Generation of radical species by one electron oxidation with metallic compounds for the construction of carbon skeletons

By oxidation of P-keto acids, cyclopropanols, and acyl chromate complexes with tris(2-pyridinecarboxylato)manganese(III) (Mn(pic)g), a-keto, p-keto, and alkyl radicals are generated respectively, which react with olefinic compounds to afford intermolecular addition products in good yield. The cation radicals can be generated from a-stannylalkyl sulfides, (1 -stannylalkyl)amino compounds, and a-stannyl carboxylic esters by oxidation with ammonium hexanitratocerate(1V) (CAN). These cation radicals of the stannyl compounds readily cleave into carbocations or carbon radicals, which are utilized as reactive intermediates for the carbon-carbon bond formation. Radical reactions, when applied to organic synthesis, frequently lead to complex mixtures of products which still remain hard to be controlled. This makes it difficult to use radical intermediates for intermolecular bond forming reactions (ref. 1). In order to carry out intermolecular radical reactions efficiently, we have tried to generate radical intermediates by the use of metallic oxidants with the expectation that both the radical species and the radical acceptors would interact with the metallic species employed (ref. 2). To find a suitable metallic oxidant for the generation of radicals leading to their intermolecular addition to olefins, we examined the oxidation of a-keto acids with various metallic oxidants in the presence of a silyl vinyl ether as an acceptor of the resulting radicals. These studies revealed that Mn(1II) 2-pyridinecarboxylate (Mn(pic)3) can be employed as an oxidant to promote the intermolecular radical addition reaction efficiently. When Mn(pic)g was added to a mixture of a p-keto acid 1 and a silyl vinyl ether 2, an addition product 6 of the resulting a-keto radical and the vinyl ether 2 was obtained in good yield even in the presence of a slightly excess amount of the vinyl ether 2 (ref. 3). In this reaction, Mn(pic)3 oxidizes the keto acid 1, generating a p-keto carboxyl radical, which cleaves into an a-keto radical 3. This a-keto radical 3 then reacts with the vinyl ether 2, to give a radical addition intermediate 4. Mn(pic)3 works effectively to terminate the radical reaction by oxidizing this radical intermediate 4 to a carbocation 5, leading to the 1P-diketone 6 . Mn(pic)3 could be further utilized to the generation of p-keto radicals from cyclopropanols and their addition reaction to olefins. Notably, the 1, 2-bond is cleaved selectively in the oxidation of 1, 2disubstituted cyclopropanols 7, giving a-substituted p-keto radicals 8, which react with olefins to form 1,5-dicarbonyl compounds 9 (ref. 4). From [n. 1 .O] bicycloalkanols such as 10, 1,5-diketones are formed with ring-expansion (ref. 5) .