Diastereomer-differentiating photochemistry of beta-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 alpha and beta-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.
Singhal
Nidhi
N
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India.
Koner
Apurba L
AL
Mal
Prasenjit
P
Venugopalan
Paloth
P
Nau
Werner M
WM
Moorthy
Jarugu Narasimha
JN
D002090:Butyrophenones / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D008958:Models, Molecular; D008968:Molecular Conformation; D010777:Photochemistry; D013237:Stereoisomerism
D002090:Butyrophenones