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.
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