Diastereomer-Differentiating Photochemistry of β-Arylbutyrophenones: Yang Cyclization versus Type II Elimination

The diastereomers of ketones 2 and 3 are shown to exhibit distinct photochemical reactivities
due to conformational preferences; while the anti isomers of 2 and 3 undergo efficient Yang cyclization in
75−90% yields with a remarkable diastereoselectivity (> 90%), the syn isomers predominantly undergo
Norrish Type II elimination. The differences in the product profiles of the diastereomers are consistent with
a mechanistic picture involving the formation of precursor diastereomeric triplet 1,4-biradicals in which the
substituents at α and β-positions stabilize the cisoid (cyclization) or transoid (elimination) geometry. The
fact that such a diastereomeric relationship does indeed ensue at the triplet-excited-state itself is
demonstrated via the nanosecond laser-flash photolysis of model ketones 1. The diastereomeric
discrimination in the product profiles observed for ketones 2 and 3 as well as in the triplet lifetimes observed
for ketones 1 can both be mechanistically traced back to different conformational preferences of the ground-state diastereomeric ketones and the intermediary 1,4-biradicals. Additionally, it emerges from the present
study that the syn and anti diastereomers of ketones 2 and 3 represent two extremes of a broad range of
widely examined butyrophenones, which lead to varying degrees of Yang photocyclization depending on
the alkyl substitution pattern.