DUAL ACTIVITY OF RUTHENIUM CATALYSTS IN CONTROLLED RADICAL REACTIONS AND OLEFIN METATHESIS

摘要

The ruthenium-arene complexes 5a-e described in this study are fine examples of readily accessible ruthenium N-heterocyclic carbene catalysts. Depending on the substituents of the carbene ligand, they can be tuned to promote atom transfer radical reactions or olefin metathesis of carbon-carbon double bonds. The quantitative ATRA of carbon tetrachloride onto styrene was achieved in the presence of complex 5c (R_1 = Mes, R_2 = Cl), but complex 5e bearing two different carbene substituents (R_1 = Cy, R_2 = H) was also a rather efficient catalyst at 90°C. Complexes 5b and 5c with R_1 = mesityl and R_2 = H or Cl, respectively, were most suitable for promoting the ATRP of methyl methacrylate. Their use resulted in well-behaved polymerizations and afforded macromolecular products with narrow molecular weight distributions, M_w/M_n, and high initiation efficiencies, f. In the case of styrene, complex 5e (R_1 = Cy, R_2 = H) was the most efficient catalyst precursor for initiating a controlled radical polymerization. A switch in the reaction pathway could be induced, however, by replacing the cycloalkyl group with a mesityl substituent on the nitrogen atoms. Thus, complexes 5a-c displayed a dual activity and afforded both the cross-metathesis and the ATRP products. Among the three species tested, the chloro derivative 5c led to the highest proportion of stilbene compared to polystyrene. When the ROMP of cyclooctene was investigated at room temperature, complex 5c lost its superiority as a metathesis catalyst precursor to the benefit of its parent 5b (R_1 = Mes, R_2 = H). At the present time, it remains difficult to put forward general guidelines to rationalize the choice of a specific ruthenium-NHC catalyst precursor for a given reaction. Depending on the substrate and the experimental conditions adopted (temperature, solvent, presence of an initiator or a cocatalyst,...) the coordination sphere around the metal center must be specifically tailored to afford the most efficient catalytic system. Fine tuning of the electronic, steric, and solubility parameters of the carbene ligand, undoubtedly contributes to these adjustments, but any correlation between well-defined 18-electron ruthenium-arene complexes and the active coordinatively unsaturated species generated in situ is blurred by the elusive nature of the actual catalytic system.