The effect of some selected ligands (L = empty, PMe3, ethylene, maleic anhydride) in the reductive elimination of the palladium complexes cis-[Pd(η1-allyl)(η1-allyl)(PMe3)L] and Pd(η1-allyl)(η3-allyl)L to form hexa-1,5-diene was computationally studied using DFT methods. Among the various possible coupling processes (C1sp3-C1′sp3, C3sp2-C3′sp2, and C1sp3-C3′sp2), C3-C3′ bond formation is the most favored in all cases, as reported before for cis-[Pd(η1-allyl)(η1-allyl)(PH3)2]. Interestingly, the activation energy for this coupling changes with the L ligand: empty (4.6 kcal/mol) < MA (5.8 kcal/mol) < CH2CH2 (12.5 kcal/mol) < PMe3 (17.3 kcal/mol). Therefore, tricoordinated Pd(η1-allyl)(η1-allyl)L complexes undergo reductive elimination at higher rates than the tetracoordinated counterparts. The order L = empty < L = MA is inverse to that found for carbon ligands (alkyl, aryl, alkenyl) that couple via direct C1-C1′ reductive elimination; the order L = empty < L = MA is also followed by the allyl groups when the disfavored C1-C1′ bond formation is considered. Structural analysis reveals that the C3-C3′ reductive elimination of cis-[Pd(η1-allyl)2(PMe3)] is particularly favored by the small distortion of the original T-shaped geometry in the transition state, which preserves the hyperconjugative dσ(C1-Pd)→π*(C2?C3)-type interaction between the metal and the allyl substituents. The activation energies for the elimination of allyl groups are intermediate between those of alkenes/arenes and alkyls when palladium complexes with similar composition cis-[Pd(R)(R)(PMe3)L] are compared. Although the effect is more moderate than in other carbon substituents, π-acceptor ligands (MA) and to a lesser extent olefins (exogenous or the same substrates/products) are efficient additives in this coupling, an electronic effect that is conveyed to the distant C3 and C3′ atoms by the entire interacting system. Consistent with this proposal, the transition state for the C3sp2-C3′sp2 reductive elimination in cis-[Pd(η1-allyl)(η1-allyl)(PMe3)L] shares both structural and electronic features with a pericyclic (homo)Cope rearrangement.
A DFT study of the effect of the ligands in the reductive elimination from palladium bis(allyl) complexes
Organometallics 2010, 29, 4983-4991.