Preparation of tungsten-based olefin metathesis catalysts supported on alumina

A new tungsten alkylidene complex, W(NAr)(CHCMe₂Ph)(OHIPT- NMe₂)(pyrrolide) {Ar=2,6-(i-Pr)₂C₆H ₃; HIPT-NMe₂=2,6-[2,4,6-(i-Pr)₃C ₆H₂]₂-4-NMe₂-C₆H ₂}, has been synthesized and shown to be highly selective for Z homocoupling metathesis of selected terminal olefins in pentane, as is W(NAr)(CH₂CH₂CH₂)(OHIPT)(pyrrolide) (5). Both 5 and W(NAr)(CHCMe₂Ph)(OHIPT-NMe₂)(pyrrolide) (6) are adsorbed onto calcined alumina. Control experiments and metathesis homocoupling of four substrates lead to the conclusions that 5 is largely adsorbed in a reaction that liberates HIPTOH, while 6 is adsorbed largely through an interaction between the dimethylamino group and an acidic site on the surface. There is no evidence that any adsorbed catalyst can give rise to Z selectivity of a magnitude equal to that found in a homogeneous reaction involving 5 or 6.