Aqueous organometallic chemistry. 3. Catalytic hydride transfer reactions with ketones and aldehydes using [Cp∗Rh(bpy)(H 2O)](OTf) 2 as the precatalyst and sodium formate as the hydride source: Kinetic and activation parameters, and the significance of steric and electronic effects

Abstract

The reaction of a precatalyst, [Cp∗Rh(bpy)(H 2O)](OTf) 2 ( 1), with sodium formate provided the hydride complex, [Cp∗Rh(bpy)(H)] + ( 2), in situ, at pH 7.0, which was then evaluated in an aqueous, catalytic hydride transfer process with water soluble substrates that encompass 2-pentanone ( 3), cyclohexanone ( 4), acetophenone ( 5), propionaldehyde ( 6), benzaldehyde ( 7), and p-methoxybenzaldehyde ( 8). The initial rates, r i, of appearance of the reduction product alcohols at 23 °C provided a relative rate scale: 8 > 7 ≈ 6 > 5 > 4 > 3, while the effect of concentration of substrate, precatalyst, and sodium formate on r i, using 7 as an example, implicates [Cp∗Rh(bpy)(H)] + formation as the rate-limiting step. The experimental kinetic rate expression was found to be: d[alcohol]/ dt = k cat[ 1][HCO 2Na]; substrate being pseudo zero order in water. The steric effects were also analyzed and appeared to be of less importance intra both the ketone and aldehyde series, but an inter series comparison appeared to show that the aldehydes had less of a steric effect on the initial rate, i.e., 7 > 4 by a factor of 3.6, while the aldehyde series appeared to have some moderate electronic influence on rates, presumably via electron donation to increase binding to the Cp∗Rh metal ion center, in accordance with these proposed concerted binding/hydride transfer reactions. A proposed catalytic cycle will also be presented.