Stereocontrol within Confined Spaces:  Enantioselective Photooxidation of Enecarbamates Inside Zeolite Supercages

Abstract

Dye-exchanged Y zeolite is shown to be an effective medium to control the stereoselectivity in the photooxygenation of chiral oxazolidinone-functionalized Z/E-1 enecarbamates. An enantioselectivity (ee) as high as 80% was observed in the methyldesoxybenzoin (MDB) product, obtained in the methylene-blue-exchanged NaY zeolite at room temperature. The efficacy of the asymmetric induction in the MDB product depends on the Z/E geometry of the alkene, the Z-isomer being more effective than the corresponding E-isomer. The stereoselectivity is rationalized in terms of conformational effects through cationic interactions between the zeolite and the substrate.

Chiral nanotechnology

“…97 The formation of left-handed Z-DNA has two requirements, a sequence that is capable of undergoing the B -Z transition, and the conditions that promote the transition. The device consists of two rigid DNA double-crossover motifs connected by a short d(GC) 10 shaft, which are capable of undergoing the B -Z transition. While still in B-promoting conditions, these nucleotides will be in the B state, but, in the presence of Z-promoting reagent, Co(NH 3 ) 6 Cl 3 , they convert to Z-DNA.…”

Chiral Nanoscience and Nanotechnology

“…This type of chiral porous hybrid solid can also catalyze the asymmetric hydrogenation of aromatic ketones and ß-keto esters 28 . Similarly, stereocontrol photooxidation of enecarbamates within confined spaces of zeolite has been reported 29 . Asymmetrically modified zeolites have also been employed to achieve high stereoselectivity in photo-oxygenation.…”

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

“…3), whereas for the corresponding E(S) ‐ 1 isomer, the S ‐MDB product is preferred with an ee value of 62% (Table 1, entry 8; see Fig. 3) (35).…”

“…2 have been investigated in this work. MB‐exchanged Y zeolites (MY‐MB) prepared by cation exchange (25, 39) were utilized for the photooxidative cleavage of the alkenyl functionality in these chiral enecarbamates, to produce the enantiomerically enriched MDB (35). No enantiocontrol would afford a racemic MDB, whereas perfect enantiocontrol would generate one of its enantiomers exclusively.…”