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

• Published in: Organometallics 2010-08, Vol.29 (15), p.3413-3417
• Main Authors: Lee, Do W, Yi, Chae S
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om100468q

The cationic ruthenium hydride complex [(η6-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 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 °C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singleton’s method (13C(recovered)/13C(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 °C led to ΔH ⧧ = 3.3(6) kcal/mol and ΔS ⧧ = −35.5(7) eu. An electrophilic addition mechanism has been proposed for the dimerization of styrene.