Cobalt group transfer reactions

Pattenden and coworkers have developed a cascade cobalt group transfer reaction by effecting consecutive cobalt-mediated radical cyclizations in a controlled... 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

The alkyl-cobalt addition-elimination (cobalt group transfer) sequence has been used by Baldwin and Li during the enantiospecific synthesis of (-)-a-kainic acid (58a) and (-)-a-allokainic acid (58b). These reactions proceed via the carbon-centered... 

The corrinoids involved in methyl group transfer do not possess the organo-ligand 5 -deoxyadenosyl [see structure (II), Section I,B] and the reaction probably proceeds via the intermediate formation of the methyl cobalt complex, but no mechanistic details have yet been established. 

Although the acylcobalt tetracarbonyls react with hydroxide ion under phase-transfer conditions, in the presence of alkenes and alkynes they form o-adducts rapidly via an initial interaction with the ir-electron system. Subsequent extrusion of the organometallic group as the cobalt tetracarbonyl anion leads to a,(J-unsaturated ketones (see Section 8.4). In contrast, the cobalt carbonyl catalysed reaction of phenylethyne in the presence of iodomethane forms the hydroxybut-2-enolide (5) in... 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

Vitamin B12 (cyanocobalamin) 3 is, in fact, not a natural product as the cyanide ligand to the cobalt ion is added during the isolation procedure. Coenzyme B12 (adenosylcobalamin) 4 and methylcobalamin 5 are the true final products of the biosynthetic pathway. Coenzyme 0,2 is the cofactor for a number of enzymic rearrangement reactions, such as that catalysed by methylmalonyl CoA mutase, and methylcobalamin is the cofactor for certain methyl transfer reactions, including the synthesis of methionine. A number of anaerobic bacteria produce related corrinoids in which the dimethylbenzimidazole moiety of the cobalamins (3 - 5) is replaced by other groups which may or may not act as ligands to the cobalt ion, such as adenine orp-cresol [12]. 

Cobalt tetraphenylporphyrin complex promotes a chain-transfer reaction in the radical polymerization of MMA to give an MMA oligomer with vinylidene unsaturation at the chain end.124 An alternative method of introducing the terminal unsaturation was disclosed by Meijs et al,125 Substituted allylic sulphides are used as chain transfer agents in which sulphide groups act as leaving group as follows ... 

The one-carbon transfer reaction (left) facilitated by the octahedral cobalt(III) complex Vitamin Bi2 (right) as coenzyme. Carbon-cobalt bonding of the substrate is involved in the mechanism, at the site designated by the R-group in the complex. 

In this group of reactions electron transfer takes place between the ligand and the metal ion in the complex. A typical example is the reaction of the cobalt(III) complex... 

These systems are complicated by the fact diat they catalyze two methyl-transfer reactions, one from MeCbl to the substrate and the other from some reagent to cobalt to regenerate MeCbl. For methionine synthase, it is known that the reactions go with overall retention of the stereochemistry of the methyl group. This shows that both transfers occur either with retention or inversion. This observation, plus the absence of EPR signals indicative of Co(II) or organic radicals, suggest that these reactions do not proceed by homolysis of the Co—C bond. 

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