作者:Torsten Malmström、Ola F. Wendt、Carlaxel Andersson
DOI:10.1039/a902288k
日期:——
The compound [Rh(DPPBTS)(NBD)][O3SCF3] 1 (DPPBTS = tetrasulfonated 1,4-bis(diphenylphosphino)butane, NBD = norbornadiene) has been prepared. On reaction of 1 with H2 different complexes [Rh(DPPBTS)(H2O)3(H)]2+, [Rh(DPPBTS)(H2O)2]+ or [Rh(DPPBTS)(OH)]2 are formed depending on the pH of the aqueous solution. Addition of α-acetamidoacrylic acid (AAA) to an aqueous solution of [Rh(DPPBTS)(H2O)2]+ affords a substrate complex in which the co-ordination mode of AAA is pH dependent, i.e. it co-ordinates via the double bond and the amide carbonyl at a pH below the pKa of AAA, or via the double bond and the carboxylate group at a pH higher than the pKa. The co-ordination mode has a dramatic effect on the rate of hydrogenation of AAA catalysed by 1, being extremely fast at a pH below the pKa of the substrate (270 000 mol h–1), but approximately 2000 times slower at a pH higher than the pKa. The hydrogenation rate is zero order in olefin concentration at pH 4.7 and a kH/kD isotope effect of 1.25 has been observed at pH 4.5. These observations indicate that the oxidative addition of H2 is the rate determining step in the hydrogenation using 1 as a catalyst, and that the mechanism is the same in water as in organic solvents.
化合物 [Rh(DPPBTS)(NBD)][O3SCF3] 1(DPPBTS = 四磺化 1,4-双(二苯基膦)丁烷,NBD = 降冰片二烯)已被制备出来。当 1 与 H2 反应时,根据水溶液的 pH 值会形成不同的络合物 [Rh(DPPBTS)(H2O)3(H)]2+、[Rh(DPPBTS)(H2O)2]+ 或 [Rh(DPPBTS)(OH)]2。在[Rh(DPPBTS)(H2O)2]+ 的水溶液中加入α-乙酰胺基丙烯酸(AAA)会产生一种底物配合物,其中 AAA 的配位模式与 pH 值有关,即当 pH 值低于 AAA 的 pKa 时,它通过双键和酰胺基羰基配位;当 pH 值高于 pKa 时,它通过双键和羧酸基配位。共配位模式对 1 催化 AAA 的氢化速率有显著影响,当 pH 值低于底物的 pKa 时,氢化速率极快(270 000 mol h-1),但当 pH 值高于 pKa 时,氢化速率约慢 2000 倍。在 pH 值为 4.7 时,烯烃浓度的氢化速率为零阶,而在 pH 值为 4.5 时,观察到的 kH/kD 同位素效应为 1.25。这些观察结果表明,在以 1 为催化剂的氢化过程中,H2 的氧化加成是决定氢化速率的步骤,其机理在水中与在有机溶剂中相同。