Chiral-at-Tungsten Dioxo Complexes─A Computational Study on Inhibiting Racemization

Abstract

Chiral cis-WO₂(acac)₂ and cis-WO₂(nacnac)₂ complexes racemize via four pathways according to DFT calculations at ωB97X-D/6–311+G(2d,f) with LANL2DZ for W and with inclusion of acetonitrile solvent. Steric congestion by N-Me and N-Ph substitution of the two nacnac ligands has a substantial geometrical impact and raises the barriers for all pathways. Despite this, even all N-Me and all N-Ph substituted derivatives of cis-WO₂(nacnac)₂ have the same four channels for racemization. For each of these complexes, the Dhimba-Muller-Lammertsma (DML) twist is preferred over the Conte-Hippler (CH) twist and significantly favored over the established Bailar (B) and Ray-Dutt (RD) twists. The favored DML barrier for WO₂(nacnac)₂^Ph4 has a large estimated ΔG barrier of 25.7 kcal/mol, suggesting it to be a viable chiral-at-tungsten complex for asymmetric catalysis.