The dimerization and cyclotrimerization of acetylenes mediated by phosphine complexes of cobalt(I), rhodium(I), and iridium(I)

The catalytic dimerization and cyclotrimerization of phenylacetylene, ethyl propiolate, and dimethyl acetylenedicarboxylate were studied by using the metal complexes [Co(PPh3)(3)Cl] (1), [Co(PPh3)(2)(CO)(2)Cl] (2), [Co(PMe3)(3)Cl] (3), [Co(PMe3)(2)(CO)(2)Cl] (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 degrees C: space group P2(1)/c (No. 14), a 8.774(4), b 22.155(3), c 20.242(2) Angstrom, beta 92.46(2)degrees, V 3931(1) Angstrom(3), Z 4, N-o 4804, N-var 469, R(F) 0.0411, R-w(F) 0.0483.