Cooperativity between steric repulsion and crossed diene coordination governs the enantioselectivity in rhodium(i)/chiral, 1,4- and 1,5-diene-catalysed 1,4-addition reaction: a DFT study

Abstract

A comparative DFT (IEFPCM/PBE0/DGDZVP) study of the Rh-catalysed, enantioselective 1,4-addition of phenylboronic acid to 2-cyclohexenone with 11 known cyclic, chiral 1,4- and 1,5-diene ligands reveals a common pathway involving a transition state-less binding (EB) of 2-cyclohexenone to a [(diene)Rh–Ph] intermediate in a multitude of orientations leading to carborhodation (CR) via two competing, diastereomeric transition states (TS), which collapse to α-rhodioketones and then by further conformational reorganization to Rh-oxa-π-allyls. The energy difference between CR-TSs determines the enantioselectivity. DFT-predicted energy difference values are in good to excellent agreement with those derived from experimental enantiomeric excess values. Enantioselectivity was shown to be determined by a cooperative action of crossed diene coordination (measured by the angle formed by the two Rh-coordinated CC bonds in the ligand) and steric repulsion of the ligand substituents (e.g., phenyl) and 2-cyclohexenone. The cooperative effect is the strongest in the hitherto unknown 3,7-diphenylbicyclo[3.3.0]octa-2,6-diene, which was predicted to give the highest enantioselectivity of all ligands studied.