The Dimerization and Cyclotrimerization of Acetylenes Mediated by Phosphine Complexes of Cobalt( <i>I</i> ), Rhodium( <i>I</i> ), and Iridium( <i>I</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 P 21/ c (No. 14), a 8.774(4), b 22.155(3), c 20.242(2) Å, ß 92.46(2)°, V 3931(1) Å3 , Z 4, N o 4804, N var 469, R ( F ) 0.0411, R w( F ) 0.0483.