Electron-rich olefins photoreactions

Mattay et al.5i suggested from the photoreaction of biacetyl with highly electron-rich olefins that an initial electron transfer from an electron-rich olefin to photoexcited ketone is the key step in the oxetane formation via the ion-radical pair (equation 26). 

A contribution to the synthesis of branched-chain aldonolactones. Helvetica Chimica Acta, 71, 981-987 (c) Mattay, J., Gersdorf, J., and Buchkremer, K. (1987) Photoreaction of biacetyls with electron-rich olefins. An excited mechanism. Chemische Berichte, 120, 307-318. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

Oxetane Formation. Reactions of carbonyl compounds such as ketones and aldehydes with electron-rich olefins results in the formation of oxetanes. The oxetane formation involves the addition of the carbonyl oxygen to the olefinic n -system to produce a biradical intermediate, which then undergoes spin inversion to produce the oxetane (Hor-spool, 1976). Typical examples of oxetane formation include the photoreaction of furan compounds with aldehyde or alkene compounds as shown in Scheme 4 (Cantrell, 1977 Whipple and Evanega, 1968). 

Another photoreaction between ketones and olefins or dienes, which has often been connected to the involvement of exciplexes, is the Patemo-Biichi reaction [4,5], i.e., the photocycloaddition of C=C double bonds to carbonyl C=0 bonds under formation of oxetanes [17,78,207-217], Especially for electron-rich olefins such as ethyl vinyl ether or 1,2-diethoxyethylene, intermediary exciplexes have often been postulated [212], with the consequence of a diminished legioselectivity and stereospecificity for oxetane formation. On the other hand, electron-deficient olefins such as a,P-unsaturated nitriles react with a high legioselectivity and stereospedlicity due to a well defined transition state, which is based on the electronic leqniranents of n,7t -excited ketones [17]. 

An empirical rule on a similar photoreactivity between mono- and di-olefin crystals (see Sect. II. a.) has been explained by correlating common molecular shape and packing with common intermolecular contacts between electron-rich and electron-deficient moieties in these olefin crystals. 

Figure 10 reveals that molecular shape and packing of photoreactive crystals are very similar. In addition, the electron-rich nitrogen or carbonyl group approaches the electron-deficient benzene ring in all the photopolymerizable crystals41,42, u-46,49,50. These two common features may govern the formation of photopolymerizable crystals and thus allow an empirical rule of similar photoreactivity between mono- and di-olefin crystals to be established (see Sect. II.a.). 

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