Cyclization of diphenylacetylene on methylchromium σ-complexes. I. Hydrogenation and transmethylation

The cyclic and linear condensations of diphenylacetylene in the presence of the methylchromium tetrahydrofuranate system are accompanied by reactions with the methyl-metal complex, namely transfer of methyl groups and hydrogen to the unsaturated ligand. cis- and trans-Stilbenes, α-methylstilbenes, α-benzylstyrene, and dibenzyl are formed among other condensation products. The additions are sterically controlled; the cis isomers initially formed isomerize to the trans olefins. The abstraction and transfer of hydrogen occur within the sysem and either the methyl groups or the solvent act as the major sources. Methyl transfers were also observed when methylmagnesium halide and diphenylacetylene were treated with halides of cobalt and nickel. Methylmagnesium chloride alone, methyllithium or trilithium hexamethylchromium failed to react with the alkyne.