title: 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 methyldesoxybenzoin (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 enecarbamates, and the type of alkali metal in the zeolite. Most significantly-the highlight of this study-is the reversed sense (R or S) in the stereoselection when the photooxygenation is run in CDCl3 solution versus inside the MY-MB zeolite. As a mechanistic rationale for this novel stereochemical behavior, we propose the combined action of spatial confinement 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.
authors.affiliation: The Department of Chemistry, Columbia University, New York, NY, USA.
authors.forename: J
authors.initials: J
authors.lastname: Sivaguru
authors.forename: Hideaki
authors.initials: H
authors.lastname: Saito
authors.forename: Marissa R
authors.initials: MR
authors.lastname: Solomon
authors.forename: Lakshmi S
authors.initials: LS
authors.lastname: Kaanumalle
authors.forename: Thomas
authors.initials: T
authors.lastname: Poon
authors.forename: Steffen
authors.initials: S
authors.lastname: Jockusch
authors.forename: Waldemar
authors.initials: W
authors.lastname: Adam
authors.forename: V
authors.initials: V
authors.lastname: Ramamurthy
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Inoue
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Turro