Synthetic, structural, and mechanistic studies on the oxidative addition of aromatic chlorides to a palladium (N-heterocyclic carbene) complex: relevance to catalytic amination.

Abstract

The oxidative addition products trans-[Pd(NHC)(2)(Ar)Cl] (NHC = cyclo-C[N(t)BuCH](2); Ar = Me-4-C(6)H(4), MeO-4-C(6)H(4), CO(2)Me-4-C(6)H(4)) have been isolated in good yields from the reactions of ArCl with the amination precatalyst [Pd(NHC)(2)] and structurally characterized. The former undergo reversible dissociation of one NHC ligand at elevated temperatures, and a value of 25.57 kcal mol(-1) has been determined for the Pd-NHC dissociation enthalpy in the case where Ar = Me-4-C(6)H(4). Detailed kinetic studies have established that the oxidative addition reactions proceed by a dissociative mechanism. Rate data for the oxidation addition of Me-4-C(6)H(4)Cl to [Pd(NHC)(2)] compared to that obtained for the [Pd(NHC)(2)]-catalyzed coupling of morpholine with 4-chlorotoluene are consistent with a rate-determining oxidative addition in the catalytic amination reaction. The relative rates of oxidative addition of the three aryl chlorides to [Pd(NHC)(2)] (CO(2)Me-4-C(6)H(4)Cl > Me-4-C(6)H(4)Cl > MeO-4-C(6)H(4)Cl) reflect the electronic nature of the substituents and also parallel observed trends in coupling efficiency for these aryl halides in aminations.