Alkylation reactions with organometallic compounds. I. the reaction of methyltris(triphenylphosphine)rhodium with diphenylacetylene

The reaction of diphenylacetylene with (Ph₃P)₃RhCH₃ was studied in order to clarify the role of the organometallic compound as an alkylating reagent. Trans-α-methylstilbene was the main product of addition to the alkyne. The reaction is mostly an insertion process as concluded from studies with D₂O. It was compared with the reaction of diphenylacetylene with the systems (Ph₃P)₃RhBr/CH₃MgBr, and RhBr₃·3H₂O/CH₃MgBr. With the first system mainly trans-α-methylstilbene was formed. In this reaction however, (Ph₃P)₃RhCH₃ is shown not to be an active intermediate. With the second system, cis-addition was observed with a high degree of stereospecificity cis-α-methylstilbene being the main product. Condensation of diphenylacetylene to products like 1,2,3-triphenylnaphthalene, 1,2,3,4-tetraphenylbutadiene and 1,2,3,4-tetraphenylcyclopentadiene was also observed.