Cobalt compounds alkyl derivatives

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). 

Reduced cobalt compounds have been of longstanding interest in bis(imino) pyridine chemistry. Following the reports of ethylene polymerization activity, efforts were focused on the preparation of various cobalt alkyl derivatives with the goal of understanding the nature of the active species formed upon treatment with methyl... 

These studies raised several fundamental questions about the effects of pincer substituents on the overall electronic structure of the resulting cobalt halide or alkyl compound. First, why are the electronic stractures of N-aryl and N-alkyl substituted bis(imino)pyridine cobalt chloride complexes different. Second, why for the N-alkyl derivatives is the chloride compound spin crossover and the methyl diamagnetic. What role does the distortion of the hgand play on the overall electronic structure ... 

For example, benzamide reacts with an alkyl chloride in the presence of cobalt compounds at room temperature to give an alkyl derivative at the ortho-posiiion in a high yield, as shown in Eq. (7.84) [166]. 

Research on alkyl and aryl derivatives of the transition metals has flourished in recent years. The last fully comprehensive review of this area was prepared by Coates (1) in 1960, although excellent reviews of individual areas have appeared recently. For example, King (2) discussed metathetical syntheses of alkyl complexes, Heck (J) has described alkyl and acyl cobalt compounds, and Cross (4) has summarized the chemistry of platinum derivatives. Tabulations of individual organometallic compounds have been assembled by Dub (5) and Kritskaya (6). 

The insertion of carbon monoxide into a C—M bond is a common reaction for alkyl derivatives of the late transition elements. It has been reported for Mo, Mn, Fe, Co, Ni, Pd, and Pt compounds. The insertion of CO into the C —Co bond of cobalt carbonyl derivatives is a key step in the 0x0 reaction and catalytic carboxylation processes (see Section IV,D). The CO insertion reaction is frequently reversible but sometimes only the reverse reaction, CO elimination, is known, e.g.,... 

Another chemically more interesting spin labeled B12 derivative involves coordinate attachment of the nitroxyl function to the cobalt atom of a cobinamide. Fig. 22 shows a reaction in which an alkyl cobin-amide is mixed with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. The nitroxide displaces water from the 6th coordination position very slowly and therefore this reaction is usually allowed to proceed for a few days with a large excess of nitroxide. From the properties of the coordinated nitroxide derivative discussed below, it is certain that the cobalt is coordinated by the N—O functional group. An analogous compound to that shown in Fig. 22 can be made with a similar nitroxide in which the 4-hydroxyl-group is missing. In this case the N—O-function is the only basic site on the molecule and therefore must be the position of attachment to the cobalt 119). 

It has already been mentioned that prochirality of the olefin is not necessary for successful enantioselective cyclopropanation with an alkyl diazoacetate in the presence of catalysts 207. What happens if a prochiral olefin and a non-prochiral diazo compound are combined Only one result provides an answer to date The cyclopropane derived from styrene and dicyanodiazomethane shows only very low optical induction (4.6 % e.e. of the (25) enantiomer, catalyst 207a) 9S). Thus, it can be concluded that with the cobalt chelate catalysts 207, enantioface selectivity at the olefin is generally unimportant and that a prochiral diazo compound is needed for efficient optical induction. As the results with chiral copper 1,3-diketonates 205 and 2-diazodi-medone show, such a statement can not be generalized, of course. 

Acyl cobalt complexes are reduced to their alkyl counterparts in good yields with Et3SiH/TFA (Eq. 248).183-310-425 Acyl ferrocene derivatives are reduced to the respective methylene compounds with Et3SiH/TFA (Eqs. 249180 and 250).179 Acylcyclopentadienylmanganese tricarbonyl is similarly reduced in good yield.351... 

Vitamin B12 derivatives and their model compounds have recently been used as recyclable electrocatalysts for the reduction of alkyl halides since low-valent Co species are good nucleophiles toward organic substrates [367-369]. Examples of such elec-trocatalysts are the vitamin B12 derivatives aquocobalamin (230), dibromo[l-hydr-oxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl-10,20-diazaoctahydropor-phinato]cobalt(III) (231), and cobaloxim (232). The above Co(I) complexes can be... 

As recently reported, cobalt-catalyzed addition of olefins to butadiene is probably an example of the addition of cobalt alkyls to butadiene (106). The catalyst was the type prepared by reaction of cobalt chloride with an aluminum alkyl in the presence of a diene. A bis-7r-allylcobalt derivative is probably formed. The unstable 7r-allylcobalt compounds probably decompose (reversibly) into cobalt hydride. The hydride would add to the olefin present to form a dialkyl, which could then add again to the diene. 

Many transition metal alkyls react with carbon monoxide to give acyl compounds. In all these cases the acyl derivatives can be detected at least by infrared methods and in most cases isolated. Molybdenum, manganese, rhenium, iron, cobalt, rhodium, nickel, palladium, and platinum alkyls, Grignard reagents, and boranes, all react with carbon monoxide, and one can explain the products from these on the basis of carbon monoxide inserting into the metal alkyl. 

Cobalt hydrocarbonyl, diborane, and aluminum hydrides add, I think, to all of these carbonyl compounds. Of course, there is the well known Grignard reagent and the alkyllithium additions to carbonyl compounds. Aluminum alkyls add, and we could have listed all the other alkali metal alkyls. Recent work has shown that the tin alkoxides add readily to all these derivatives, and similarly, a tin amide adds to most of these carbonyl compounds. 

Because such alkylation proceeds by S l mechanism, even cobalt complexes derived from unreactive (in an SN2 sense) halides can be formed. The cobalt complexes are air-stable compounds, but are affected by direct daylight. The incorporated Co—C bond is weak and, therefore, photolysis of 33 sets free the anomeric radical 11. In the presence of olefins 12 this radical adds to the double bond, followed by subsequent combination to give the insertion product 35 (Scheme 9). 

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