pH Effects on reaction rates in rhodium catalysed hydrogenation in water

Abstract

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 pK_a of AAA, or via the double bond and the carboxylate group at a pH higher than the pK_a. 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 pK_a of the substrate (270 000 mol h^–1), but approximately 2000 times slower at a pH higher than the pK_a. The hydrogenation rate is zero order in olefin concentration at pH 4.7 and a k_H/k_D 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.