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Synthesis of N-heterocyclic carbene ligands and derived ruthenium olefin metathesis catalysts


We describe the synthesis of commonly used free N-heterocyclic carbenes (NHCs), 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), and of the two corresponding ruthenium-based metathesis complexes. The complex containing IMes was found to be highly efficient in macrocyclizations involving ring-closing metatheses (RCM), whereas the complex featuring the IPr ligand shows excellent activity in both RCM and cross metathesis because of its greater stability. The free carbenes IMes and IPr are isolated in four steps, with an overall yield of 50%. They are then used to replace a labile phosphine in precatalysts belonging to two families of ruthenium-containing complexes, benzylidene and indenylidene types, respectively. Such complexes are isolated as analytically pure compounds with 77% and 95% yield. The total time for the synthesis of the free NHCs is 56 h, and incorporation in complexes requires an additional 4–5 h.

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Figure 1: NHC and complexes described in this contribution.
Figure 2: Synthesis of free IMes and IPr.
Figure 3: Synthesis of olefin metathesis complexes.


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We are grateful to the European Community for funding through the seventh framework program (CP-FP 211468–2-EUMET).

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X.B. carried out experimental procedure development, synthetic work and assembly of the manuscript; S.P.N. performed ligand synthesis and manuscript assembly.

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Correspondence to Steven P Nolan.

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The authors declare no competing financial interests.

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Bantreil, X., Nolan, S. Synthesis of N-heterocyclic carbene ligands and derived ruthenium olefin metathesis catalysts. Nat Protoc 6, 69–77 (2011).

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