Migratory Insertion of Allyl Groups across the Pd−C Bond in Palladium(II) Isocyanide Complexes. The Fundamental Interplay between Temperature and Allyl Hapticity

The 2,6-dimethylphenyl- and tert-butylisocyanides (DIC and TIC, respectively) react in chlorinated solvents with the allyl dimers [Pd(μ-Cl)(η3-C3H3Me2)]2 and [Pd(μ-Cl)(η3-C3H5)]2, giving the insertion products trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCMe2)Cl], trans-[Pd(TIC)2(Cd N(CMe3)CH2CHCMe2)Cl], and trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCH2)Cl], respectively. In particular, the reaction between the complex [Pd(μ-Cl)(η3-C3H3Me2)]2 and DIC was studied in detail, and a number of different species involved in the insertion process were identified. A mechanistic network taking into account all the involved derivatives was proposed on the basis of independently measured equilibrium and rate constants. In this respect, two independent equilibria, both involving the formation of η1-allyl intermediates, were detected and the related constants determined. The formation of such intermediates bearing the η1-allyl fragment in cis position to the isocyanide is crucial when the subsequent insertion is considered. The intermediates, however, do not display the same reactivity owing to the different thermodynamic parameters governing their formation. In particular the formation of [Pd(η1- C3H3Me2)(DIC)2Cl] (MD2Cl) represents the privileged path to the product trans-[Pd(DIC)2(Cd N(2,6-Me2C6H3)CH2CHCMe2)Cl] (P) when the insertion process is carried out at RT. The alternative intermediate [Pd(η1-C3H3Me2)(DIC)3]þCl- (MD3) hardly contributes to the formation of the product P since it is disfavored by increasing the temperature.