Cobalt catalysis alkylation

The total yield of 2-vinylpyridine formed from 2-methylpyridine can be as high as 90%. 2-Vinylpyridine may also be obtained in almost quantitative yields by heating 2-alkylaminopyridine derivatives (which are directly available by cobalt catalysis) with a supported (e.g., AI2O3) alkali metal hydroxide [Eq.(8) R = R = alkyl, cyloalkyl, etc., RR N = heterocycle] (76SZP14399 78MI1). 

The same reaction scheme can be written for (Z) -2-phenyl-2-butene, except that paths B and E would lead to erythro and threo aldehydes. In cobalt catalysis this isomerization could explain both the lack of stereospecificity and the lack of influence of the sterochemistry of the starting olefin on the distribution of aldehydes 26 and 27. This hypothesis agrees well with results with a-ethylstyrene. On the other hand, when rhodium is used, extensive isomerization occurs less readily probably because of a better stability of alkyl- and acylrhodium carbonyls, and one can thus achieve a high degree of stereospecificity. 

Although cobalt catalysis is somewhat less developed compared to other transition metal catalysis for cross-coupUngs, but has shown rather unconventional, novel reactivity profiles [50]. For example, Oshima reported a cobalt-catalyzed tandem radical cyclization/cross-coupling reaction between an aryl Grignard reagent and an alkyl halide bearing an (o-alkenyl group (Equation 5.44) [51]. 

In addition to iridium and cobalt catalysis, and following the work initiated by Cavell et al. [138] on the alkylation of azolium salts, nickel-catalyzed alkylations of various heteroarenes (i.e., indoles [139], benzimidazoles [139], benzothiazoles [139], benzoxazoles [139], 1,3,4-oxadiazoles [140]) with olefins have been reported (Scheme 19.95 and Scheme 19.96). These reactions proved complementary to other methods because they proceeded with the Markovnikov regioselectivity with respect to the olefin. 

Originally, the pyridine construction reaction was based on cobalt catalysis and restricted to the use of acetonitrile or alkyl nitriles as one of the cycloaddition partners. However, recent advancements in this area have led to the development of certain ruthenium or rhodium catalysts, allowing the use of methylcyanoformate as an electron-deficient nitrile component in crossed [2 - - 2 - - 2]-cycloaddition reactions [39]. From the point of view of applications, the use of methylcyanoformate in transition-metal-catalyzed pyridine formation reaction is quite beneficial because the ester moiety might serve as a functional group for further manipulations. It might also serve as a protective group of the cyanide moiety, because cyanide itself cannot be used in this reaction. These considerations led to the design of a quite flexible approach to substituted 3-(130)- and y-carbolines (131) based on transition-metal-catalyzed [2 -f 2 -I- 2] cycloaddition reactions between functionalized yne-ynamides (129) and methylcyanoformate (Scheme 7.28) [40]. 

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Although mechanistically different, a successful kinetic resolution of cyclic allyl ethers has recently been achieved by zirconium catalysis [2201. Other metals such as cobalt [221], ruthenium [222], and iron [2231 have been shown to catalyze allylic alkylation reactions via metal-allyl complexes. However, their catalytic systems have not been thoroughly investigated, and the corresponding asymmetric catalytic processes have not been forthcoming. Nevertheless, increasing interest in the use of alternative metals for asymmetric alkylation will undoubtedly promote further research in this area. 

The substituent R may be alkyl, cydoalkyt. or benzyl. Catalysts are selected from transition metals which can form carbonyl complexes. Ruthenium and especially cobalt form active catalysts, although other metals like Rh. Pd. Ft. Os, Ir, Cr, Mn, Fe, and Nt have also been examined. If metals like ruthenium or iron catalysis are used, carbon dioxide is formed instead of water as the by-product. 

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 immobilization of an active species into a conducting polymer layer allows one to obtain active electrodes for the reduction of various organic halides. Polypyrrole containing viologen electrodes appear to be active for the reduction of alkyl dibromide [177] or hexachloroacetone [178], Cobalt-bipyridyl-polypyrrole films are active electrodes for the reduction of alkyl chloride [107], The mechanism of this reaction is similar to that observed in the homogeneous phase. This confinns one of the major interests of the modified conductive polymer electrodes, i,e. the possibility of performing catalytic reactions with smaller amounts of active catalyst in comparison to homogeneous catalysis, and then to avoid problems related to the separation of products from the solution which contains this catalyst. 

This chapter is concerned with highlighting some of the more notable advances that have come to light as a result of identifying key factors that influence catalyst performance, particularly those related to precatalyst structure. The importance of the initiator and the role played in chain fransfer is probed. Current mechanistic understanding is examined from both a spectroscopic and a computational viewpoint while efforts to prepare well-defined iron or cobalt alkyl catalysts are discussed. Efforts to heterogenize these homogeneous catalysts are briefly reviewed, as is their use in multi-component catalysis. 

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