Hydrogenolysis versus Methanolysis of First- and Second-Generation Grubbs Catalysts: Rates, Speciation, and Implications for Tandem Catalysis

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Abstract

An unexpected “generation gap” is uncovered between the Grubbs catalysts RuCl2(L)(PCy3)(═CHPh) (1a, L = PCy3; 1b, L = IMes, N,N′-bis(mesityl)imidazol-2-ylidene) in their reactions with hydrogen versus methanol, in the presence of base. Treatment of the first-generation catalyst 1a with H2 and NEt3 (CH2Cl2, 60 °C, 1000 psi H2) affords RuHCl(H2)(PCy3)2 (2a) in 75% yield within 30 min, as determined by in situ NMR analysis. Complex 2a is in turn efficiently converted (96%; 2 h) into the important hydrogenation catalyst RuHCl(CO)(PCy3)2 (3a) by mild thermolysis with methanol and NEt3 (4:1 CH2Cl2−MeOH, 60 °C), 72% net yield for the 1a−3a transformation. In comparison, subjecting the second-generation catalyst 1b to this two-step process effects <40% net conversion to RuHCl(CO)(IMes)(PCy3) (3b) (hydrogenolysis of 1b: ca. 60% RuHCl(H2)(IMes)(PCy3) (2b) (1 h); carbonylation of isolated 2b: 65% 3b (2.5 h)), owing to the susceptibility of the dihydrogen derivative 2b to disproportionation and decomposition. The opposite trend in efficiency for 1a versus 1b is found for methanolysis under argon in the presence of base: 1b undergoes 83% conversion to 3b, versus <60% for the 1a−3a transformation (4:1 CH2Cl2−MeOH, 60 °C). This difference reflects the longer duration of the methanolysis reaction (8 h for 1a vs 4 h for 1b; cf. 30 min and 1 h, respectively, for hydrogenolysis) and the lower thermal robustness of 1a. These findings highlight the importance of tailored, catalyst-specific approaches in devising efficient tandem catalysis methodologies based on the first- and second-generation Grubbs complexes. They are directly relevant to the improved synthesis of advanced polymer materials via tandem ROMP−hydrogenation and potentially relevant to RCM- and CM-functionalization processes.

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