Cobalt hydride elimination

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

A mechanism was proposed in which entry into the catalytic cycle is achieved via Et2AlCl-mediated cobalt hydride generation. Diene hydrometallation affords the cobalt-complexed -jr-allyl A-5, which inserts the tethered alkene to furnish intermediate B-4. Elimination of LnCoOBn provides the cyclization product. Reduction of LnCoOBn by Et2AlCl regenerates cobalt hydride to complete the catalytic cycle (Scheme 17). 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Similar a-hydride elimination reactions resulting in metal carbenes were studied extensively in the case of tungsten [Eq. (6.77)] and tantalum [Eq. (6.78)] complexes. An a-hydrogen bond interaction of this type was proposed to rationalize the stereospecificity of reactions catalyzed by cobalt-containing coenzyme B12. ... 

On the basis of these observations the draft mechanism shown in Scheme 3.160 has been proposed for the catalytic reaction, by analogy with the previous reaction. The reaction of CoCl2(dpph) with M( jSiCH2MgOl gives complex 168, which is electron-rich, because of coordination of the Grignard reagent. Complex 168 effects single-electron transfer to an alkyl halide to yield an anion radical of the halide and cobalt complex 169. Immediate loss of halide from the anion radical affords an alkyl radical intermediate, which adds to styrene to yield a benzyl radical. Cobalt species 169 would then recombine with the carbon-centered radical to form cobalt species 170. Finally, /i-hydride elimination provides... 

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... 

The cobalt hydride in turn reacts with a new monomer molecules to regenerate the Co(II) [235]. One pubUcation describes a chain transferring agents that can be used in controlled polymerization of methacrylate monomers where reductive elimination of cobalt hydride from the neighboring methyl group deflects further chain growth [236]. The agent was illustrated as follows ... 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

An even more active version of the catalysts containing Lewis acids and metal anions allows the reactions to be conducted at ambient temperatures with 1 atm of CO. This catalyst appears to suppress the competitive formation of ketones from isomerization of the epoxide ° that possibly occurs through p-hydride elimination by the cobalt-alkyl intermediate generated prior to CO insertion. Reactions of a range of epoxides catalyzed by [(salph)Cr(THF)2] [Co(CO)J occur with balloon pressures of carbon monoxide, even on the multigram scale. ... 

In this context, it is notable that while solutions of 1-pentene and Co2(CO)8 proved to be essentially unreactive when irradiated, solutions of Co2(CO)g, 1-pentene and HSiEt3 were active for pentene isomerization with small concentrations of silylalkenes also found as an end product [75]. However, a chain reaction appears unlikely for the hydrosilation pathway the apparent fate of the cobalt radicals is to form 18 e species such as HCo(CO)4 and Et3SiCo(CO)4 which themselves are photo or thermal catalysts for pentene isomerizations. The silyl alkene products appear to result from the formation of Et3SiCo(CO)3( 1-pentene) followed by insertion of pentene into the Si-Co bond, then P-hydride elimination. 

As a Generator of Alkyl Radicals from Alkyl Halides in the Presence of Cobalt Catalysts. A conceptually novel use of Me3SiCH2MgCl recently emerged in cobalt-catalyzed Heck-type transformations. The palladium-catalyzed Heck reaction always etT5)loys aryl- or alkenyl halides. In eontrast, alkyl halides are not available for use as starting material, due to predominant 0-hydride elimination from the corresponding alkylpalladium... 

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