The origin of enantioselectivity in photochemical reactions with diarylprolinol silyl ether catalysts – Ching Ching Lam, University of Cambridge

January 26, 2022 @ 2:30 pm – 3:00 pm
via zoom
Lisa Masters

Diarylprolinol silyl ethers are important organocatalysts that enable stereoselective functionalisation at positions neighbouring the carbonyl group. However, applications of the catalysts in photochemical pathways have not yet been widely explored. Here, the stereoselective photochemical synthesis reaction of 1,4-dicarbonyls with diarylprolinol silyl ether catalysts was studied with force field and density functional theory calculations. A procedure has been developed for investigations of large and flexible chemical systems based on the conformation labelling system, ONIOM calculations and Python scripting. The procedure allows efficient and effective use of computational resources and tackles the challenges in processing outputs from conformational searching calculations.

We explain the enantioselectivity from the perspective of conformational changes and structural deformations based on the radical addition step of the reactions. The iminium in the most stable SR transition state (TS) takes up the conformation of the most stable ground state iminium (EE). The iminium diene in the SS TS adopts an EZ conformation to avoid potential structural deformations due to radical attacks from the more sterically hindered position. For systems with simpler catalysts, the iminium is EE for both, as the steric hindrance imposed by the substituent is not sufficient to cause this large structural deformation, and so the reaction shows poor enantioselectivity.

Leave a Reply