Cobalt I nucleophilicity

All cobalt(I) nucleophiles (Eqn. 4) as well as their conjugate acids (Eqn. 5) react rapidly with alkyl halides to produce the appropriate alkyl cobalt complexes. 

Kinetic studies of the reactions of cobalt(I) nucleophiles with alkyl halides [45] are consistent with an 8, 2 mechanism for these reactions. Indeed, inversion of configuration at the displacement center has been observed both for secondary [46] and primary [47,48] alkylating agents. However, for some alkylating agents for which inversion of configuration is not possible [44,49] as well as in certain other instances... 

With the possible exception of hydridocobalamin [42], olefins must be suitably activated with electron-withdrawing substituents to undergo these reactions. Hence, acrylonitrile (X = CN ) and methylacrylate (X = COOCH3) are active alkylating agents [51] but allyl alcohol (X = CH2OH) is not [52]. The mechanism of these reactions is not as well studied, but Schrauzer et al. [53] have presented spectral evidence that the alkylation of cobalt(I) nucleophiles with activated olefins proceeds via a IT complex intermediate (Eqns. 10 and 11). 

Cobalt(I) nucleophiles and hydridocobalt complexes also add to acetylene and substituted alkynes to yield alkenyl cobalt complexes (Eqns. 13 and 14), again with a change in addition mode, although mixtures of products are obtained for some X [56]. 

The reaction of pyridine cobaloxime(I) with phenylacetylene has been shown to occur via direct attack of the cobalt(I) nucleophile on the unsubstituted sp carbon... 

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 hahde 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, -trifluorotoluene, to produce the expected substituted phenyl(pyridine)[Pg.441]

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. 

The stereochemistry of the product is determined by step a, in which the proton is transferred from the quinine to the coordinated carbonyl, and the quinine is considered associated with the substrate making it more susceptible to nucleophilic attack by the cobalt(I) (Fig. 3). The mechanism is analogous to some biological oxidoreductase systems, where the site that determines the stereochemistry is remote from the active cata-... 

Another possible precursor to conduct free radical reactions is the glycosyl-cobait(III) dimethylglyoximato complex 33 [22,23], These organometallic compounds can readily be prepared by the displacement of the halide atom in 17 with the highly nucleophilic cobalt(I) anion 32. The latter can be generated from the dimeric Co(II) complex 31 under reducing conditions. 

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). 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

Such a reactivity has been observed previously during the pyridine-promoted solvolysis of acetyltetracarbonyl cobalt(I) (56). Both effects in the case of ruthenium suggest that the carbonylated product does not leave the metal center by reductive elimination of acyl iodide, but rather by nucleophilic displacement. 

In contrast to the behavior of the foregoing nickel(I) complexes as catalysts, the catalytic reactions of alkyl halides with cobalt(I) species such as vitamin Bi2s, cobaloximes(I), and cobalt(I) salen exhibit a significant difference. Cobalt(I) species, acting as potent nucleophiles in Sn2 reactions with alkyl halides, give stable alkylcobalt(III) intermediates. Lexa and coworkers [318] have discussed this mechanistic scheme for the catalytic reduction of l-bromobutane by the electrogenerated cobalt(I) tetraphenylpor-phin complex, where TPP denotes the ligand. Reversible one-electron reduction of the parent cobalt(II) species... 

Transition metal complexes, either as anions or as neutral molecules, are often very good nucleophiles for alkyl halides and sulfonates. Some of them, such as vitamin B12s, a cobalt (I) species, and the solvent-separated ion pair, Na+ S Fe(CO)42-, where S is N-methylpyrrolidinone, are among the most reactive known (1, 2). Reactions are of several types, for example... 

The cobalt(I) species that is generated is a very powerful nucleophile and a highly unstable entity. It is required as a substrate for the adenosyltransferase, which adds the upper adenosyl ligand to the centrally chelated cobalt ion. The enzyme responsible for this process is encoded by cobO The enzyme catalyzes the adenosyl transfer from ATP to a range of corrins, including cob(I)yrinic tf,c-diamide, cob(I)yrinic acid, and cob(I)inamide, generating the cobalt(III) corrinoid and triphosphate. The P. denitriftcans CobO is a homodimer with a... 

Indeed, a plot of log ha versus log Keq for the proton transfer (log( oL(h)2+) = 11.3) is hnear, with a slope of ca. 0.5. Most remarkably, the rate constants for both CO and CO2 addition to the macrocycle fall on the same plot. Protonation and formation of the metal-carbon bond appear to lend themselves to a description in terms of an associative reaction (Sn2), in which the low-spin d cobalt(I) metal center serves as the nucleophile. 

Epoxy olefins 67a-b can be converted to cycloalkanols 69a b respectively on treatment with cobalt(I) dimethylglyoxime using a sunlamp. These reactions proceed via the cyclization of the intermediate j8-hydroxycobaloximes 68a-b, which are produced by a nucleophilic opening of epoxides with cobalt(I) (Scheme 26) [27, 28]. 

Such cobalt(I) complexes are extremely air-sensitive, low spin d systems, some of which are believed to be 4-coordinate square-planar and all of which are highly nucleophilic reagents due to the lone pair behavior of the d electrons. These nucleophilic cobalt(I) chelates are the conjugate bases of hydrido-cobalt complexes (Eqn. 1) [23,26] which in turn are unstable, decomposing to hydrogen and cobalt(II) (Eqn. 2) [23,27,28], a reaction which is known to be reversible, at least in some cases. 

Despite the sensitivity of cobalt(I) reagents to air oxidation, the extremely high nucleophilicity of these reagents makes them attractive substrates for alkylation reactions. Since cobalt(I) reagents must always be prepared in situ from complexes in a higher oxidation state, this route is commonly referred to as reductive alkylation. 

Finally it should be pointed out that a number of studies have been published purporting to show the nucleophilic displacement of cobalt(I) chelates from alkylcobalt complexes by thiolate anions in base (Eqn. 44) [77,78]. 

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