METATHESIS OF ALKANES CATALYZED BY SILICA-SUPPORTED TRANSITION-METAL HYDRIDES

The silica-supported transition metal hydrides (=Si-O-Si=)(=Si-O-)(2)T a-H and (=Si-O-)(x)M-H (M, chromium or tungsten) catalyze the metathes is reaction of linear or branched alkanes into the next higher and low er alkanes at moderate temperature (25 degrees to 200 degrees C). With (=Si-O-Si=)(=Si-O-)(2)Ta-H, ethane was transformed at room temperatur e into an equimolar mixture of propane and methane. Higher and lower h omologs were obtained from propane, butane, and pentane as well as fro m branched alkanes such as isobutane and isopentane. The mechanism of the step leading to carbon-carbon bond cleavage and formation likely i nvolves a four-centered transition state between a tantalum-alkyl inte rmediate and a carbon-carbon sigma-bond of a second molecule of alkane .