Highly active, homogeneous catalysis by polyoxometalate-assisted N-heterocyclic carbene gold(I) complexes for hydration of diphenylacetylene

Abstract

Monomeric, N-heterocyclic (NHC) carbene/carboxylato/gold(I) complex [Au(RS-pyrrld)(IPr)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (1, 0.01 mmol) in a mixed solvent of 1,4-dioxane : water (4 : 1 v/v) showed high activity in the catalysis of the hydration of diphenylacetylene to deoxybenzoin in the presence of protonic acid forms of Keggin polyoxometalates (POMs). The reaction proceeded homogeneously. The catalytic activities were superior to that of the previously reported reaction using the PPh3 derivative of gold(I) complex, [Au(RS-pyrrld)(PPh3)] (3). An important difference is that the reaction of (3) + H-PW12 proceeded in a heterogeneous/suspension system, because of the poor solubility of (3). The present catalytic systems were experimentally stable: re-addition of the substrate (1.5 mmol) to the reaction systems of (1) + H-PW12, (1) + H-SiW12, and (1) + H-AlW12, after the first 2-h reaction, revealed no loss of activity. The effect of the NHC ligand was also examined using the [Au(RS-pyrrld)(NHC)] complexes (NHC = IMes (6), BIPr (7), IF3 (8) and ItBu (9) (see Abbreviations and Scheme 1). We also isolated a novel compound, [Au(H2O)(IPr)]3[α-PW12O40] (2), in 42% yield from a 1 : 3 M ratio mixture of (1) in EtOH and H-PW12 in water. Compound (2) showed almost the same catalytic activity and stability as the mixed system of (1) + H-PW12, suggesting that (2) is the actual catalyst precursor. Complexes (1), (2), and (6)-(9) were characterized by elemental analysis, IR, TG/DTA, and (1H and 13C{1H}) NMR, in addition to X-ray crystallography and 31P{1H} NMR for (2). The role of POM in the reactions, i.e., the formation of ion-pair species, was elucidated based on a study of the effects of strong protonic acids such as HBF4, HPF6 and CF3SO3H, instead of POMs, on (1). Quantum-mechanical (QM) calculations indicated that both the catalytically unsaturated species [LAu]+ and the important intermediate [LAu(C2Ph2)]+ in the catalytic cycle are energetically more stable for L = IPr than for L = PPh3, in good agreement with the experimental finding that (1) has much greater catalytic activity and stability than (3).