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

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.