Cobalt complexes Co

Samarium diiodide serves as an effective reductant for a variety of functional groups. Conjugated double bonds are readily reduced by Sml2 in the presence of proton donors such as methanol (Kagan and Namy, 1986). Inanaga (1990) has shown that alkynes are reduced efficiently to alkenes by samarium diiodide in the presence of catalytic amounts of cobalt (Co) complexes. Selective synthesis of (Z)-alkenes is possible due to the chemoselective nature of... 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

The principle cost determinant in typical hydrolytic or phenolic resolutions is the cobalt catalyst, despite the relatively low catalyst loadings used in most cases and the demonstrated recyclability with key substrates. From this standpoint, recently developed oligomeric (salen)Co complexes, discussed earlier in this chapter in the context of the hydrolytic desymmetrization of meso-epoxides (Scheme 7.16), offer significant advantages for kinetic resolutions of racemic terminal epoxides (Table 7.3) [29-31]. For the hydrolytic and phenolic kinetic resolutions, the oligo-... 

But one must not be too facile with sweeping generalizations concerning concentration dependences or their absence. For example, consider the reaction between the cobalt(II) complex known as Co(sep)2+ and molecular oxygen. With these reagents only, the two-step reaction in acidic, aqueous solution is5... 

Thus, the interaction of the ground state cobalt(ii) complex with long Co-N bonds and the ground state cobalt(iii) complex with shorter Co-N bonds initially... 

If this complex now collapses, it will be the labile Co-Cl bond which is broken, as opposed to the inert Cr-Cl bond. The labile cobalt(ii) complex reacts further with bulk water to generate [Co(H20)6] (Eq. 9.37). The key feature is that a necessary consequence of this inner-sphere reaction is the transfer of the bridging ligand from one center to the other. This is not a necessary consequence of all such reactions, but is a result of our choosing a pair of reactants which each change between inert and labile configurations. In the reaction described above, the chloride... 

Cobalt atom reactions with ethylene were also studied (121). By using techniques similar to those described for Cu (122) and Ni (101), it has proved possible to synthesize a novel series of mononuclear and binuclear cobalt-ethylene complexes, Co(C2H4) , = 1, or 2, and... 

The alleged preparation of the supposed cobalt(II) complex Na[Co(Et2dtc)3] described by D Ascenzo and Wendlandt (305) has been repeated by Holah and Murphy (306), who identified the product as [Co(Et2dtc)3]. Complexes of cobalt(III), nickel(II), and palladium(II) salts with cationic, dithiocarbamate ligands have been synthesized (307). Reaction of the secondary amine (Et2N(CH2)2)2NH with CS2 produces... 

Cobalt(III) complexes containing mixed chelating ligands have been produced. Reaction of potassium bis[biuretocobaltate(III)], K2[Co(bi)2] with R2dtc or Rxant at 0° produces (313) the blue-violet [Co(bi)2(S—S)] ion (S—S = R2dtc or RXant). If the reaction is performed above 0° in the presence of water, the products are [Co(bi)2(S—S>2] and biuret. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

Cobalt carbonyl complexes involving InBr form when InBr inserts into the Co—Co bond in CojtCoJg ... 

Very recently, a series of trihalomethyl cobalt porphyrin complexes Co(OEP)CX was prepared either by the reaction of Co(OEP) with CBrCl, CBr4 or CI4, or by the reaction of fCo(OEP)] with CCI4 or CB14. The dihalomethyl complexes, formed in small amounts in these reactions, were prepared in larger amounts from lCo(OEP)P with In a similar fashion, R SnH reacted with Co(OEP)CH ,... 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

Because of structural similarities to the vitamin B,2 coenzyme, cobalt(III) complexes of the type RCo(L4) or RCo(L2)2 [L4 = bis(salicylaldehyde)-ethylenediiminate and L2 = dimethylglyoximate, inter alia] have been actively investigated 40, 76, 77, 125). Corresponding acyl complexes have been synthesized 40, 76). However, neither the CO insertion into the Co—-R linkage nor the decarbonylation of the Co—COR moiety has been achieved (77, 125). A probable reason for this was presented in Section II. 

By studying the NMR spectra of the products, Jensen and co-workers were able to establish that the alkylation of (the presumed) [Co (DMG)2py] in methanol by cyclohexene oxide and by various substituted cyclohexyl bromides and tosylates occurred primarily with inversion of configuration at carbon i.e., by an 8 2 mechanism. A small amount of a second isomer, which must have been formed by another minor pathway, was observed in one case (95). Both the alkylation of [Co (DMG)2py] by asymmetric epoxides 129, 142) and the reduction of epoxides to alcohols by cobalt cyanide complexes 105, 103) show preferential formation of one isomer. In addition, the ratio of ketone to alcohol obtained in the reaction of epoxides with [Co(CN)5H] increases with pH and this has been ascribed to differing reactions with the hydride (reduction to alcohol) and Co(I) (isomerization to ketone) 103) (see also Section VII,C). 

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