title: Structural dependence of thermodynamics of alkene binding to yttrium alkyl complexes and of kinetics of alkyl migration to coordinated alkenes.
abstract: Agostic interactions in yttrium alkyls are structure dependent. Primary alkyl yttrium complexes have beta-CH(2) agostic interactions at low temperature, but a shift toward alpha-agostic interactions occurs on warming. For the more crowded beta-disubstituted yttrium alkyls, an alpha-CH(2) agostic interaction is seen. The thermodynamics of alkene binding to the primary alkyl yttrium complex Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) depend strongly on the structure of the alkene. A single allylic substituent on the alkene has a small effect on alkene binding, but a second allylic substituent has a large destabilizing effect. Propene binding to yttrium alkyls is largely independent of the nature of the alkyl ligand. Equilibrium constants for propene binding to n-, gamma-substituted, beta-substituted, and secondary alkyl yttrium complexes are similar. The rate of migration of an alkyl group to a coordinated alkene depends strongly on the structure of the alkyl group: n-alkyl approximately gamma-substituted >> beta-substituted >> alpha-substituted. The approximately 200-fold slower insertion of propene into Cp(2)YCH(2)CH(CH(3))(2) (6) than that into Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) is therefore due to kinetically slow migration of the beta-disubstituted alkyl group of 6 and not to differences in the equilibrium binding of propene. Processes related to chain transfer and site epimerization at the metal center are also reported.
authors.affiliation: University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA. casey@chem.wisc.edu
authors.forename: Charles P
authors.initials: CP
authors.lastname: Casey
authors.forename: Jon A
authors.initials: JA
authors.lastname: Tunge
authors.forename: Ting-Yu
authors.initials: TY
authors.lastname: Lee
authors.forename: Maureen A
authors.initials: MA
authors.lastname: Fagan