Synthesis and Evaluation of Molybdenum and Tungsten Monoaryloxide Halide Alkylidene Complexes for Z-Selective Cross-Metathesis of Cyclooctene and Z-1,2-Dichloroethylene

Abstract

Molybdenum complexes with the general formula Mo(NR)(CHR′)(OR″)(Cl)(MeCN) (R = t-Bu or 1-adamantyl; OR″ = a 2,6-terphenoxide) recently have been found to be highly active catalysts for cross-metathesis reactions between Z-internal olefins and Z-1,2-dichloroethylene or Z-(CF₃)CH═CH(CF₃). In this paper we report methods of synthesizing new potential catalysts with the general formula M(NR)(CHR′)(OR″)(Cl)(L) in which M = Mo or W, NR = N-2,6-diisopropylphenyl or NC6F5, and L is a phosphine, a pyridine, or a nitrile. We also test and compare all catalysts in the cross-metathesis of Z-1,2-dichloroethylene and cyclooctene. Our investigations indicate that tungsten complexes are inactive in the test reaction either because the donor is bound too strongly or because acetonitrile inserts into a W═C bond. The acetonitrile or pivalonitrile Mo(NR)(CHR′)(OR″)(Cl)(L) complexes are found to be especially reactive because the 14e Mo(NR)(CHR′)(OR″)Cl core is accessible through dissociation of the nitrile to a significant extent. Pivalonitrile can be removed (>95%) from Mo(NAr)(CHCMe₂Ph)(OHMT)(Cl)(t-BuCN) (Ar = 2,6-diisopropylphenyl; OHMT = 2,6-dimesitylphenoxide) to give 14e Mo(NAr)(CHCMe₂Ph)(OHMT)Cl in solution as a mixture of syn and anti (60:40 at 0.015 M) nitrile-free isomers, but these 14e complexes have not yet been isolated in pure form. The syn isomer of Mo(NAr)(CHCMe₂Ph)(OHMT)Cl binds pivalonitrile most strongly. Other Mo(NR)(CHR′)(OR″)(Cl)(L) complexes can be activated through addition of B(C₆F₅)₃. High stereoselectivities (>98% Z,Z) of ClCH═CH(CH₂)₆CH═CHCl are not restricted to tert-butylimido or adamantylimido complexes; 96.2% Z selectivity is observed with boron-activated Mo(NC₆F₅)(CHR′)(OHIPT)(Cl)(PPhMe₂). So far no Mo═CHCl complexes, which are required intermediates in the test reaction, have been observed in NMR studies at room temperature.