Structural Dependence of Thermodynamics of Alkene Binding to Yttrium Alkyl Complexes and of Kinetics of Alkyl Migration to Coordinated Alkenes

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Abstract
Agostic interactions in yttrium alkyls are structure dependent. Primary alkyl yttrium complexes have β-CH2 agostic interactions at low temperature, but a shift toward α-agostic interactions occurs on warming. For the more crowded β-disubstituted yttrium alkyls, an α-CH2 agostic interaction is seen. The thermodynamics of alkene binding to the primary alkyl yttrium complex Cp*2YCH2CH2CH(CH3)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-, γ-substituted, β-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 ≈ γ-substituted ≫ β-substituted ≫ α-substituted. The ∼200-fold slower insertion of propene into Cp*2YCH2CH(CH3)2 (6) than that into Cp*2YCH2CH2CH(CH3)2 (2) is therefore due to kinetically slow migration of the β-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.

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