(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

摘要

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)] (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 NaN_3, KSCN, KOCN, and CF_3COOAg to produce the corresponding mononuclear (π- allyl)palladium-NHC pseudohalogen complexes, [(π-allyl)- Pd(X)(NHC)] (X = N_3, NCS, SCN, NCO, CF_3COO) (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 NaN_3, KSCN, KOCN, or CF_3COOAg, followed by cleavage with the NHCs. Reactions of [(π-allyl)Pd(N_3)(NHC)] with organic isothiocyanates (R- NCS) or CH_3O(CO)CΞCO(CO)CH_3 resulted in selective 1,3- dipolar cycloaddition into the Pd-azido bond to give heterocyclic compounds. By contrast, analogous reactions of [(η~3- allyl)Pd(N_3)(IPr)] with an organic isocyanide (R-NC: R = tertbutyl, benzyl) gave the adduct [(η~3-allyl)Pd(N_3)(IPr)]?(R-NC) as the only product or a mixture of the adduct and a dipolar cycloaddition product, [(η~3-allyl)Pd{CN_4(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.