCHEME 1 RSeX oxidant^ i ) base + RSeOH r-o r-0. r-o r-o 0 n RSeX v° oxidant ^ ii ) > - Y base r a X SeR b X ^eR x Y iii ) b . A RSe" (H)0 SeR 0x (H)0 ^SeR (H)0 iv ) J \ > i 1 V ^\ (v_ , y \—r„ L Scheme 1 confd: R2^0H PhSeX oxidant^ v ) 0 9 SePh 'SePh R2 = H2 or 0 RSeX ox.1 vi ) 0 SeR 0 Ov RSeX vii ) 0 > 0 0. ^ RSeX3 ^^^ 0 SeR only. viii ) 0=SeR OSeR 0 0H(R) aA suitable electrophilic selenenylating reagent, commonly PhSeCl or PhSeBr but many others have been used (see also : later discussions on oxyselenenylation). ^Commonly 30% H202, 03, NalO^, or m-CPBA. 32 499 5 97 17 Briefly, these include the sz/rc-nature of the 1,2-elimination 5 step preferentially towards the less substituted carbon to produce the (E)-olefin4 9 5 where applicable in acyclic substrates, while the 4 5 preferential formation of the erafo-olefin 9 over the eajo-olefin (see entry (vii), Scheme 1) occurs in relevant cases when attainment of sj/rc-geometry is unhindered. Allylic |2,3| sigmatropic rearrangement, where possible, also occurs preferentially^9^ over sz/n-el imination. 49 59 19 In addition, the chirality of the selenium centre in selenoxides is important and affects the course of the reaction in sterically hindered substrates 49 95 15* 20. Limitations of the reaction include the tendency of the selenenic acids (RSeOH) formed in the syn-elimination 49 59 196 29 1 22 step to add to the olefin products unless secondary amines • -I /- 9-i 9 like ^r2NH or Et2NH are present to remove the selenenic acids as less reactive selenenamides. However, this tendency may itself be 22 120 exploited synthetically to obtain allylic alcohols 9 (see later). Unwanted epoxidations of the olefin product may also occur when hydrogen peroxide is involved as oxidant especially when highly substituted olefins are being formed, although this reaction may also be utilised synthetically 22 (see later). No epoxidations were observed when 22 tert-buty] hydroperoxide was used as an alternative oxidant of the 4 5 16 selenides. In addition to earlier work 99 , a clearer definition of the various side-reactions and firmer guidelines for the optimisation of reaction conditions for the sz/rc-elimination have recently been 21 21 given : inter alia, it was found that protic solvents slowed down the sz/n-elimination; that a- or 3-chloro or phenyl, or a-alkyl substituents accelerated the elimination, while 3-alkyl or methoxy substituents slowed it down. 5 In contrast, selenoxides lacking a 3-hydrogen are somewhat more stable 4-11 1 23 but nevertheless decompose when heated sufficiently ' to give carbonyl 24 o compounds. Benzyl phenyl selenoxide does not decompose below 110 C, or racemise^4 at selenium below 100°C, although heating at 110°-130°C for a mere 2-3 minutes gave pc benzaldehyde (78%) together with diphenyldiselenide. Various other benzyl phenyl selenoxides also gave the corresponding benzaldehydes on similar treatment 25 . 0 'I 110-130° PhCH2—SePh — > PhCHO -j- P.h£e2 2 ~ 3 m 78% It is of note in connection with the oxidation of selenides to selenoxides and in their decomposition that selenoxides themselves destroy hydrogen peroxide 21 '2 2 , especially in aqueous methanol 21 .
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