Organic Syntheses Using Recyclable Organocobalt Complexes as a Controller

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(1)Electrochemical Radical Cyclization of Oxime Ethers.We have developed a new procedure for electroreduction with organocobalt complex by using a sacrificial anode in an undivided cell system and examined the cobaloxime-mediated radical cyclization of the oxime ethers. The cyclization of the bromo oxime ethers proceeded cleanly to give the amino alcohol derivatives. The primary, secondary, and tertiary bromides led to the cyclization products as a results of trapping of the corresponding radicals with the oxime function, respectively. The stereochemistry of the cyclic products is independent of the geometry of the starting oxime ethers. Carbon-nitrogen double bond was found to be reactive for formation of cyclopentane structures.(2)Successive Radical Cyclization and Coupling Reaction with Aldehyde.Intermolecular radical coupling of bromo alkynes with aldehyde was examined by using the electrochemically generated low valency coblat complex. The formation of the coupling product can be explained by the coupling of the vinyl radical, produced as a result of radical cyclization, with keyl radical from benzaldehyde.(3)Preparation of Alkenyl-and Alkyl-cobalt Complexes and Their Reactivities.The reaction of chloropyridinecobaloxime with vinylmagnesium bromide (4 equiv.) afforded the vinylcobaloxime in 36% yields. The C=C double bond of the vinylcobaloxime are conjugated with the cobalt atom according to the IR and NMR data, and bence are less reactive toward the 1,3-dipolar addition and Diels-Alder reaction. Syntheses of more reactive alkenyl-cobalt complexes were also studied. Alternatively, the alkyl-cobalt complexes were prepared by the addition reaction of cobalt-hydride complex to acrylates. The ester group of the product is also conjugated with the cobalt atom. It seems that highly electron deficient compounds such as chlorosulfonylisocyanate are amenable to the reverse-electron demand cycloaddition.