Organocobalt complexes 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). 

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). 

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). 

The homogeneous catalytic [2+2+2]-cycloaddition of alkynes and nitriles was first discovered by Yamazaki and Wakatsuki [3] using the phosphine-stabilized cobalt(III) complex (Stmcture 5). At the same time, Bdnnemann and co-workers [5] observed the co-cyclization (eq. (2)) at cobalt catalysts prepared in situ, as well as using phosphine-free organocobalt(I) diolefm complexes. 

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