Cobaltporphycenes as Catalysts. The Oxidation of Vinyl Ethers via the Formation and Dissociation of Cobalt−Carbon Bonds

Abstract

Cobalt(III) porphycenes, structural isomers of cobalt(III) porphyrins, catalyze the addition of alcohols to vinyl ethers in a toluene−dioxane solution containing alcohol and Et3N as well as the autoxidation of the resultant metal-bound adduct; this produces two acetals, namely, those of β-formylacetaldehyde and β-hydroxyacetaldehyde, with a total turnover number of 34−77 after 21 h at 20 °C. Under identical reaction conditions, analogous cobalt(III) porphyrins were found to undergo rapid decomposition. A salient feature of this catalytic process is the formation of an alkylcobalt(III) complex, a species that was characterized using 1H NMR and UV−vis spectroscopic methods. The pseudo-first-order rate constants for the addition of n-butyl vinyl ether to the chloride adducts of octaethylporphycene cobalt(III) and octaethylporphyrin cobalt(III) complexes were determined to be 8.8 × 10-4 and 1.9 × 10-4 s-1 at 20 °C, respectively. Irradiating a solution of the alkylcobalt(III) complex resulting from the addition of vinyl ether to a tetra-n-propylporphycene cobalt(III) complex under aerobic conditions yielded an alkylperoxocobalt(III) species as the result of insertion of dioxygen into the cobalt−carbon bond. This intermediate species was characterized by the presence of diagnostic proton resonances, ascribed to the axial ligand, in the upfield region of the 1H NMR spectrum; specifically, the Co−CH2 and Co−OOCH2 proton resonances appear at −3.91 and −1.22 ppm, respectively, in toluene-d8 at −40 °C. Breakdown of the alkylperoxocobalt(III) intermediate, via O−O bond homolysis, produces the two observed acetal products. The exact product ratio of these acetals, aldehyde/alcohol, depends on the viscosity of the solution. This result is taken as support for the idea that alkoxy radicals that diffuse away from the cobalt complex give the alcohol, whereas the aldehyde is produced as the result of oxidation occurring within a solvent cage. The Co(III) complex that results after (1) O−O bond homolysis and then (2) hydrogen atom abstraction from the medium is a five-coordinate species that is capable of continuing the reaction cycle in the presence of excess vinyl ether. As a consequence, the starting octaethylporphycene cobalt(III) complex acts as an efficient catalyst for the oxidation of vinyl ethers.