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

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.