Cobalt alkyls

It lias also been shown that the alkyl cobalt (III) initiator can be generated in im by adding a fast-decomposing azo-initiator [2,2,-azo-bis(4-mcthoxy-2,4-dimethyl valeronitrilej to a solution of the cobalt (11) complex in monomer. Very narrow dispersily PMA and PMA-WocA-PBA were prepared. 

Prior equilibrium. Consider the net reaction between certain metal halide and alkyl cobalt complexes, RCo + MX = Co+ + RM + X". There is a rapid equilibrium ... 

Alkali Metal Derivatives of Metal Carbonyls, 2, 1S7 Alkyl and Aryl Derivatives of Transition Metals, 7, 1S7 Alkyl cobalt and Acylcobalt Tetracarbonyls, 4, 243 Allyl Metal Complexes, 2, 32S... 

Bis(aryl)cobalt(II) compounds have been prepared by reaction of R MgX (where R = C6H6 Cl n = 2-4) with Co(PR3)2Cl2.203 They undergo both thermally and oxidatively induced decomposition, with the corresponding biphenyl a product. The reactions of alkyl-cobalt complexes have been reviewed recently, and include thermolysis, photolysis, oxidation, and reduction reactions.25 Homolysis of the Co—C bond is a feature of reactions. 

In contrast with the role of cofactor B12 in methionine synthase (methyl group transfer to a thiol), functional Bi2 model complexes have provided a formidable challenge. Several oxime alkyl-cobalt (structural) B12 models when reacted with arene- and alkanethiolates lead only to... 

Lenhert and Hodgkin (15) revealed with X-ray diffraction techniques that 5 -deoxyadenosylcobalamin (Bi2-coenzyme) contained a cobalt-carbon o-bond (Fig. 3). The discovery of this stable Co—C-tr-bond interested coordination chemists, and the search for methods of synthesizing coen-zyme-Bi2 together with analogous alkyl-cobalt corrinoids from Vitamin B12 was started. In short order the partial chemical synthesis of 5 -de-oxyadenosylcobalamin was worked out in Smith s laboratory (22), and the chemical synthesis of methylcobalamin provided a second B 12-coenzyme which was found to be active in methyl-transfer enzymes (23). A general reaction for the synthesis of alkylcorrinoids is shown in Fig. 4. 

A documentation of the physical and chemical properties of alkylcorrinoids and alkyl-cobalt Bi2 model compounds is presented in a recent review by Hill (41). In this report we shall deal with only those properties of alkylcorrinoids which have proved to be useful when applied to a study of B 12-enzyme mechanisms. 

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

Co(CO)4] ) dehydrohalogenation [Eq. (15)] followed by addition of HCo(CO)4, or whether splitting out of HCo(CO)4 occurs from the alkyl-cobalt [Eq. (14)], which is the malonate precursor, followed by HCo(CO)4 addition in the opposite direction. In one case [Eq. (15)], olefin formation proceeds directly from the bromide and no reversibility of any steps is required, while according to Eq. (14) olefin formation proceeds from elimination of HCo(CO)4. 

These radical species may (slowly) combine to form the ju-peroxodico-balt species or, as in the preparation of [CH3Co(NH3)5]2+, react (more rapidly) with the alkyl hydrazine to form the alkylated cobalt(III) species and dinitrogen. Schiff base complexes such as these are discussed in more detail later in this article. 

Like dimethylbenzimidazole, it is combined with ribose in the unusual a linkage. A compound called factor A is the vitamin B12 analog with 2-methyladenine. Related compounds have been isolated from such sources as sewage sludge which abounds in anaerobic bacteria. It has been suggested that plants may contain vitamin B12-like materials which do not support growth of bacteria. Thus, we may not have discovered all of the alkyl cobalt coenzymes. 

Vitamin B12s reacts rapidly with alkyl iodides (e.g., methyl iodide or a 5 -chloro derivative of adenosine) via nucleophilic displacement to form the alkyl cobalt forms of vitamin B12 (Eq. 16-31). These reactions provide a convenient way of preparing isotopically labeled alkyl cobalamins, including those selectively... 

Alginates 170,178 Alkali metal ions 206 Alkaline phosphatases 645 active site of 645 occurrence 645 Alkane(s) 382 Alkenes, hydration of 683 Alkyl shift, rearrangements with 527 Alkyl cobalt 867 Alkyl ethers 382 Alkyl cobalamms... 

When simple terminal alkenes are used as acceptors, the cyclic primary alkyl cobalt species are stable, and can often be isolated and purified by standard techniques.145 Scheme 32 shows some of the transformations that Pattenden has accomplished with the cyclic alkylcobalt complex (55).146 In addition to standard elimination to an alkene, the complexes can be converted to alcohols, halides, oximes, and phe-... 

