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-(CF)CH═CH(CF). 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 NCF, 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)(CHCMePh)(OHMT)(Cl)(t-BuCN) (Ar = 2,6-diisopropylphenyl; OHMT = 2,6-dimesitylphenoxide) to give 14e Mo(NAr)(CHCMePh)(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)(CHCMePh)(OHMT)Cl binds pivalonitrile most strongly. Other Mo(NR)(CHR')(OR″)(Cl)(L) complexes can be activated through addition of B(CF). High stereoselectivities (>98% Z,Z) of ClCH═CH(CH)CH═CHCl are not restricted to tert-butylimido or adamantylimido complexes; 96.2% Z selectivity is observed with boron-activated Mo(NCF)(CHR')(OHIPT)(Cl)(PPhMe). So far no Mo═CHCl complexes, which are required intermediates in the test reaction, have been observed in NMR studies at room temperature.

Synthesis of E- and Z-trisubstituted alkenes by catalytic cross-metathesis

“…1b), revealing that initiation entails reaction of Mo complex i with 1a (not Z - 2 ) to give disubstituted alkylidene ii . Reaction of ii with Z - 2 may subsequently lead to the putative chloro-substituted alkylidene iii 27 , which may then react with 1a to give methylidene v and 3a via metallacyclobutane iv , with the quaternary carbon center at the less hindered Cβ 28 (for more detailed analysis, see Extended Data Fig. 1).…”

Functional Precision Polymers via Stereo‐ and Regioselective Polymerization Using Group 6 Metal Alkylidene and Group 6 and 8 Metal Alkylidene N‐Heterocyclic Carbene Complexes

“…Molybdenum‐based catalysts are preferred over tungsten‐based ones since the metallacyclobutane in the latter can either be TBP or a square‐pyramidal (SP) while molybdacyclobutanes usually TBP. Nonetheless, numerous examples for Z‐selective tungsten‐based catalysts exist, too . In an extension of the work outlined above, Schrock et al.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[224] Some of the first active homogeneous metathesis catalysts were based on molecular Mo-imido-alkylidene complexes of general type Mo(=NR 1 )(=CHR 2 )(X) 2 (X = alkoxide,a ryloxide,o ra mide). [569,570] Themost active monoalkoxide-pyrrolide (MAP) [571][572][573][574][575] and monoalkoxidechloride (MAC) [576] catalysts also exploit electronic dissymmetry at the metal center-a key factor in generating highly active metathesis catalysts (see Section 7.2.2.1). [569,570] Themost active monoalkoxide-pyrrolide (MAP) [571][572][573][574][575] and monoalkoxidechloride (MAC) [576] catalysts also exploit electronic dissymmetry at the metal center-a key factor in generating highly active metathesis catalysts (see Section 7.2.2.1).…”