NHC Ruthenium Complexes as Second Generation Grubbs Catalysts

Introduction

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.

References

• For relevant reviews see:
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• 1b Arduengo AJ. Acc. Chem. Res.  1999,  32:  913 
• 1c Herrmann WA. Angew. Chem. Int. Ed.  2002,  41:  1290 
• For recent papers see:
• 2a Jafarpour L. Heck MP. Baylon C. Lee HM. Mioskowski C. Nolan SP. Organometallics  2002,  21:  671 
• 2b Díez-Barra E. Guerra J. Rodríguez-Curiel RI. Merino S. Tejeda J. J. Organomet. Chem.  2002,  660:  50 
• For relevant reviews see:
• 3a Trnka TM. Grubbs RH. Acc. Chem. Res.  2001,  34:  18 
• 3b Fürstner A. Angew. Chem. Int. Ed.  2000,  39:  3012 
• 3c See also reference 1c
• 3d A Spotlight article dealing with olefin metathesis: Chauder BA. Synlett   1999.  p.267 
• 4 Weskamp T. Schattenmann WC. Spiegler M. Herrmann WA. Angew. Chem. Int. Ed.  1998,  37:  2490 . Corrigenda. Angew. Chem. Int. Ed.  1999,  38: 
• 5 Huang J. Stevens ED. Nolan SP. Petersen JL. J. Am. Chem. Soc.  1999,  121:  2674 
• 6a Scholl M. Trnka TM. Morgan JP. Grubbs RH. Tetrahedron Lett.  1999,  40:  2247 
• 6b Scholl M. Ding S. Lee CW. Grubbs RH. Org. Lett.  1999,  1:  953 
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• 7b Weskamp T. Kohl FJ. Hieringer W. Gleich D. Herrmann WA. Angew. Chem. Int. Ed.  1999,  38:  2416 
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• 8 For an exhaustive study of the mechanism see: Sandford MS. Love JA. Grubbs RH. J. Am. Chem. Soc.   2001.  123:  p.6543 
• 9 Sold by Strem Chemicals Inc. (Cat. No. 44-7770); Acros Organics (Cat. No. 35616-2500); Sigma-Aldrich Co. (Cat. No. 56,974-7)
• 10 Lee CW. Choi T.-L. Grubbs RH. J. Am. Chem. Soc.  2002,  124:  3224 
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• 14 Vastenamer^® and Norsorex^® are polymers made by ROMP from cyclooctene and norbonene respectively: Bhaduri S. Mukesh D. In Homogeneous Catalysis. Mechanisms and Industrial Applications   John Wiley & Sons; New York: 2000. 
• 15 Bielawski CW. Grubbs RH. Angew. Chem. Int. Ed.  2000,  39:  2903