NHC Ruthenium Complexes as Second Generation Grubbs Catalysts

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Abstract
Developments in two seemingly unconnected fields have given rise to a new family of powerful catalysts with many applications in olefin metathesis. On the one hand, the progress made over the past decade in the synthesis of N-heterocyclic carbenes (NHCs) and their complexes, as well as the catalytic applications of these systems, has been impressive. [ 1 ] [ 2 ] On the other hand, the achievements in the evolution of homogeneous catalysts for olefin metathesis have turned this reaction into a common C-C coupling process [ 3 ] with various possibilities including ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), acyclic diene metathesis polymerization (ADMET), ring-opening metathesis (ROM), and cross-metathesis (CM or XMET). Herrmann et al. [ 4 ] were the first to synthesize a modified Grubbs catalyst that included N-heterocyclic carbene ligands. The bis(NHC)-substituted derivatives 2 showed a minor improvement in terms of catalytic activity compared to conventional Grubbs catalyst 1. Soon after, papers concurrently published by Nolans’, [ 5 ] Grubbs’, [ 6 ] and Herrmanns’ [ 7 ] groups described the synthesis of mono(NHC)-substituted complexes such as those of type 3 and 4. This new family of compounds has been called the second generation Grubbs catalysts. The strong σ donor character of NHC ligands facilitates the dissociation of the phosphine in compounds 3 and 4 due to the trans effect, although the key step for olefin metathesis consists of the coordination of the olefin to the unsaturated ruthenium complex. [ 8 ] These new compounds (3 and 4), which are reasonably air- and water-stable, are more active than 2 or 1 and can also tolerate the presence of different functional groups (with the exception of basic ones like nitriles and amines). [ 3a ] The enhanced activity of these compounds means that the reactions can be carried out under mild conditions. Three years after its synthesis, [ 6b ] complex 4a [R = Mes (mesityl), R′ = Cy] has become commercially available [ 9 ] owing to its wide-ranging applications. Compound 4a was synthesized from 1 and the saturated 1,3-dimesityl-2,3,4,5-tetrahydro-1H-imidazol-2-ylidene, which was protected as the alkoxide to generate the free carbene ligands in situ.

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