Highly efficient molybdenum-based catalysts for enantioselective alkene metathesis

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Abstract
Alkene metathesis (also called olefin metathesis or transalkylidenation) is an organic reaction widely used to synthesize products including medicines, polymers and fuels. Its importance was recognized in 2005, when Yves Chauvin, Robert Grubbs and Richard Schrock shared the Nobel prize for work on metathesis. A new class of molybdenum-based chiral catalyst, capable of initiating alkene metathesis with exceptional efficiency and enantioselectivity, is reported in this issue. The new catalysts bear a stereogenic metal centre and carry only monodentate ligands. Their effectiveness was demonstrated in an enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, via a metathesis reaction that cannot be promoted by any of the previously reported catalysts. This paper discloses a new class of chiral catalysts that initiate olefin metathesis with exceptional efficiency and enantioselectivity. These new catalysts bear a stereogenic metal centre and carry only monodentate ligands; the molybdenum-based complexes are rendered non-racemic by a stereoselective ligand exchange process involving an enantiomerically pure aryloxide, a class of ligands rarely used in asymmetric catalysis. Discovery of efficient catalysts is one of the most compelling objectives of modern chemistry. Chiral catalysts are in particularly high demand, as they facilitate synthesis of enantiomerically enriched small molecules that are critical to developments in medicine, biology and materials science1. Especially noteworthy are catalysts that promote—with otherwise inaccessible efficiency and selectivity levels—reactions demonstrated to be of great utility in chemical synthesis. Here we report a class of chiral catalysts that initiate alkene metathesis1 with very high efficiency and enantioselectivity. Such attributes arise from structural fluxionality of the chiral catalysts and the central role that enhanced electronic factors have in the catalytic cycle. The new catalysts have a stereogenic metal centre and carry only monodentate ligands; the molybdenum-based complexes are prepared stereoselectively by a ligand exchange process involving an enantiomerically pure aryloxide, a class of ligands scarcely used in enantioselective catalysis2,3. We demonstrate the application of the new catalysts in an enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, through an alkene metathesis reaction that cannot be promoted by any of the previously reported chiral catalysts.

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