Organocobalt radical cyclizations

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

Hisaeda and coworkers subsequently reported radical (macro)cyclizations of oo-bromoalkyl acrylates 300 catalyzed by modified vitamin B12 301a under photo-lytic conditions (Fig. 71) [334], The active Co(I) catalyst is generated by electroreduction at platinum cathodes and subsequently substitutes the halide in 300. The resulting organocobalt(III) intermediate was characterized by UV and MS techniques in the dark. Photolysis triggers 6-endo, 10-endo, or 16-endo cyclizations affording the products 302 in 95, 15, and 43% yield, respectively. The reaction was inhibited by addition of the radical trap PBN 303 and the derived radical adducts were detected by ESR-spectroscopy. 

These reactions are believed to proceed via a reductive process to give an organocobalt complex 37a, which undergoes an intramolecular cyclization of the radical 37b generated by homolytic cleavage of C-Co bond (Scheme 15). 

The cleavage of the Co—C bond in organocobalt compounds may occur by the reverse of the three pathways shown in equations (29)-(31). Bond homolysis has been particularly intensively studied, and may occur by photolysis or by thermolysis. Photolysis of the Co—C bond in methylcobalamin is accelerated by the presence of dioxygen, to give B,2a and formaldehyde. The slower rate of the anaerobic photolysis is suggested to result from recombination of Bjjr and methyl radicals. The photolysis of adenosylcobalamin gives 5 -deoxy-5, 8-cycloadenosine and adenosine-5 -carboxaldehyde in the presence of Oj, and the cyclized product only in the absence Of02. 

---