Substituents cobaloxime

Steric factors play an important role in reactions of bromomethylaqua-cobaloxime with substituted imidazoles - 1-methylimidazole reacts at approximately the same rate as imidazole itself, but the presence of an alkyl substituent in the 2-position of the incoming imidazole reduces the rate dramatically (72). 

Another type of isomerization, that of cis-trans isomerization of an axial Co substituent has also been observed to occur in a single crystal-to-single crystal manner in cobaloximes. Compound 42 was monitored by x-ray crystallographic determination of the unit cell dimensions at 9 points over 2 h [121]. Molecular... 

FIGURE 18.5. Solid-state X-ray induced reaction of a cobaloxime. The Co-C bond is broken, and racemization of the upper axial substituent occurs, as shown. 

Table 2. Influence of Cobaloximes 6 Substituents on CCT Activity in Styrene077...

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

In addition, as part of a kinetic examination of the rates of ligand displacement by H20 in trifluoromethyl cobaloximes, the pKa of the CF3—Co(III) aquo derivative (—1.36) was found to be significantly lower than that of alkylcobaloximes (0.0 to 0.67) (65). The differences between the results for the CH3 and CF3 substituents in both sets of experiments was attributed to the greater electron-withdrawing ability of the CF3 group. (Trifluoromethyl) cobaloxime was chosen for the second study because it is much more stable toward strong aqueous acids than the alkylcobaloximes 63). 

Such reactions are formally the reverse of the alkylation of cobalt(I) nucleophiles by suitably activated olefins (Eqn. 8). Indeed, Schrauzer et al. [53] have presented spectroscopic as well as other evidence that for cobaloximes where X = —CN or —COOCH2CH3 these reversible reactions proceed via intermediate formation of a cobaloxime(I)-olefin w complex, i.e. the microscopic reverse of Eqns. 10 and 11. However, Barnett et al. [73] have studied the kinetics of the analogous base-catalyzed elimination of 2-cyanoethylcobalamin to produce cob(I)alamin and acrylonitrile. These authors found no general base catalysis and a rate law which was first order in organocobalamin and first order in hydroxide ion and determined a second-order rate constant of 230/M/min. As these authors pointed out, this rate constant is several orders of magnitude greater than the second-order rate constant for ionization of acetonitrile so that the mechanism must either by a concerted E2 elimination (or possibly direct elimination of hydridocobalamin) or, if stepwise, the rate of /8-proton dissociation must be substantially enhanced by the cobalt-containing substituent. 

Table 1.5 Effect of substituents on cobaloximes methyl methacrylate polymerization.

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... 

Finally, some successful reductive arylations have been reported via reaction of aryl halides with cobalt(I) nucleophiles. Generally speaking these reactions are only successful when the aryl halide has an electron-withdrawing substituent and yields are generally less than 15% [58,59]. Thus, pyridinecobaloxime(I) reacts with p-bromoacetophenone, methyl-m- and methyl-/>-bromobenzoate, and p-bromo-a,a,a-trifluorotoluene, to produce the expected substituted phenyl(pyridine)cobaloximes in yields of 11.5, 10, 7, and 5% respectively. 

Cobaloximes(I) and hydridocobaloximes can be arylated with aryl halides with electron-withdrawing substituents, consistent with both electron transfer and Sn2 displacement mechanisms. 

The synthesis and characterization of cobaloximes that have fluorine-containing alkyl ligands, especially those with fluorine substituents at the carbon atom (a-C) attached to the cobalt ion, are the topic of this report. As noted above, the cobaloximes have been intensely examined spectroscopically and numerous high quality solid-state structural investigations have been previously accomplished (7-5). We have been interested in exploiting these properties of the cobaloxime system in order to assess the effect of fluorine substitution on spectroscopic and structural parameters of this class of compounds. 

Vinylallenes can be hydrometallated to produce 1-cobaloxime-substituted dienyl complexes (Equation (25)). Cobaloxime-substitued 1,3-diene without a substituent in the 3-position will participate in thermal and Lewis acid-catalyzed Diels-Alder reactions, but the resulting cycloadducts apparently rearrange and give products with... 

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