Control of chirality by cations in confined spaces: photooxidation of enecarbamates inside zeolite supercages

Abstract

On photooxygenation of the optically active Z/E enecarbamates 1 (X =i-Pr) and 2 (X = Me) equipped with the oxazolidinone chiral auxiliary in methylene-blue (MB)-incorporated, alkali-metal (M = Li, Na, K, Cs, Rb), exchanged Y-type zeolites (MY-MB), oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyl-desoxybenzoin (MDB) product. The extent (%ee) and/or the sense (R or S) of the stereoselectivity in the formation of the MDB product depends on the choice of the alkyl substiuent (i-Pr or Me) at the C-4 position of the oxazolidinone chiral auxiliary, the Z/E configuration of the alkene functionality in the enecarhamates, and the type of alkali metal in the zeolite. Most significantly-the highlight of this stuy-is the reversed sense (R or S) in the stereoselection when the photooxygena tion is run in CDCI₃ solution versus inside the MY-MB zeolite. As a mechanistic rationale for this novel stereochemical behavior, we propose the combined action of spatial confine ment and metal-ion coordination (assessed by density-functional calculations) of the substrate within the zeolite supercage, both of which greatly reduce the freedom of the substrate and entropically manipulate the stereochemical outcome.