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2.10.4.1 Catalyst and Mechanism

DOI: 10.1055/sos-SD-222-00346

Lee, D.; Reddy Sabbasani, V.Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (20162650.

The availability of catalysts with desirable features is one of the key requirements for their practical use. All alkyne metathesis catalysts known to date belong to either ill-defined [the so-called Mortreux system consisting of hexacarbonylmolybdenum(0) and various phenols] or well-defined single-component catalyst systems (Schrock-type complexes). The latter have played a major role in the development of new air-stable yet alkyne-metathesis-active catalysts. Well-defined single-component catalysts are generally viewed as variants of Schrock-type alkylidyne complexes of high oxidation state molybdenum (e.g., 1 and 555560) or tungsten (e.g., 8 and 554) (Scheme 120). Except for the Mortreux system, all classical Schrock-type alkylidyne complexes are highly moisture- and oxygen-sensitive, and molybdenum complex 555 is even sensitive to molecular nitrogen. The first well-defined and widely used alkyne metathesis catalyst is Schrock-type tungsten complex 8 containing bulky alkoxide ligands that prevent or slow down bimolecular decomposition of the alkylidyne intermediates in the catalytic cycle.[‌200‌] Other bulky ligands having alkyl, thiolate, and amide groups do not show much improvement in catalyst activity.[‌201‌] Significant improvements of the performance of Schrock-type complexes have emerged mainly due to the systematic studies of Fürstner and coworkers, and their broad range of applications have included complex natural product synthesis.[‌202‌]

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