Cobaloxime hydride

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

The development of new radical initiators is an area of active investigation. Tada et at. have described a (triphenyltin)cobaloxime complex as a reagent for radical generation from bromides [95JOC6635]. This is an alternative to reductive processes involving tin hydride and allows for the introduction of an additional functional group in the product. 

A general route for synthesizing a great variety of cobaloximes is by reaction of Co complexes with electrophiles. Co precursors are generated by reduction of Co or Co electrochemically, by Na/Hg, or with boro-hydride. The reaction has been extensively investigated and rates have been compared with those found for various organic nucleophiles. It was found that Co cobaloximes are among the... 

Hydrides of cobalt chelates are well documented in inorganic chemistry,89 163 and cobaloximes received particular attention in the I960—70s because their chemical behavior was somewhat similar to that of vitamin Bi2.85,86,164 171 Although the hydrides of cobaloximes are very reactive, two instances in which (DH)2CoH was isolated have been reported.88172 It is believed that in the absence of radicals or monomer, the main mode of decomposition of (DH)2CoH... 

The addition of LCoH to olefinic double bonds is well documented in the chemistry of cobaloximes. Because LCoH has the ability to dissociate to H+ and LCo1, both the hydride and Co1 form of the cobalt chelate must be considered in any system where LCoH can be obtained. 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

Subsequently, Cl was observed in polymerization of MMA with high concentrations of cobaloximes,77 293 phthalocyanines,14 136 294 295 and porphyrins.295 Cl is observed only when the cobaloxime was in the 2+ oxidation state and can catalyze chain transfer.296 Thus, Cl is a phenomenon that is closely connected to CCT. The first explanation involved a hydride (as in eq 31), where P is a polymer obtained from the propagating radical which has been terminated by hydrogen atom donation by Co111—H.161... 

Square planar cobalt complexes including porphyrin and cobaloxime derivatives rapidly and reversibly add carbon-centred radicals. An alternative pathway to the reverse reaction is an elimination to form the olefin and a cobalt hydride species, which can initiate polymerization by donating hydrogen to monomer.24... 

Many of the radical reactions use tri-n-butyltin hydride as the reagent of choice, but the tin-derived residues are often difficult to remove. Torii et al. have shown that cobaloxime I, generated by electrochemical means, can replace the tin hydride and converts the... 

In some cases the second step does not take place, and the counterion never binds to the metal. This makes the reaction an electrophilic addition, rather than an oxidative addition to the metal, although the latter term is sometimes seen in the literature to describe this type of reaction. An example is the reaction of the highly nucleophilic Co(I) anion, cobaloxime, with an alkyl triflate, a reaction known to go with inversion. Protonation of metal complexes to give metal hydrides is also very common (Eqs. 3.30-3.31). 

Tin Hydride-Mediated Reaction Allyltin Method Photochemical Fragmentation of Barton Esters Direct Photolysis of Cobaloximes Direct Photolysis of Organotellurium Compounds Miscellaneous... 

The nature of the reduced cobaloxim species in aqueous solution is unknown but it is suggested that there may be both a hydride and an anionic complex in equilibrium [130], e.g. 

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