The reaction of some endo-5-acetyl norborn-2-enes with benzene thiol: a thermally induced non-allylic [1,3]-acetyl shift, as anticipated by a photochemical precedent

Abstract

The thermal reaction of benzene thiol with rac-1-(4aS*,5R*,8S*,8aR*-5,8-methano-1,2,3,4,4a,5,8,8a-octahydro-4a-naphthyl)ethanone (10b), an endo-5-acetylnorborn-2-ene with a particularly close distance between one olefinic carbon and the keto carbon, and with its 8aS* epimer 10a, has been investigated. Besides the conventional unrearranged adducts 16a, b and 17a, b of the thiol to the C=C bonds of 10a, b, an adduct 18 was formed from 10b that involved a [1,3]-acetyl shift; no other products were formed. The dependence of adduct ratios on thiol concentration is consistent with two competing reactions of an intermediate radical 14b formed by addition of thiyl radical to the C=C bond of 10b, namely, abstraction of hydrogen from thiol (which is the conventional reaction path) vs. intramolecular attack of the radical on the keto group inducing the [1,3] shift of the latter. This shift constitutes an intramolecular variety of the known alkyl transposition reaction of ketones initiated by a reversible attack of an alkyl radical on a keto carbonyl group to generate an intermediate tert-alkoxyl radical; however, it is very much faster than the intermolecular reaction and corresponds to an effective molarity between 107 and 1010. These findings have a bearing on the mechanism of photorearrangements homologous to the ODPM (oxa-di-π-methane) photorearrangement, such as ODPE (oxa-di-π-ethane) and higher homologues, in that they support a two-step mechanism involving this type of acyl shift. Keywords: oxadi-π-methane photorearrangement; oxadi-π-ethane photorearrangement; acetyl shift, non-allylic [1,3]; thiol, addition to C=C bonds; norbornenes.