Spectroscopic properties of axially and equatorially substituted β-trimethylstannyl ketones and compounds with related chromophores

Abstract

The synthesis of two optically active cyclohexanones, with a β-trimethylstannyl substituent, are described and their c.d. spectra are reported. Whereas the axial SnMe₃ group confers a dissignate (anti-octant) contribution on the n→π* transition relative to the parent unsubstituted ketone, the equatorial SnMe₃ group contributes strongly in a consignate (octant) sense. This contrasting behaviour is rationalized in terms of a conformationally dependent through-bond coupling between the lone pair on the carbonyl oxygen atom and the electron-donating C–Sn bond which gives rise to a higher energy transition in the equatorially substituted compound. This band is assigned to the second component of the split n→π* transition. The spectral properties of some βγ-unsaturated ketones, in which the p orbital on the β-carbon atom is in the same orientation with respect to the carbonyl group as the equatorial C–Sn bond, are identical. Thus the high energy band, formerly assigned to a charge transfer transition, should be reassigned as a second n→π* transition.