Alkylated cobaloximes

It is also known that alkyl cobaloximes are subject to radical-induced decomposition.2 7 This suggests an alternative to the mechanism shown in Scheme 9.28 involving reversible chain transfer (Section 9.5). 

The dehydration and deamination reactions appear to operate in a parallel fashion and will be considered together. Schrauzer and Silbert propose a base-catalyzed cleavage of the carbon-cobalt bond in the B12-catalyzed diol dehydration reaction as shown in Fig. 17 (81), based on demonstrated lability of the beta hydrogens in alkyl cobaloximes with electronegative groups in this position. 

Alkylated cobaloximes yield the corresponding dimeric species of alkyl radicals by photolysis under acidic conditions. But the BDHC complex with a hexyl or benzyl group at its axial site does not yield the corresponding dimeric species by photolysis (dodecane and bibenzyl, respectively). Consequently, the hydrogenation product must be obtained through the formation of a carbanion by heterolytic cleavage of the Co-C bond, followed by its protonation. 

Stoichiometric Processes. In 1964, Schrauzer published the first of many papers on the synthesis and properties of alkyl cobaloximes. This work led to the development of cobaloximes and related compounds as vitamin B12 model compounds, e.g. (l)-(4), summarized in a review in 1976. By the mid 1970 s, many of the fundamental reactions of vitamin B12 and model complexes were well established. This work is summarized in several reviews. ... 

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. 

In another elegant study, Branchaud and coworkers have demonstrated an efficient cross-coupling between alkyl cobaloximes 54 and nitroalkyl anions 55 to give nitroalkanes 56 (Scheme 20). 

Scheme 20. Cross-coupling between alkyl cobaloximes and nitro alkyl anions...

First, upon irradiation, alkyl cobaloximes react with trichloromethanesulphonyl chloride, at 10 °C. A new carbon-sulphur bond is formed and the alkanesulphonyl chloride is produced in excellent yield546 (equation 141). 

The photoreactions of alkyl-cobaloximes are discussed in the Organometallic Compounds section. 

Another paper on the insertion of oxygen into the Co—R bond of alkyl-cobaloximes confirms that this reaction proceeds non-stereospecifically.277 Other reports contain brief discussions of Co—C bond homolysis in cobaloximes 278 and related compounds.279... 

It is claimed that multiple regression analyses, involving Taft s polar substituent constant or and Dubois steric parameter E, can satisfactorily account for the properties of alkyl cobaloximes (R = alkyl) and alkyl cobalamines in terms... 

The kinetics and equilibria for axial ligation of pyridine to a number of neutral and anionic alkyl cobaloximes have been studied by stopped-flow techniques. The neutral complexes appear to exist in solution as an equilibrium mixture of a six-coordinate aquo-species [RCoCDaHaXHOH)] and the five-co-ordinate compound, [RCo(D2H2)] and the mechanism of the formation of [RCo(D2H2Xpy)] is considered to involve initial dissociation of H2O according to the equation... 

If a cobaloxime is coordinated with a suitable basic group (B) in one of the axial positions, it behaves in many ways like vitamin Bi2- When reduced to the Co(II) state it reacts with alkyl halides to form alkyl cobaloxime, analogous in many properties and chemical reactivity to vitamin B12 coenzyme. 

The thermal and photochemical insertions of dioxygen into the Co—C bond of alkyl cobaloximes [CoR(dmg)2L] [R=CHgC6H4X or CHgCH=CHY (thermal), R=Me, Et, MegSiCHg, PhCHgCHg, etc. (photochemical), L=pyridine or water] follow a second-order rate law (23), in all cases studied. The rates are little affected... 

In a temperature-dependent n.m.r. study of alkyl cobaloximes it has been established that two differwt dynamic processes may occur, [equations (4) and (5)]. Both processes interchange the two possible allyl structures, (differentiated... 

Finally, we note as a general observation for the fluorinated alkyl cobaloximes that i F NMR resonances for the a-F atoms generally are broadened due to incomplete averaging of coupling to the quadrupolar 59Co (I = 7/2) nucleus. This broadening is evident for the CF=CF2 complexes as well, where the F atom on the carbon atom attached to cobalt is severely broadened and the F atom trans to the cobalt is slightly broadened (25),... 

Synthetic methods have been devised that lead to moderate to good yields of a-fluorinated alkyl cobaloximes. NMR studies and structural comparisons to the nonfluorinated analogs suggest that there is a significant ionic character in the bonding of the fluorinated alkyl ligands to the cobalt center. 

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