Cross-Metathesis of Vinyl Halides. Scope and Limitations of Ruthenium-Based Catalysts

The phosphine-free “second-generation” Blechert/Hoveyda−Grubbs catalyst Ru(═C(H)C 6 H 4 - o -O- i -Pr)(H 2 IMes)Cl 2 (H 2 IMes = 4,5-dihydro-1,3-dimesitylimidazol-2-ylidene) and Piers catalyst [Ru(═CHPCy 3 )(H 2 IMes)Cl 2 ]BF 4 (Cy = cyclo -C 6 H 11 ) for olefin metathesis effected cross-metathesis (CM) of vinyl chloride and 1,2-dichloroethene with several unhindered terminal and internal alkenes in up to 95% yield (5 mol % catalyst). In most cases, 1,2-dichloroethene was more successful than vinyl chloride. Ring-opening CM of cyclooctene provided greater yields than CM: with vinyl chloride, 93% yield; with 1,2-dichloroethene, >95%. Other common Ru-based catalysts failed to effect CM under similar conditions, but instead underwent rapid decomposition. The dimeric ruthenium-monochloromethylidene complex [Ru(═CHCl)(H 2 IMes)Cl(μ-Cl)] 2 was isolated as a thermally unstable intermediate. CM reactions with 1,2-dibromoethene afforded 22% CM product in the best case; halide exchange with the catalyst was significant. CM reactions involving vinyl fluoride typically led to <3 turnovers, but the dimeric intermediate [Ru(═CHF)(H 2 IMes)Cl(μ-Cl)] 2 was sufficiently long-lived to be characterized by single-crystal X-ray diffraction. Ring-opening CM of cyclooctene with vinyl fluoride (55% yield) was more favorable than CM.