Organocobalt alkyl complexes

Brief mention has been made of the use of Li, Na, B, and A1 alkyls to form organocobalt-DMG complexes, but no details have been given except that tri-n-butylboron will react with either the dimeric [Co"(DMG)2py] or [Co "(DMG)2pyCl] to give [BuCo(DMG)2py] and that the yield is greater with the former [161) it is difficult to know what the mechanism is. 

Organocobalt porphyrin complexes have also yielded useful mechanistic information. The reactions of (tetrakis(p-methoxyphenyl)porphyrinato)cobalt(II) ((TAP)Co(II)) with radicals derived from dialkylazo thermal initiators with acrylic monomers provide evidence for the intermediacy of Co(III)-H species in CCTP. Reaction of (TAP)Co (II) with tertiary alkyl radicals, for example, as derived from AIBN in the presence of monomers that form stable Co-alkyl complexes, such as methyl acrylate, results in quantitative formation of Co(III)-alkyl. Whereas with monomers leading to tertiary C-Co bonds, such as MMA, the Co (II) is very much a spectator as normal polymerization ensues. Thermodynamic and activation parameters have been measured for the dissociation of (TAP)Co(III)-C(CH3)2CN to Co(II) and organic radical in solution as a probe into CCTP mechanism by low-spin Co(II). ... 

Another series of coenzyme B12 model has attracted a lot of attention both for their syntheses and their characterization. F or example, an organocobalt(m) complex with a bulky Schiff base on the equatorial positions or with secondary alkyl ligands led to a rarely observed five-coordinate organocobalt(m) complexes 164 and 165 (Figure These... 

Addition of sodium hydroxide to a mixture of [Co"(DMG)2] and an alkylating agent in methanol-water yields up to 50% of the organocobalt product. This appears to be the most convenient method for the preparation of organocobalt(III) derivatives, provided that the maximum yield is not required 161,163). Methods involving the reaction of Co with electrophiles are, at least with DMG complexes, more convenient than the main alternative route via Co(III) and Grignard reagents. 

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). 

Decay of Organocobalt Complexes The dramatic effect of the nature of the alkyl group on the mechanism of decomposition of macrocyclic Co(III) complexes containing a Co-C bond was demonstrated in an early pulse radiolytic study of the formation and decay of LComR,18 where L is rac -Me6-diene. 

The addition of an alkyl halide to the d7 complex [Co(CN)6]3- results in a one-electron oxidation of the metal and proceeds by homolytic abstraction of halogen. For a methyl or benzyl halide, an organocobalt product is formed,... 

Primary alkyl iodides formed stable organocobalt complexes in good yield, accompanied by minor quantities of the corresponding alkane and alkene. Isopropyl and t rf-butyl iodides yielded mainly mixtures of alkenes and alkanes. 

Besides organocobalt complexes, organostibine has also been found to be able to mediate controlled/ diving polymerization of many vinyl monomers. " For example, Yamago et al reported that at 60 °C the polymerization of VAc mediated by a-dimethylstibanyl ester reached 92% conversion in 5 h and produced narrowly dispersed PVAc (PDl = 1.26), but no detailed reaction kinetics was provided in their study.Kamigaito et al. very recently found that a manganese carbonyl complex [Mn2(CO)io] coupled with an alkyl iodide ( -I)... 

As exemplified by Eqn. 19, cobalt(II) complexes can efficiently trap organic radicals to form organocobalt complexes. Hence, organic radicals generated by means other than the atom-transfer step of Eqn. 18 may also be used to alkylate cobalt(II) complexes. Thus, Roche and Endicott [63] generated organic radicals by photolysis of carboxylatopentaammine-cobalt(III) complexes in acidic media (Eqn. 20)... 

Although many examples of reactions of cobalt-bound organic ligands exist, most of these have been carried out for synthetic purposes rather than as studies of mechanism or of the effect of the covalently bound cobalt moiety. Although many such transformations are quite simple, they frequently lead to organocobalt complexes which are difficult, or impossible, to obtain by direct alkylation. For example, Schrauzer and Windgassen [51] prepared carboxymethyl- and carboxyethyl-(pyri-dine)cobaloxime by hydrolysis of the corresponding methyl esters in concentrated sulfuric acid. Similarly, Brown and coworkers [58] prepared p- and w-carbo-xyphenyl(aquo)cobaloximes from their methyl esters by hydrolysis in 0.5 N KOH in aqueous methanol. 

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