Unexpected Deactivation of PdCl(cinnamyl)(NHCCl) Precatalysts Mediated by Alkylamines

Chlorinating the backbone of N-heterocyclic carbene ligands (i.e., NHCCl) yielded Pd catalysts (i.e., Pd-NHCCl) with dramatically improved performance in cross-coupling catalysis. Unexpectedly, precatalysts of the type PdCl(cinnamyl)(NHCCl) at low catalytic loading (0.1 mol %) turned out to be not applicable in Buchwald-Hartwig C–N coupling with alkylamines. This is caused by the reaction of the precatalyst with alkylamines, leading to free NHCCl and PdCl(cinnamyl)(alkylamine); similar reactivity was observed with ammonia. PdCl(cinnamyl)(alkylamine) is not activated to a zerovalent Pd species; therefore, catalysis cannot begin. Other precatalysts, like PdCl2(morpholine)(NHCCl) and PdCl2(3-chloropyridine)(NHCCl) (Pd-PEPPSI-NHCCl), as well as precatalysts of the type PdCl(cinnamyl)(NHC) bearing nonchlorinated ligands, are stable under these conditions, activated, and perform well in catalytic C–N coupling.