Kinetically controlled catalytic cross-metathesis reactions that generate (Z)-α,β-unsaturated esters selectively are disclosed. A key finding is that the presence of acetonitrile obviates the need for using excess amounts of a more valuable terminal alkene substrates. On the basis of X-ray structure and spectroscopic investigations a rationale for the positive impact of acetonitrile is provided. Transformations leading to various E,Z-dienoates are highly Z-selective as well. Utility is highlighted by application to stereoselective synthesis of the C1-C12 fragment of biologically active natural product (-)-laulimalide.

Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph₄C₆H (STPP) and S-2,6-(mesityl)₂C₆H₃ (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.

Reactions between Mo(NAr)(CHR)(Me₂Pyr)-(OTPP) (Ar = 2,6-i-Pr₂C₆H₃, R = H or CHCMe₂Ph, Me₂Pyr = 2,5-dimethylpyrrolide, OTPP = O-2,3,5,6-Ph₄C₆H) and CH₂=CHX where X = B(pin), SiMe₃, N-carbazolyl, N-pyrrolidinonyl, PPh₂, OPr, or SPh lead to Mo(NAr)(CHX)-(Me₂Pyr)(OTPP) complexes in good yield. All have been characterized through X-ray studies (as an acetonitrile adduct in the case of X = PPh₂). The efficiencies of metathesis reactions initiated by Mo(NAr)(CHX)(Me₂Pyr)(OTPP) complexes can be rationalized on the basis of steric factors; electronic differences imposed as a consequence of X being bound to the alkylidene carbon do not seem to play a major role. Side reactions that promote catalyst decomposition do not appear to be a serious limitation for Mo=CHX species.