Miller, R.L., Pinkerton, A.B., Hurlburt, P.K., Abney, K.D. 1995. Extraction of cesium and strontium into hydrocarbon solvents using tetra-C-alkyl cobalt dicarbollide. Solvent Extr. IonExch, 13 (5) 831-827. 

Chamberlin, R.M., Abney, K.D. 1999. Strontium and cesium extraction into hydrocarbons using alkyl cobalt dicarbollide and polyethylene glycols. J. Radioanal. Nucl. Chem. 240 (2) 547-553. 

M 1)>CH3 (366 M 1)>C6H5 (66 M ) k7 varied in a similar manner, R = CH3CH2>C6H5 CH3>C6H5CH2, CFj. Thus the tendency for styrene and fluoroolefins to be hydrogenated could result from either the thermodynamic or kinetic difficulty with which the corresponding alkyl-cobalt carbonyls undergo carbonylation. 

When compared to all literature A values (17-41 kcal moD1) for alkyl-cobalt bonds, the methyl-cobalt bond in methylBi2 and the adenosyl-cobalt bond in coenzyme B i2 and Ado-Cbi+ are relatively stable. Although methylBi2 exhibits the largest A value reported [98] to date, 41 kcal mol-1, coenzyme B12 and Ado-Cbi+ both have large A (33 and 37.5 kcal mol-1, respectively) [76,102],... 

Another example is provided by the alkyl group exchange between the pentacoor-dinated alkyl cobalt porphyrins and tetracoordinated cobalt(II) hydroporphyrins59 that takes place readily at room temperature as observed by H NMR (Equation 8.121). 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Alkyl cobalt(lII)bis(dimethylglyoxime anion)pyridine, [R-Co(l,l)(dmgH)2py,l]... 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

The direct extension of similar approaches to other systems, notably alkyl-cobalamins and related alkyl-cobalt compounds, is constrained by the instability of the corresponding hydrides, which are consequently unsuitable as radical traps. Modification of the approach to utilize other radical traps (e.g., 02) potentially is feasible but needs further investigation in view of possible complicating features such as reaction between the trapping agent and the parent metal-alkyl and ambiguities between homolytic dissociation and other (e.g., concerted) mechanisms (26, 27). 

However, some indirect indication of the strength of alkyl-Co bonds in organo-cobalamin, relative to those in other alkyl-cobalt compounds, is provided by observations concerning the stability of ben-zylcobalamin. Attempts to prepare benzylcobalamin by either the Bi2s route (Reaction 31) (32) or the Bi2r route (Reactions 32-34) (15) have yielded spectroscopic evidence for its initial formation in solution. However, benzylcobalamin proved to be too unstable for isolation and... 

Mechanistically, alcohol carbonylation reactions catalyzed by the HCo(CO)4/ Co(CO)4 system appear to be governed by several features which are unique to this system. In particular, the high inherent acidity of the HCo(CO)4 species (45), coupled with the nucleophilicity of the conjugate base (55), is responsible for the activation of the substrate and formation of the alkyl-cobalt bond. In addition, the facility of homolytic cleavage of cobalt-carbon bonds (46, 47) may be responsible for the complications in selectivity not normally observed with other systems. 

The Co1 arene complex [Co(C6Me6)2]+ has a sandwich structure, with two ij6-bound arene ligands. The complex is paramagnetic, with two unpaired electrons.39 Alkyl cobalt(I) carbonyls, generally made by the reactions... 

Early experiments using simple alkyl cobalt compounds as models for AdoCbl fostered the notion that steric crowding around the CooC bond might be an important factor that enzymes could exploit to effect the remarkable rate enhancements for homolysis of AdoCbl (Ng et al, 1983). These studies demonstrated that the more bulky the alkyl group attached to cobalt, the faster the rate of homolysis, leading to the idea that the enzymes might distort the coenzyme and thereby weaken the CooC bond. 

The discovery that carbon monoxide can be inserted into alkyl-cobalt tetracarbonyl to yield unstable acylcobalt tetracarbonyl (which... 

R. F. Heck Advan. Organometal. Chem. 4,243(1966) Synthesis and reactions of alkyl-cobalt and acylcobalt tetracar-bonyls 24 (30) ... 

Alkylcobalt(III) complexes can also be synthesized in aqueous solution. Two of the best-known systems are methylcobalamin and a group of related cobaloximes, and alkylcobalt(III) complexes having ancillary cyanide ligands. As with the chromium(III) system, alkyl cobalt(III) complexes having dimethylglyoxime (DMG) or cyanide ligands can be synthesized by reaction of the cobalt(II) precursor with alkyl halides (Scheme... 

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