Cobalt-mediated reactions in synthesis. The degradation of carboxylic acids to functionalised noralkanes via acylcobalt salophen intermediates

Abstract

Arylmethyl- and allyl-carbonylcobalt salophen complexes, e.g.(7), (8), (9), and (22), readily undergo carbon-to-cobalt-bond homolysis and in situ decarbonylation, producing new alkyl radical centres which can be intercepted with oxygen-, nitrogen-, halogen-, sulphur-, and selenium-containing radical-trapping agents leading to functionalised noralkanes. The sequence constitutes a useful, and in some cases more flexible, variant of the classical Hunsdiecker reaction, and amounts to a cobalt equivalent of the Barton radical decarboxylation reaction of carboxylic acids via their corresponding thiohydroxamic esters. In a similar manner, irradiation of the oxy-substituted acylcobalt salen reagents (25) and (26) in the presence of tetramethylpiperidine oxide produces the products (14) and (27), respectively, resulting from homolysis–decarboxylation and alkyl-radical trapping. In the absence of radical-trapping agents, irradiation of (7c) produces (18), and irradiation of (29) leads to the but-2-enolide (32).