Organocobalts reactions with

Organocobalt(III) complexes are stable towards oxygen unless they undergo spontaneous fission of the Co—C bond to give Co(II), Co(I), or CoH complexes (see Section VI,B,l,a and D). Reactions with halogens are dealt with in Section VI,C,l,c. 

Jhe reaction mechanism of the various metal complexes clearly have much in common. The recently recognized insertion reaction appears to be a particularly good example of a reaction which is general among the metal compounds. In the following discussion I intend to point out the generality of the insertion reaction with examples from the literature and from our own wrork. The most complete series of substantiated insertion reactions involves the organocobalt carbonyl complexes, and these reactions will form the nucleus of the discussion. 

Cobalt(II) compounds also undergo transalkylation reactions with organocobalt(III) compounds, probably via a transition state in which the two cobalt centres are bridged by the alkyl group. 

The treatment of diorganozincs or organozinc halides with cobalt bromide in THF NMP mixtures provides blue solutions of organocobalt reagents, which have a half-life of several hours at -20 °C.25,26 Carbonylation reactions with these new cobalt reagents proceed well simply by bubbling carbon monoxide... 

B. Giese, A. Ghosez, T. Gobel, J. Hartung, O. Huter, A. Koch, K. Kroder, and R. Springer, Radical reactions with organocobalt complexes. Free Radicals in Synthesis and Biology (F. Minisci, ed.), KJuwer Academic Publishers, 1989, p. 97. 

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

Continuing their investigations of conjugated polymers incorporating (ip-cyclopentadi-enyl)(T]4-cyclobutadienyl)cobalt complexes, Endo has discovered a new route to polythiophenes that involves reaction of organocobalt polymers with S8 (Scheme 4.15).50 When polymer 49... 

This is, in fact, a particular example of a general class of reactions in which organocobalt complexes with )8-oxy substituents (including 2-hydroxy-, 2-alkoxy-and 2-alkyloxyalkyl-cobalts) [52,91-93] are decomposed in acid via elimination of aquocobalt complexes (Eqn. 55). 

Although the previously described polymerization with organocobalt porphyrins (7) is the first example of controlled radical polymerization, the applicability of this system is only limited to acrylic esters (20). Use of 7 for free-radical polymerization of methacrylic esters (21) results in a chain-transfer reaction with respect to the a-methyl group, to give oligomers with terminal unsaturation. 

Alkylzinc reagents undergo transmetallation in the presence of Co(II) salts to provide the corresponding organocobalt derivatives. These intermediates are stable below —10 °C in mixtures ofTHF and NMP and have proved useful for the synthesis of symmetrical ketones [310, 311]. Cross-coupling reactions with alkylzincs can also be performed using catalytic amounts of Co(II) salts. In contrast to arylzinc reagents, however, disclosed examples [312, 313] are still rather scarce. 

Continuing the organocobalt theme, two groups have reported using the Pauson-Khand reaction with carbohydrate templates to construct bis-annulated pyranosides. Propargylic ether substrates such as 148 and 150 provided the isomeric tricyclic systems 149 and 151, while the propargyl malonate substrates 152 and 154, cyclized to the isomeric tricyclic systems 153 and 155 (Scheme 38). ... 

Co2(CO)g, the basic compound of organocobalt compounds, reacts with many compounds to afford the variety of organocobalt compounds as shown in Scheme 17.1. In these products, NaCo(CO)4 [12] or HCo(CO)4 [16], obtained by reaction with sodium or hydrogen, are also used as the raw materials for the synthesis of organocobalt compounds. 

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