Cobaltporphycenes as catalysts. The oxidation of vinyl ethers via the formation and dissociation of cobalt-carbon bonds

Cobalt(III) porphycenes, structural isomers of cobalt(III) porphyrins, cata lyze the addition of alcohols to vinyl ethers in a toluene-dioxane solution containing alcohol and Et3N as well as the autoxidation of the resultant m etal-bound adduct; this produces two acetals, namely, those of beta -formyl acetaldehyde and beta -hydroxyacetaldehyde, with a total turnover number of 34-77 after 21 h at 20 degreesC. Under identical reaction conditions, anal ogous cobalt(III) porphyrins were found to undergo rapid decomposition. A s alient feature of this catalytic process is the formation of an alkylcobalt (III) complex, a species that was characterized using H-1 NMR and UV-vis sp ectroscopic methods. The pseudo-first-order rate constants for the addition of n-butyl vinyl ether to the chloride adducts of octaethylporphycene coba lt(III) and octaethylporphyrin cobalt(III) complexes were determined to be 8.8 x 10(-4) and 1.9 x 10(-4) s(-1) at 20 degreesC, respectively. Irradiati ng a solution of the alkylcobalt(III) complex resulting from the addition o f vinyl ether to a tetra-n-propylporphycene cobalt(III) complex under aerob ic conditions yielded an alkylperoxocobalt(III) species as the result of in sertion 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 upheld region of the 1H NMR spectrum; specific ally, the Co-CH2 and Co-OOCH2 proton resonances appear at -3.91 and -1.22 p pm, respectively, in toluene-d(8) at -40 degreesC. Breakdown of the alkylpe roxocobalt(III) intermediate, via O-O bond homolysis, produces the two obse rved acetal products. The exact product ratio of these acetals, aldehyde/al cohol, depends on the viscosity of the solution. This result is taken as su pport for the idea that alkoxy radicals that diffuse away from the cobalt c omplex 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 t he medium is a five-coordinate species that is capable of continuing the re action cycle in the presence of excess vinyl ether. As a consequence, the s tarting octaethylporphycene cobalt(III) complex acts as an efficient cataly st for the oxidation of vinyl ethers.