A well-defined Pd(ii)-NHC aqua complex for ambient Buchwald–Hartwig amination in tert-butanol

Abstract

Upon treatment of the sterically hindered Pd(NHC)Cl(allyl/cin) precursors with HCl, two monomeric aqua complexes, Pd(NHC)Cl₂(OH₂) bearing the bulky and electron-rich IPr^# and BIAN-IPr^# ligands, were isolated instead of the expected chloride-bridged dimers [Pd(μ-Cl)(NHC)Cl]₂. This unexpected observation of Pd(NHC)Cl₂(OH₂) formation suggests that enhanced steric bulk of the NHC ligand redirects the reaction pathway of Pd(NHC)Cl(allyl/cin) complexes upon HCl treatment, effectively suppressing dimerization. Notably, the air- and moisture-stable Pd(II)-BIAN-IPr^# aqua complex functions as a highly efficient and versatile catalyst for Buchwald–Hartwig amination, facilitating the coupling of (hetero)aryl chlorides with (hetero)aryl amines at room temperature under low catalyst loadings (0.1–1 mol%) in the environmentally benign solvent ^tBuOH. Remarkably, Pd(BIAN-IPr^#)Cl₂(OH₂) exhibits excellent chemoselectivity, preferentially converting aryl chlorides with primary amines into secondary amines, whereas secondary amines remain unreactive under identical conditions. The catalyst demonstrates broad substrate scope and functional group tolerance across 82 examples, including 18 newly synthesized compounds—a performance attributed to the superior σ-donating ability and tunable steric profile of the BIAN-IPr^# ligand. DFT calculations reveal that this catalytic system selectively favors the formation of secondary amines over tertiary amines. Moreover, the Pd-BIAN-IPr^# catalyst exhibits a lower energy barrier in the rate-determining step and a narrower HOMO–LUMO gap relative to IPr^#-based analogues, consistent with its enhanced catalytic efficiency and selectivity.