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.