Organocobalt complexes preparation

Thiapyran derivatives can be prepared from preformed organoiron and -cobalt complexes as in Scheme 160 the organocobalt complex (136) can be used as an intermediate in the synthesis of 1,2-dithia cyclopent-4-en-3-thione (See Scheme 122 in Section IV,H.)... 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

In addition to the work described here, experiments with styrenes and a,)8-unsaturated acids, esters and ketones, as well as related halides (24), suggest certain generalizations concerning the effect of structure on organocobalt complex stability and catalysis. Table IX lists several organocobalt complexes, whose preparations, either by reaction of the halide with pentacyanocobaltate(II) or by addition of hydrido complex to the corresponding unsaturated substrate, have been attempted. In many cases, both methods were used. 

Although many examples of reactions of cobalt-bound organic ligands exist, most of these have been carried out for synthetic purposes rather than as studies of mechanism or of the effect of the covalently bound cobalt moiety. Although many such transformations are quite simple, they frequently lead to organocobalt complexes which are difficult, or impossible, to obtain by direct alkylation. For example, Schrauzer and Windgassen [51] prepared carboxymethyl- and carboxyethyl-(pyri-dine)cobaloxime by hydrolysis of the corresponding methyl esters in concentrated sulfuric acid. Similarly, Brown and coworkers [58] prepared p- and w-carbo-xyphenyl(aquo)cobaloximes from their methyl esters by hydrolysis in 0.5 N KOH in aqueous methanol. 

Co(II) complexes react rapidly with free radicals but, because of the difficulty of handling radicals, most of the evidence is indirect. The radicals and Co(II) complexes can both be prepared in situ by photolysis of organocobalt(III) complexes, i.e.,... 

The experimental details are mentioned as they arise often air must be excluded from the reactions but there are many examples when this exclusion is not important (or may even be fatal ) to the reaction. A general point is that nearly all of these organocobalt(III) complexes are unstable to visible light, and so light should be excluded during the preparations at all times whenever the organocobalt(III) complex is in solution. The solid complexes are stable to visible light. 

The Preparation and Characterization of Organocobalt(III) Complexes of the General Formula [RCo" (L )X] where L4 is the equatorial ligand (Corrin, etc.) and X is the unidentate axial ligand (.S,6-dimethylbenzimidazole, HjO or absent, where L is corrin otherwise as stated in the table). 

Addition of sodium hydroxide to a mixture of [Co"(DMG)2] and an alkylating agent in methanol-water yields up to 50% of the organocobalt product. This appears to be the most convenient method for the preparation of organocobalt(III) derivatives, provided that the maximum yield is not required 161,163). Methods involving the reaction of Co with electrophiles are, at least with DMG complexes, more convenient than the main alternative route via Co(III) and Grignard reagents. 

Preparations from Other Organocobalt III) Compounds a. Addition, Exchange, or Removal of an Axial Ligand. This reaction is frequently used to prepare organocobalt(III) complexes with different ligands (usually neutral bases) in the sixth coordination position. The... 

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