(π‐Allyl)Pd Complexes Containing N‐Heterocyclic Carbene and Pseudohalogen Ligands – Synthesis, Reactivity toward Organic Isothiocyanates and Isocyanides, and Their Catalytic Activity in Suzuki–Miyaura Cross‐Couplings

Abstract

AbstractDinuclear (π‐allyl)palladium chlorides, [(π‐allyl)Pd(μ‐Cl)]2, were cleaved by N‐heterocyclic carbenes (NHCs) to give mononuclear (π‐allyl)palladium–NHC chlorides, [(π‐allyl)Pd(Cl)(NHC)] (1–6) [NHC = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPR), 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidine (SIPR), 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes)]. Complexes 1–6 were subsequently treated with aqueous NaN3, KSCN, KOCN, and CF3COOAg to produce the corresponding mononuclear (π‐allyl)palladium–NHC pseudohalogen complexes, [(π‐allyl)Pd(X)(NHC)] (X = N3, NCS, SCN, NCO, CF3COO) (7–30). These products could also be obtained by treating dinuclear pseudohalogen‐bridged Pd complexes, [(π‐allyl)Pd(μ‐X)]2, which were prepared by replacing the μ‐Cl ligand in [(π‐allyl)Pd(μ‐Cl)]2, with aqueous NaN3, KSCN, KOCN, or CF3COOAg, followed by cleavage with the NHCs. Reactions of [(π‐allyl)Pd(N3)(NHC)] with organic isothiocyanates (R–NCS) or CH3O(CO)C≡CO(CO)CH3 resulted in selective 1,3‐dipolar cycloaddition into the Pd–azido bond to give heterocyclic compounds. By contrast, analogous reactions of [(η3‐allyl)Pd(N3)(IPr)] with an organic isocyanide (R–NC: R = tert‐butyl, benzyl) gave the adduct [(η3‐allyl)Pd(N3)(IPr)]·(R–NC) as the only product or a mixture of the adduct and a dipolar cycloaddition product, [(η3‐allyl)Pd{CN4(R)}(IPr)], depending on the isocyanides used. Finally, a series of (π‐allyl)Pd–NHC pseudohalogen complexes, [(π‐allyl)Pd(X)(NHC)], exhibited high catalytic activity in Suzuki–Miyaura cross‐coupling reactions of aryl chlorides with arylboronic acids.