The title complex, MeO2CCH2CH(CO2Me)Co(CO)4 (1), was found to decompose in alkanes or chlorinated hydrocarbons as solvents quantitatively to Co2(CO)8 and a 1:1 mixture of dimethyl fumarate and dimethyl succinate at room temperature under 1 atm of CO. HCo(CO)4 was trapped in this process either by 1-heptene or Proton Sponge in the form of n-octanoylcobalt tetracarbonyl or a [BH]+[Co(CO)4]− salt, respectively. As model reactions confirmed, the decomposition of 1 consists of two distinct steps such as (1) β-hydride elimination and (2) subsequent fast consumption of HCo(CO)4 in its reaction with 1. Comparative 1H NMR experiments on deuterium labeled MeO2CCHDCH(CO2Me)Co(CO)4 (2) proved that the β-elimination reaction is stereospecific and syn. Consistent with kinetic studies, a modified mechanism of β-elimination is suggested, which includes a pre-equilibrium CO dissociation from 1, a fast intramolecular transformation of the resulting intermediate into a tricarbonylhydridoolefincobalt species, which may transfer a hydrogen atom to Co(CO)4 radicals (formed by the homolytic dissociation of Co2(CO)8) in a rate-determining step to yield HCo(CO)4. Together with earlier mechanistic studies on the formation of 1, we showed for alkylcobalt carbonyls for the first time the reversibility of the β-elimination reaction on a molecular level. ΔH‡ = 25.1 (±0.9) kcal mol−1 and ΔS‡ = 5.3 (±0.3) eu were obtained as overall activation parameters. The mechanism of CO dissociation was investigated by kinetic measurements of the 13CO exchange reaction of 1. These studies provided the activation parametes ΔH‡ = 7.7 (±0.6) kcal mol−1 and ΔS‡ = −44.2 (±0.1) eu, which could be best explained by an intramolecular associative pathway. Accordingly, the gain in energy and the considerably more structured transition state resulting from the weak coordination of a carboxylic oxygen to cobalt to form a five-membered metallacycle may account for the particularly low values of activation enthalpy and entropy, respectively.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry