REGIOSELECTIVE AND STEREOSELECTIVE DIMERIZATION OF PHENYLACETYLENE TO (Z)-1,4-DIPHENYLBUT-3-EN-1-YNE BY RUTHENIUM(II) CATALYSIS - REACTION-MECHANISM INVOLVING INTERMOLECULAR PROTONATION OF SIGMA-ALKYNYL BY 1-ALKYNE

The Ru(II) cis-dihydride [(PP3)RuH2] (1) is a catalyst precursor for the regio- and stereoselective dimerization of HC=CPh to (Z)-PhC=CCH=C(H)Ph in toluene solution [PP3 = P(CH(2)CH(2)PPh(2))(3)]. The real catalyst of the 1-alkyne coupling reaction is the bis(alkynyl) complex [(PP3)Ru(C=CPh)(2)] (6), which is formed by treatment of 1 with 4 equiv of HC=CPh. A study under various experimental conditions, the detection of some intermediates, and the use of isolated complexes in independent reactions, taken altogether, permit mechanistic conclusions which account for both the conversion of 1 to 6 and the catalysis cycle. In particular, the transformation of 1 into 6, which is accompanied by production of 2 equiv of styrene, is traversed by a number of intermediates among which are Ru(H)(CH=CHPh), Ru(pi-CH2=CHPh), Ru(H)(C=CPh), and Ru(CH=CHPh)(C=CPh). It is proposed that key steps in the catalytic cycle of HC=CPh dimerization are the protonation of a terminal phenylethynyl ligand in 6 by external HC=CPh and the subsequent C-C bond-forming reaction between cis vinylidene and alkynyl groups. X-ray diffraction analyses have been carried out on [(PP3)RuH(C=CPh)].C6H6(5.C6H6), 6, and (E)-[(PP3)RuH{eta(1)-(MeO(2)C)C=CH(CO2-Me)}].EtOH(10.EtOH). Crystal data for 5.C6H6: orthorhombic, Pbca with a = 27.285(7) Angstrom A, monoclinic, P2(1)/a with a 10.660(2) Angstrom A, b = 40.342(9) Angstrom A, c = 11.082(4) Angstrom A, beta = 93.27(3)degrees, V = 4758 Angstrom A(3), Z = 4, R = 0.059. Crystal data for 10.EtOH: triclinic, P $($) over bar$$ 1 with a 11.611(2) Angstrom A, b = 22.377(2) Angstrom A, c = 9.490(3) Angstrom A, a = 93.87(2)degrees, beta = 109.08(2)degrees, gamma = 76.26(1)degrees, V = 2263 Angstrom A(3), Z = 2, R = 0.047.