Acid and Polarity Effects in Benzene Photoreactions with Alkenes and Alkynes D. Bryce-Smith*, A. Gilbert, N. Al-Jalal, R. R. Deshpande, J. Grzonka, M. A. Hems, and P. Yianni Chemistry Department, University of Reading, Whiteknights, Reading, Berkshire, RG6 2 AD Z. Naturforsch. 38b, 1101-1112 (1983); received May 2, 1983 Photoreactions, Benzene, Alkenes, Alkynes, Acid Effects The effects of polar solvents and proton donors on various photoreactions of the benzene ring with electron-acceptor alkenes and alkynes are described and discussed in relation to the various electronic excitation mechanisms involved. Proton donors prove valuable as mechanistic probes for polar intermediates in such processes, and can in some systems both initiate and divert reaction pathways. In 1970 we observed that the course of the photo- anhydride and certain maleimides, a second mole- chemical cycloadditions of certain ethylenic and cule may add thermally to give the remarkably acetylenic compounds to benzene, earlier reported stable 2:1 adducts (2) and (3) [1, 12]. The ortho- by us [1, 2], was profoundly altered in the presence adducts of acetylenes (4) undergo valence isomeri- of proton donors, leading to a type of photochemical zation to give cyclo-octatetraenes; e.g. dimethyl Friedel-Crafts reaction [3]. Since then, we have acetylenedicarboxylate gives exclusively the isomer described, mostly in preliminary form, numerous (5) having a single bond between the substituents, further examples of acid catalysis in the photo- apparently for steric reasons [2], Addition mode (a) reactions of benzene, viz. with amines [4], ethers shows the greatest sensitivity to proton donors and [5], a ketone [6], a carboxylic acid [6], and hexa- polar solvents, and discussion of these effects forms fluorobenzene [7]. Our objective in this present the main part of the present paper. paper is to bring together the previous preliminary Addition mode (b) is observed with alkenes having reports on acid catalysis in the photoreactions of an ionization potential similar to that of benzene; ethylenic and acetylenic compounds with benzene e.g. cis-cyclooctene gives an approximately 4:1 in conjunction with unpublished recent findings, mixture of the meta adducts (6) and (7) with the and to provide a mechanistic rationale together preparatively useful combined quantum yield of with relevant experimental details. 0.38 [13]. No example of analogous meta photo- But before we proceed to consider the acid-cata- addition of an acetylene has yet been convincingly lysed processes, it is desirable to summarise the main established. types of reaction that have been shown to occur in Addition mode (c) is observed less frequently, the absence of acids. Ethylenes and/or acetylenes though para photoadducts (8) have occasionally photoadd to benzene in the following four main been reported as minor products, e.g. from cis-but- ways: (a) ortho cycloaddition; (b) meto-cycloaddi- 2-ene [14] and cyclobutene [15] (but not cyclopro- tion; (c) £>ara-cycloaddition; and (d) para-ene addi- pene, which adds meta [13]). Only allenes have so tion [8, 9]. The mode of addition varies with the far been found to undergo para cycloaddition as the addend, and in some cases more than one pathway predominant mode; e.g. allene and benzene mainly may be followed. In the case of (a), the adducts (1) give the adduct (9), with a minor proportion of the of ethylenes may be obtained as such when the meta adduct (10) [16]. ethylenic addend is not a strong thermal dienophile, Ene-addition, mode (d), is only observed with e.g. acrylonitrile [10], 2,3-dihydropyran [11]. When relatively electron-donor alkenes having the group it is a strong dienophile, as in the case of maleic Me2C=, and is the predominant mode in the photo- addition of tetramethylethylene to benzene, giving the adduct (11) [13]. * Reprint requests to Prof. D. Bryce-Smith. The orbital symmetry aspects of these processes 0340-5087/83/0900-1101/$ 01.00/0 have been analysed by one of us [17], and the con- 1102 D. Bryce-Smith et al. • Acid and Polarity Effects in Benzene Photoreactions elusions reached have been found to have good pre- terion (13) or a related dipolar exciplex species [13], dictive value [8]. It is particularly relevant to the With CH3OD as solvent, we have shown that D is mechanism of acid effects that the analysis shows incorporated at the 4-position of the ring in (11). the three new bonds formed in meta cycloadditions We come now to the ortho cycloadditions, mode to arise in a partially or fully concerted manner (a), which are by far the most interesting in their from Si benzene plus So alkene, whereas concerted susceptibility to polar factors. ortho cycloaddition can only arise from So benzene As mentioned above, orbital symmetry con- plus Si alkene, or via charge transfer either to or siderations suggest that homopolar ortho cyclo- from the arene. additions of So ethylene or acetylene to Si benzene Not