Photochemistry of 7-syn-isobutylnorborn-2-en-2-yl methyl ketone. Intramolecular hydrogen abstraction in an α,β-unsaturated ketone with geometrically equivalent α- and β-carbon atoms

Abstract

Preparation and photolysis of 7-syn-isobutylnorborn-2-en-2-yl methyl ketone (7) and its deuteriated isomer (8) are described. Irradiation of (7) in its n,π* band (λ > 3 400 Å) leads to two volatile isomers (11)(55%) and (13)(10%), whilst (8) gives (11d) and (13d). The same products were obtained on irradiation of (7) and (8) in the π,π* band (λ > 2 500 Å), along with two additional ketones, the photoreduction product (16) and the dienone (25). These results indicate that isomerization of (7) occurs through hydrogen abstraction by the β-carbon atom of the enone system, but not the α-carbon atom. Thus minimization of stereochemical and geometric differences between α and β carbons relative to the abstractable hydrogen does not suffice to elicit abstraction at both centres. The structure of the brexane photoproduct (13) was secured through independent synthesis from olefinic ester (38a). Synthesis of (7), (8), and (13) made use of the novel reaction of a norbornene with trichloroacetyl isocyanate to form an unsaturated nitrile [as (31)→(34) and (38)→(39)].