On a mechanism for the trans-addition of alkylmagnesium and alkylrhodium derivatives to diphenylacetylene

The reaction of diphenylacetylene with tris(triphenylphosphine)rhodium bromide yields 1,2-diphenylpropene mostly as the trans-isomer. When reactions are terminated with D₂O it is shown that trans-1,2-diphenylpropene is deuteriated at the vinylic position and at the ortho-position of the aromatic ring adjacent to the methyl group. The cis-isomer is deuteriated at the vinylic position only. These results offer an explanation of the mechanism of trans-addition in this case. Possible implications for other cases of trans-addition are discussed.