The Mechanism by Which Ionic Liquids Enable Shilov-Type CH Activation in an Oxidizing Medium

Quantum mechanical studies on methane CH activation catalyzed by PtCl_2 in concentrated H_2SO_4 and ionic liquid solution show that the effect of the ionic liquid is to enable Shilov-like chemistry in an oxidizing medium, by solvating the otherwise insoluble PtCl_2(s) in H_2SO_4. Other possible mechanisms have been investigated and discarded

Inaccessibility of β-Hydride Elimination from −OH Functional Groups in Wacker-Type Oxidation

Quantum mechanics calculations (B3LYP and MPW1K density functional theory) on mechanisms relevant to the Wacker process for dehydrogenation of alcohol to ketone show that the commonly accepted mechanism for product formation (β-hydride elimination (BHE) leading to Pd−H formation) is not energetically feasible (36.2 kcal/mol). An alterative pathway involving a five-bodied reductive elimination (RE) leads to an activation enthalpy of 18.8 kcal/mol, which is just half that of the BHE from the −OH group usually assumed for the Wacker process. We find that a water molecule catalyzes both processes, reducing the barrier to 17.2 for RE and 25.0 for BHE, but will not change the relative ordering of the two mechanisms. This suggests that assumptions of BHE mechanisms should be reexamined for cases in which the β atom is not an alkyl group

Hydrovinylation of Olefins Catalyzed by an Iridium Complex via CH Activation

Olefin dimerizations are typically proposed to proceed via a Cossee−Arlman type migratory mechanism involving relatively electron-rich metal hydrides. We provide experimental evidence and theoretical calculations that show, in contrast, relatively electron-poor O-donor Ir complexes can catalyze the dimerization of olefins via a mechanism that involves olefin CH bond activation and insertion into a metal−vinyl intermediate

Unraveling the Wacker Oxidation Mechanisms

The mechanisms for the aqueous PdCl2 mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl^-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl^-] leading to anti-addition aldehyde products is only accessible when CuCl_2 is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl_2, and the prerequisite dissociation of Cl^- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl^-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl_2]