Chain-Selective and Regioselective Ethylene and Styrene Dimerization Reactions Catalyzed by a Well-Defined Cationic Ruthenium Hydride Complex: New insights on the Styrene Dimerization Mechanism

The cationic ruthenium hydride complex [(eta(6)-C6H6)(PCy3)(CO)RuH]+BF4- was found to be a highly regioselective catalyst for the ethylene dimerization reaction to give 2-butene products (TOF = 1910 h(-1), >95% selectivity for 2-butenes) The dimerization of styrene exclusively produced the head-to-tail dimer (E)-PhCH(CH3)CH=CHPh at an initial turnover rate of 2300 h(-1) A rapid and extensive Hill exchange between the vinyl hydrogens of styrene-d(8) and 4-methoxystyrene was observed within 10 min without forming the dimer products at room temperature. The inverse deuterium isotope effect of k(H)/k(D) = 0.77 +/- 0.10 was measured from the first-order plots on the dimerization reaction of styrene and styrene-d8 in chlorobenzene at 70 degrees C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singleton's method (C-13(recovered)/C-13(virgin) at C-1 = 1 096 and C-2 = 1.042) indicates that the C-C bond formation is the rate-limiting step for the dimenzation reaction The Eyring plot of the dimerization of styrene in the temperature range of 50-90 degrees C led to Delta/H+/- = 3.3(6) kcal/mol and Delta S+/- = -35.5(7) cu An electrophilic addition mechanism has been proposed for the dimerization of styrene.