Dimethylglyoximato cobalt

The reactions are of the first-order in each reactant and, for example, k (25 °C) for the reaction with pyridinatobis(dimethylglyoximato)cobalt(II) in benzene is 0.30 l.mole . sec. k increases slightly with the basicity of B but is relatively insensitive to changes in DH. A radical mechanism identical with equations (110, 111) is proposed " . 

Cages derived from oxime precursors. In the initial synthesis of a cage (Boston Rose, 1968), tris(dimethylglyoximato)cobalt(m) [where dimethylglyoximato (dmgH) = (147)] was reacted with boron trifluoride... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

The preparation of oximes from olefins is a valuable approach for the synthesis of nitrogen-containing compounds such as amino acids and heterocycles. Okamoto and colleagues have reported that a catalytic reduction-nitrosation of styrenes 31 with ethyl nitrite and tetrahydroborate anion by the use of bis(dimethylglyoximato)cobalt(II) complex afford the corresponding acetophenone oximes 32 (Scheme 23). 

Since bromo(pyridine)cobaloxime(III) was not commerically available and its synthesis was not convenient36, we utilized chloro(pyridine)bis(dimethylglyoximato)-cobalt(III) (Equation 3) (also known as chloro(pyridine)cobaloxime (III)) instead. It has four cathodic waves in polarography when observed in acetonitrile. Its half wave potentials are located at -0.65, -1.45, -2.42, and -2.92 volts vs the Ag/AgNC>3 electrode, corresponding to the reduction of the cobalt from +3 to +2, +1, and 0, and the reduction of the ligand, respectively. 

More successful asymmetric reductions have been based on amine (particularly alkaloid) complexes of bis(dimethylglyoximato) cobalt(II), also known as cobaloxime(II) and represented Co(dmg)2 (compound VII). Cobaloxime-chiral amine complexes have been used to catalyze the hydrogenation of both olefinic and ketonic substrates (Fig. 24). It has been determined that hydroxyamine modifiers, for example, alkaloids such as quinine, quinidine, and cinchonidine, are most effective. The highest optical purity obtained thus far has been 71%, observed for reduction of benzil in benzene solution at 10° using quinine as the... 

Further results on asymmetric hydrogenations of activated carbonyl compounds catalyzed by bis(dimethylglyoximato) cobalt (Il)-chiral amine complexes have been reported (55,56). Some chiral reductive dimerizations were observed (55). 

A number of these trans compounds have been prepared by Ablov and his co-workers 4-6, 9, 11) who have recently described ci5-[Co-(DH)2H20(NCS)] (5), whereas the previous discussion shows that S-bonding predominates in the trans series unless modified by solvent effects. Although no evidence was observed for the N-bonded isomer in the study of a frequency which is difficult to observe in these compounds due to ligand vibrations. The difficulties in determining the coordination of NCS in bis(dimethylglyoximato)-cobalt III) compounds are illustrated by the report of hydrolysis studies of twenty-six new salts of the bisthiocyanate anion in which the results were discussed on the basis of N-bonding 284) in spite of the available X-ray evidence to the contrary 649). 

Bis (dimethylglyoximato) cobalt complexes are of interest as models of vitamin Bi2. The cobalt atom in the square planar ligand field of the four sp -hybridized nitrogen atoms of dimethyl-glyoxime has a pronounced tendency to form stable organo-cobalt derivatives, in analogy to the cobalt atom in the... 

The complexity of vitamin B12 led to the preparation and investigation of model compounds, of which derivatives of bis(dimethylglyoximato)-cobalt(III), termed cobaloximes, have been the most extensively studied (29, 235-237). These derivatives, especially methylaquocobaloxime, have helped cast considerable light on the mechanisms involved in the biological action of vitamin B g. 

This reaction is very rapid and produces air-stable, crystalline i -alkyliron complexes in excellent yields from a variety of substituted i/ -allyliron complexes. Other electrophilic olefins, including dichlorodicyanoquinone, dimethyl methylene malonate, and / ,/ -dicyano-o chlorpstyrene undergo similar cycloaddition reactions with // -allylFp complexes. jy vAllyl complexes of j/ -CsHjWCCOyj, j -C5H5Mo(CO)3, i/ -CjHj-Cr(NO)2 and (pyridine)bis(dimethylglyoximato)cobalt react in a similar fashion but are much less thoroughly studied . 

Fig. 2. Examples of cobalt chelates capable of stabilizing carbon-cobalt bonds. For additional examples see ref. 40. A, bis(dimethylglyoximato)cobalt = cobaloxime, CofDjHj) B, bis(acetylacetone)ethyIenedi-iminecobalt, Co(BAE) C, bis(salicylaldehyde)ethylenediiminecobalt, Co(SALEN) D, tetraphenyl-porphyrincobalt, Co(TPP) E, bis(salicylaldehyde)-o-phenylenediimit, ecobalt, CofSALOPH) F, oc-tamethylporphyrincobalt, Co(OMP).

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