Catalytic Metathesis of Conjugated Diynes

摘要

Alkyne metathesis has recently witnessed a significant growth in the number of homogeneous catalysts that are able to efficiently promote the formation and cleavage of carbon-carbon triple bonds.All well-defined catalyst systems reported to date can be regarded as variants of Schrock-type molybdenum(VI) or tungsten(VI) alkylidyne complexes in which the metal-alkylidyne moiety, M≡∞C-R, is supported by alkoxide, arlyoxide, amido, imidazolin-2-iminato phosphoraneiminato,or silanolate ligands. With a few exceptions,lkyne metathesis usually requires the presence of internal alkynes, such as RC≡CMe (R = alkyl or aryl) affording the symmetric alkynes RC≡CR and MeC≡CMe To drive these equilibrium reactions to completion, the latter 2-butyne (b.p. =27°C), can be continuously removed from the reaction mixtures under vacuum-driven conditions or as recently shown by Furstner and co-workers, advantageously by addition of molecular sieves (5 A M.S.) to adsorb 2-butyne. For our part, the molecular-sieve-promoted method was also clearly superior when alkyne homocouplins and ring-closing alkyne metathesis (RCAM) reactions were studied in the presence of catalytic amounts of the tri(tert-butoxy)silanolate-supported tungsten benzylidyne complex 1 (Scheme 1).