Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst

Grafting of [W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)2] on a silica partially dehydroxylated at 700°C (SiO2- (700)) generates the corresponding monosiloxy complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)] as the major species (≈90%) along with [(≡SiO)W(≡NAr)(CH2tBu)(2,5-Me2NC4H2)2], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)], which shows that they are the key intermediates of alkene metathesis

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands

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 <i>Z</i>-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