Stereoelectronic effect in the capture reaction of the 1,2-dimesityl-2-phenylvinyl radical

Abstract

The reduction of (E)-1,2-dimesityl-2-phenylvinyl bromide (E-4-Br) to the corresponding ethene (4-H) under radical conditions (Bu₃SnH–AIBN–benzene–hν) proceeds with predominant inversion of configuration to give an E-4-H/Z-4-H ratio of ≥9∶1. This suggests that the intermediate 1,2-dimesityl-2-phenylvinyl radical (3) is captured preferentially from the mesityl face. The reason for this is clarified by theoretical calculations which show that in the conformation of 3 the β-mesityl ring is close to perpendicular to (78.2°) and the β-phenyl ring is nearly coplanar (12.0°) to the plane of the double bond (at BLYP/6-31G*). The calculated transition state energy for hydrogen transfer from the model hydrogen donor SiH₄ is 5.7 kcal mol^–1 lower from the mesityl than from the phenyl face.

Comparison with the analogous ketene MesC(Ph)CO and the vinyl cation MesC(Ph)C⁺–Mes systems shows the same preference for reaction from the formally bulkier, but in fact easier to approach mesityl face, which is likewise governed by steric effects.