Alpha- and beta-migratory insertion and elimination processes for alkyl complexes of permethyl-scandocene and permethyltantalocene

Abstract Beta-migratory insertion and elimination processes for alkyl derivatives of permethylscandocene, (η 5 -C 5 Me 5 ) 2 Sc-R, have been investigated. Ethylene is rapidly polymerized at −80 °C by (η 5 -C 5 Me 5 ) 2 Sc-R (R = H, CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 , etc.) without the need for a co-catalyst. The relative rates of ethylene insertion into scandium-alkyl bonds and the propensity for β-H elimination to afford olefin and (η 5 -C 5 Me 5 ) 2 Sc-H are found to depend critically on the nature of R. With less sterically encumbered scandocene compounds ( e.g. [(η 5 -C 5 Me 4 )SiMe 2 (η 5 -C 5 Me 4 )]Sc-H), α-olefins are oligomerized selectively to the 'head-to-tail' dimers, and catalytic cyclization of 1,5-hexadiene or 1,6-heptadiene to methylenecyclopentane or methylenecyclohexane is observed. Both α- and β-migratory insertion and elimination processes, which interconvert alkyl with alkylidene hydride or olefin hydride derivatives of permethyltantalocene ( i.e. [(η 5 -C 5 Me 5 ) 2 -Ta-X-R] ⇌ (η 5 -C 5 Me 5 ) 2 Ta(=X)R (X = alkylidene, olefin; R = H, alkyl)), have been examined. For a complex with the alkyl substituent appended from a cyclopentadienyl ligand, [(η 5 -C 5 Me 5 )(η 5 ,η 1 -C 5 Me 4 CH 2 CH 2 CH 2 )Ta], the relative rates of α-H and β-H elimination have been measured, and, significantly, α-H elimination is found to be 10 8 times faster than β-H elimination at 100 °C. The relative rates of the hydrogen and methyl migrations to alkylidene, benzyne and olefin ligands have been determined. Related α and β elimination processes have been examined for some alkoxide, thiolate and amide derivatives, [η 5 -C 5 Me 5 ) 2 Ta-YCH 3 ] (Y = O, S, NCH 3 ).