Stereoelectronic effects in the nucleophilic addition to the sp-hybridized carbon of a ketene and vinyl cation: When is a mesityl effectively smaller than a phenyl ring?

Addition of MesMgBr to mesitylphenylketene 3 yields a ca. 3:1 mixture of the magnesium enolates 5-(E) and 5-(Z), whereas reaction of enol 1-(Z) with MesMgBr results in the exclusive formation of enolate 5-(Z). Acetylation of the configurationally stable 5-(E) and 5-(Z) yields the enol acetates 4-(E) and 4-(Z), respectively. The preference for the formation of the (E)-enolate from 3 indicates that the attack occurs preferentially from the side of the formally bulkier mesityl ring. Ketene 3 and the two diastereomeric transition states obtained by attack of MeLi on its C=O were calculated by ab initio calculations. 3 adopts a conformation with a planar Ph-C=C moiety while the mesityl is nearly perpendicular to this plane. Since the attack on the C=O group occurs in the plane of the C=C double bond, the coplanar Ph is effectively bulkier to the approaching nucleophile than the twisted Mes and the preferred attack is from the mesityl side. Similar stereoelectronic effects operate in 1,2-dimesityl-2-phenylvinyl cation (10).