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

Abstract

The cationic ruthenium-hydride complex [(eta(6)-C(6)H(6))(PCy(3))(CO)RuH](+)BF(4) (-) 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(CH(3))CH=CHPh at an initial turnover rate of 2300 h(-1). A rapid and extensive H/D 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-d(8) in chlorobenzene at 70 degrees C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singleton's method ((13)C(recovered)/(13)C(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 dimerization reaction. The Eyring plot of the dimerization of styrene in the temperature range of 50-90 degrees C led to DeltaH(double dagger) = 3.3(6) kcal/mol and DeltaS(double dagger) = -35.5(7) e.u. An electrophilic addition mechanism has been proposed for the dimerization of styrene.