(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

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Abstract

Dinuclear (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)] (16) [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 16 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) (730). 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.

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