Methanol as a Hydrogen Source in the PdII−Bis(N-Heterocyclic Carbene)-Mediated Catalytic Semireduction of Alkynes Under Mild Reaction Conditions

G, Prabusankar and Arruri, S and Gembali, R and Chatla, N B (2017) Methanol as a Hydrogen Source in the PdII−Bis(N-Heterocyclic Carbene)-Mediated Catalytic Semireduction of Alkynes Under Mild Reaction Conditions. Asian Journal of Organic Chemistry. ISSN 2193-5807 (In Press)

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Abstract

A series of six new mono-organopalladium complexes of the form [LPdX2] (L=1,2-bis[(N-methylimidazol-2-ylidene)methyl]ethane (6), 1,2-bis[(N-vinylimidazol-2-ylidene)methyl]ethane (7), 1,2-bis[(N-isopropylimidazol-2-ylidene)methyl]ethane (8), 1,2-bis[(N-2,6-dimethylphenylimidazol-2-ylidene)methyl]ethane (9), 1,2-bis[(N-4-ethyl benzoate imidazol-2-ylidene)methyl]ethane (10), 4,5-bis[(N-isopropylimidazol-2-ylidene)methyl]acridine (11); X=Br for complexes 6–9 and 11, X=Cl for complex 10) were synthesized and characterized by using FTIR spectroscopy, 1H and 13C NMR spectroscopy, elemental analysis, and X-ray crystallography. In the solid state, complexes 6–11 were of neutral charge and each contained one bis(NHC) ligand (L) and two halide ligands in an almost-ideal square-planar geometry. These molecules were catalytically competent for the semihydrogenation of diphenylacetylene. Notably, MeOH was used as a hydrogen source in catalytic processes for the first time. Among these six new catalysts, the acridine-bridged palladium−bis(N-heterocyclic carbene) complex gave excellent selectivity for the formation of Z alkenes from internal alkynes. A range of 15 different alkynes were tested under our optimized conditions and the reactions all proceeded with very high diastereoselectivities under mild conditions.

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