Electroorganic syntheses of macrocyclic lactones mediated by vitamin B₁₂ model complexes: Part 17. Hydrophobic vitamin B₁₂

The electrolyses of bromoalkyl acrylates were carried out in N,N-dimethylformamide in the presence of a catalytic amount of heptamethyl cobyrinate perchlorate under various reaction conditions. A large-membered cyclic lactone was obtained in a moderate yield. A photo-sensitive intermediate having a cobalt-carbon bond formed during the electrolysis has been characterized using electronic and electrospray ionization mass spectroscopies in the dark. The formation of a cyclic lactone was inhibited by the addition of the spin-trapping reagent, α-phenyl N-(t-butyl)nitrone. Based on various spectroscopic results, the electrolysis proceeds as follows: the Co(II) complex as a catalyst is electrochemically reduced to the Co(I) species, and the corresponding alkylated complex is generated by the reaction of the supernucleophilic Co(I) species with a bromoalkyl acrylate. The alkylated complex is subsequently decomposed to form the substrate radical, and the intramolecular addition of the radical affords the cyclic product. During the catalytic reaction, the cobalt complex repeatedly acts as a mediator.