Diastereoselective induction in radical coupling reactions: photolysis of 2,4-diphenylpentan-3-ones adsorbed on faujasite zeolites

Abstract

The photochemistry of d, l and meso isomers of 2,4-diphenylpentan-3-one (DPP), adsorbed onto various cation-exchanged X and Y faujasites, was investigated. LiY and NaY show greater diastereoselectivity for the formation of the d,l isomer of diphenylbutane over the meso isomer than LiX and NaX. KY displays a significantly attenuated selectivity relative to LiY and NaY. The magnitude of the diastereoselectivity is a function of the substrate configuration and the difference between the magnitude of diastereoselection exhibited by LiY and NaY over LiX and NaX is greater when the substrate is d,l-DPP rather than meso-DPP. All the zeolites studied show substantially more diastereomerization relative to decarbonylation, as measured by the s parameter ( vide infra), when compared with other supercage media such as micelles and porous glass. However, substrate configuration is irrelevant when comparing the s parameters. The different cation-exchanged Y zeolites all show similar s parameters, whereas the X zeolites display a non-monotonic behavior with respect to the s parameter and cation size. The stereoselectivities for NaX and NaY are independent of loading and these media display cage effects of 100% for d,l-DPP.