Organocobalt compounds

See also page 421, Section 2.8 page 427, Section 2.9 page 432, Section 2.10 page 434, Section 2.11 and page 460, Section 2.30. 

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We recently synthesized the organocobalt compounds containing a vinyl group represented in 28, 29, and 30. They were not homopolymerized by the redox... 

Organocobalt compounds may be prepared by several routes.833 The Co1 species is a useful starting point. This may be obtained by reduction of Co11 or Com compounds using hydrazine, thiols, chromium(II), borohydride and other reagents. The Co1 species will give a Com hydride species by reacting with a proton. Both the Co1 and the hydridocobalt(III) species are alkylated readily. 

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

Figure 13 Possible reactions of thiolates with organocobalt compounds.

Bonnemann, H. (1985) Organocobalt compounds in pyridine syntheses - an example for structure-activity relations in homogeneous catalysis. Angew. Chem. Int. Ed. Engl, 24, 248. 

Acyl radicals are very useful synthetic intermediates. Their preparation is not simple since the corresponding halides are highly electrophilic and cannot be used as radical precursors. Organocobalt compounds were proposed as suitable source of acyl radicals [44]. However, the use of acyl selenides proved to be more general [45, 46]. These radical precursors can be efficiently prepared from the corresponding carboxylic acids and esters [47]. Acyl phenyl selenides should be preferred, when possible, relative to acyl methyl selenides due to the consumption of two equivalents of tin hydride with this last system (Scheme 1) [4]. Acyl selenides have found many applications in tandem radical additions to alkenes. Examples of intermole-cular [Eq. (18)] [48,49] and intramolecular reactions [Eq. (19)] [50a] are reported. The enoyl selenide 68 give the unsaturated acyl radicals 69. This intermediate... 

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