Protonation of Unsaturated Hydrocarbon Ligands: Regioselectivity, Stereoselectivity, and Product Specificity

Abstract

Understanding the mechanisms of protonation of hydrocarbon ligands is fundamental t o a wide range of chemistry including organic synthesis, organometallic chemistry, and even bioinorganic chemistry. Protonation at carbon or metal sites is often slow. with the result that in species containing both types of sites, initial protonation can be at either the metal or the carbon. This has fundamental consequences on the reactivity of hydrocarbon ligands, which are highlighted in this article. In particular, many reactions are apparently the result of a regioselective protonation on the basis of structural analysis of the isolated products. In fact, these products are often formed by an indirect route involving kinetically controlled protonation at the "wrong" site followed by rearrangement to form the thermodynamically controlled, apparently regioselective. product. Other aspects of the protonation mechanisms of complexes containing hydrocarbon ligands are discussed. with an emphasis on the manner in which competitive protonation at metal or ligand can be exploited to select which hydrocarbon is produced and to control the stereochemistry of the hydrocarbon.

Effect of the acidity and chemical nature of the protonating agent on the rate of acetylene reduction catalyzed by the nitrogenase active site isolated from the enzyme

“…Thus, if the reaction rate is described a compli cated equation, including the case in which the reac tion rate as a function of proton concentration is not a first order equation, the mechanism of protonation in the system is considered to be indirect. In this case, the reaction rate depends on the nature of the protonating agent and the reduction reaction yields a cis product [30]. The totality of data for reactions involving FeMoco isolated from the protein suggests that the protonation of the acetylene molecule coordinated to the reduced cofactor occurs intramolecularly in the [FeMoco-C 2 H 2 -proton donor] complex.…”

“…In principle, the substrate in a [catalyst-substrate] complex can be protonated directly by the medium or via the intermediate protonation of catalyst atoms fol lowed by proton transfer to the substrate. There are empirical criteria for assigning a mechanism to one type or the other [30]. If the proton binds directly to the atom that is the "destination" of protonation, the dependence of the reaction rate on the acid concen tration will be described by a first order equation.…”

Protonation mechanisms of Nickel Complexes Relevant to Industrial and Biological Catalysis

“…Unfortunately the kinetics cannot distinguish whether the nickel or the allyl site is protonated preferentially. Earlier studies on a variety of different complexes 1,2 show that it is impossible to reach a decision based on precedent: preferential protonation at metal or ligand can dominate depending on the complex being studied. Irrespective of where the initial protonation occurs, the important point is that the proton rapidly equilibrates between the allyl and nickel sites.…”

“…Protonation at the metal is thermodynamically less favourable, and usually kinetically slower than protonation of a stereochemical lone pair of electrons. 1,2 If it is essential during the enzyme's action that proton transfer to the metal occur, then this is more favourable with alkyl thiolates than aryl thiolates. Analysis of the kinetic data for the reactions of [Ni(SR)(triphos)] + shows that the acidities of the corresponding coordinated PhSH and EtSH vary by less than a factor of 40.…”

“…Only two rules need to be remembered. 1,2 (1) In ligands, thermodynamically-favourable proton-transfer reactions to stereochemical lone pairs on atoms are diffusion-controlled (k diff = 1 × 10 10 dm 3 mol -1 s -1 ). 3 (2) However, even thermodynamically-favourable proton transfer reactions to carbon and metal sites can be appreciably slower than the diffusion-controlled limit.…”