Control of Chirality by Cations in Confined Spaces: Photooxidation of Enecarbamates Inside Zeolite Supercages


  • This paper is pant of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday.


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.