surprisingly, we and others have observed are forbidden as concerted processes. The only that the largely homopolar concerted meta addition, allowed ortho cycloadditions involve Si ethylene or mode (b), usually shows little or no sensitivity acetylene plus So benzene. Charge-transfer excita- either to the presence of proton donors or to solvent tion is also fully "allowed', though polar effects pre- polarity [13, 18]. The only exception to this so far dominate in practice. These predictions have proved noted is in the photoaddition of ethyl vinyl ether to valid to a remarkable degree, and no clear excep- anisole, giving adducts (12), which is markedly pro- tions have yet been discovered. Thus ortho cyclo- moted by solvent acetonitrile in comparison with additions, when they occur normally in the pre- cyclohexane [19]. There is however no diversion of sence of a excess of benzene, have involved (i) the reaction pathway. The only mechanistic con- ethylenic or acetylenic addends having an Si state clusion we have yet reached on this apparent below Si benzene (in practice having significant anomaly is that the transition state(s) for formation absorption near to or beyond the benzene solvent of adducts (12) is or are markedly more polar than front beyond ca. 280 nm but not showing charge- the system at earlier stages along the reaction co- transfer absorption in benzene at wavelengths ordinate, and more polar than the corresponding >280nm, (ii) charge-transfer excitation at wave- transition states for formation of analogous adducts lengths > 280 nm, (iii) systems where the reactants where one or both -OEt groups are absent. The comprise a marked donor acceptor pair (in practice most odd feature of this anomalous case is that the having an ionisation potential difference > ca. 1 eV) sensitivity to solvent polarity is manifest in a but show no charge-transfer or other absorption reaction which is expected a priori to be essentially beyond the benzene solvent front. homopolar in view of the closely similar ionization N-(w-Butyl)maleimide and dimethyl acetylene- potentials of anisole and ethyl vinyl ether (8.54 and dicarboxylate could in principle fall either in 8.6 eV respectively). The observed meta rather than category (i) or category (iii): the ortho adduct ortho cycloaddition is however in accord with the (4:R=R'=C02Me) from the latter can be trapped orbital symmetry analysis [17]. with tetracyanoethylene, but otherwise undergoes As for the rather rare para cycloaddition, mode rapid valence isomerization to the corresponding (c), no studies of solvent polarity or proton donor cyclo-octatetrane (5) [20] (which does not appear effects have yet been reported, and we ourselves to undergo the expected bond exchange to the con- have not yet investigated these matters. former/tautomer (14) to any significant extent [21]). Maleimide and its N-alkyl derivatives give the 2:1 On the other hand, the para ene-addition, mode adducts (3) [22], via the 1:1 adducts (15): the latter (d), is markedly promoted by proton donor solvents can be trapped by tetracyanoethylene (which does such as methanol, but not by aprotic polar solvents not itself photoadd to benzene) and maleic anhy- such as acetonitrile. As we have discussed our studies on this process in some detail elsewhere [13], dride as the 1:1:1 adducts (16) and (17) respectiv- it will be sufficient for the present purposes to point ely [23]. out that para-ene photoaddition is non-stereospe- The maleimides and the acetylene photoadd to cific and occurs by a non-concerted mechanism, in benzene on irradiation at wavelengths >290nm, contrast with the concerted mechanism normally so direct photoexcitation of benzene can be ruled attributed to conventional thermal ene-additions. out. Furthermore, although both the maleimides We have suggested the intermediacy of the zwit- and dimethyl acetylenedicarboxvlate have electron 1103 D. Bryce-Smith et al. • Acid and Polarity Effects in Benzene Photoreactions acceptor properties relative to benzene, the ultra- to acid implies the substantial absence of a polar violet spectra of these in benzene show no signs of intermediate capable of being intercepted by tri- significant charge-transfer absorption at wave- fluoroacetic acid, but the mechanistic significance lengths beyond the benzene solvent front at ca. of this observation is better appreciated by com- 280 nm: the absorption due to the maleimides and parison with the effects, described below, of acids the acetylene above 280 nm is virtually identical in on the seemingly analogous photoaddition of maleic benzene, cyclohexane, and chloroform, and extends anhydride to benzene. almost to the visible region, though the acetylene In contrast, irradiation of dimethyl acetylene- absorbs much more weakly than the maleimides. dicarboxylate in benzene in the presence of tri- Direct photoexcitation of the addends therefore fluoroacetic acid gave the results shown in Table I. clearly occurs.