The Dimerization and Cyclotrimerization of Acetylenes Mediated by Phosphine Complexes of Cobalt(I), Rhodium(I), and Iridium(I).

Abstract

The catalytic dimerization and cyclotrimerization of phenylacetylene, ethyl propiolate, and dimethyl acetylenedicarboxylate were studied by using the metal complexes [Co(PPh₃)₃Cl] (1), [Co(PPh₃)₂(CO)₂Cl] (2), [Co(PMe₃)₃Cl] (3), [Co(PMe₃)₂(CO)₂Cl] (4), [Rh(dppe)(CO)Cl] (5), [Rh(PPh₂Me)₂(CO)Cl] (6), [Ir(dppe)(CO)Br] (7), and [Ir(PPh₂Me)₂(CO)Cl] (8). For all complexes studied, with terminal acetylene substrates, there is competition between head-to-head dimerization to form 1,4-disubstituted butenynes and cyclotrimerization to form 1,3,5- or 1,2,4-trisubstituted benzenes. The metallocyclopentadiene [Ir(dppe)(CO)(L)Br] (L= 1,4- diethoxycarbonylbuta-1,3-diene-1,4-diyl) was formed in the reaction of diethyl acetylenedicarboxylate with [Ir(dppe)(CO)Br] (7), and this stable complex is a possible intermediate in the cyclotrimerization of the acetylene. The metallocyclopentadiene was fully characterized by X-ray crystallography. Crystal data for the benzene solvate of (33), C₄₃H₄₂BrIrO₅P₂ at 21°C: space group P2₁/c (No. 14), a 8.774(4), b 22.155(3), c 20.242(2) Å, β 92.46(2)˚, V 3931(1) ų , Z 4, N_o 4804, N_var 469, R(F) 0.0411, R_w(F) 0.0483.