A general method for preparation of metal carbenes via solution- and polymer-based approaches

A new general, synthetically simple, and safe method for the preparation of metal carbene complexes, which is based on diphenyl sulfonium salts as carbenoid precursors, has been developed, and its scope and applications were studied. In general, deprotonation of a sulfonium salt with a base results in a sulfur ylide, which, in turn, reacts with an appropriate metal precursor to give the corresponding metal carbene complex. Thus, starting from benzyldiphenylsulfonium salt, the complexes (PCX)Rh=CHPh (X = P, N) were prepared in quantitative yield. Syntheses of Grubbs' catalyst, (PCy3)(2)Cl2Ru=CHPh, and of Werner's carbene, [Os( CHPh)HCl(CO)((PPr3)-Pr-i)(2)], were achieved by this method. Novel trans-bisphosphine Rh and Ir carbenes, ((Pr3P)-Pr-i)(2)(Cl)M=CHPh, which could not be prepared by other known methods, were synthesized by the sulfur ylide approach. The method is not limited to metal benzylidenes, as demonstrated by the preparation of the Ru vinyl-alkylidene, (PCy3)(2)Cl2Ru=CH-CH=CH2, methoxycarbonyl-alkylidene, (PCy3)(2)Cl2Ru=CH(CO2Me), and alkyliclene (PCy3)(2)Cl2Ru CH(CH3), (PCy3)(2)Cl2Ru=CH2 compounds. The problem of recycling of starting materials as well as the issue of facile purification of the product metal carbene complex were addressed by the synthesis of a polymer-supported diarylsulfide, the carrier of the carbenoid unit in the process. Based on the sulfur ylide route, a methodology for the synthesis of metallocarbenes anchored to a polymer via the carbene ligand, using a commercial Merrifield resin, was developed.