Cobalt complexes amides

Iluxional cobalt complex, 25 70 of cobalt complexes, 12 281-288 of cobalt(lll) complexes, 12 389-391 " Cs NMR spectra, of cesium amide, 25 247... 

The products isolated from reactions of amides with transition metal halides usually contain coordinated halide (e.g. the formulations in Table 2). In some cases such as [Co(NMF)6][CoCLt], halide and amide are coordinated to different metal atoms, but when such compounds are dissolved in the neat ligand, halide can be replaced and at high dilution all the metal ions may be fully coordinated by the amide alone. The electronic spectrum resulting when this cobalt complex is dissolved in nitromethane has been interpreted as relating solely to the tetrahedral complex [CoC12(NMF)2]. 

Co(OAc)2 in the presence of sodium hydride and a sodium alkoxide has been used to catalyze the carbonylation of aryl bromides, giving mixtures of carboxylic acids and esters, again at normal pressure. When amines were present, amides were formed. Unfortunately, nothing is known about the nature of the cobalt complexes involved. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Co(in) complexes promote similar reactions. When four of the six octahedral positions are occupied by amine ligands and two cis positions are available for further reactions, it is possible to study not only the hydrolysis itself, but the steric preferences of the complexes. In general, these compounds catalyze the hydrolysis of N-terminal amino acids from peptides, and the amino acid that is removed remains as part of the complex. The reactions apparently proceed by coordination of the free amine to cobalt, followed either by coordination of the carbonyl to cobalt and subsequent reaction with OH or H2O from the solution (path A in Figure 12-15) or reaction of the carbonyl carbon with coordinated hydroxide (path B). As a result, the N-terminal amino acid is removed from the peptide and left as part of the cobalt complex in which the a-amino nitrogen and the carbonyl oxygen are bonded to the cobalt. Esters and amides are also hydrolyzed by the same mechanism, with the relative importance of the two pathways dependent on the specific compoimds used. 

Chlorodiphenylacetonitrile formed tetraphenylsuccinonitrile as well as a diflFerent type of complex. The infrared spectrum of the latter contained no nitrile absorption band (2210 cm." ) but exhibited a cyanide stretch at 2130 cm. S characteristic of inorganic pentacyanocobaltate(III) complexes 14,22), and a carbonyl band at 1575 cm. Its PMR spectrum indicated the presence of the (C6H5).)CH group. Diphenylacetamide precipitated on heating an aqueous solution of the complex. Apparently the resonance-stabilized radical initially formed in the reaction of this a-halonitrile with pentacyanocobaltate(II) may either dimerize or combine with pentacyanocobaltate(II) to form an N-keteniminocobalt complex. The latter is unstable in water, being converted to an N-amido-cobalt complex, which may be further hydrolyzed to the free amide (Reaction 23, paths d and e). Presumably the radical cannot add penta-... 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

PC8HM, Phosphine, dimethylphenyl-, 22 133 iridium complex, 21 97 PC 2H27, Phosphine, tributyl-, chromium complexes, 23 38 PC18H1S, Phosphine, triphenyl-, 21 78 23 38 cobalt complexes, 23 24-25 cobalt, iridium, and rhodium complexes, 22 171, 173, 174 iridium complex, 21 104 palladium complex, 22 169 palladium and platinum complexes, 21 10 ruthenium complex, 21 29 PNOC 2Hl2, Phosphinic amide, diphenyl-, lanthanoid complexes, 23 180 PNAH.2, Propionitrilc, 3,3, 3 -phosphinidy-netri-,... 

Cobalt complexes, 635-882 ADP, 760 amides, 682 arsenates, 774 arsenic ligands, 767-775 ATP, 760 bipyridyl, 691 bis(dithiolates), 876 carboxylates, 790 cyanates, 679 cyanides reduction, 646 disulfides, 829... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

The heptanuclear iron carbonyl cluster [Fe3(CO)u(/u-H)]2-Fe(DMF)4 (178) acted as an efficient catalyst in the reduction of carboxamides by l,2-bis(dimethylsilyl)benzene in toluene to the corresponding amines in high yields. Several tertiary and secondary amides including a sterically crowded amide were also reduced smoothly A review of the development of optically active cobalt complex catalysts for enan-tioselective synthetic reactions has addressed the applications of ketoiminatocobalt(II) complexes such as (5)-MPAC (179) and (5)-AMAC (180), transition-state models for borohydride reduction, halogen-free reduction by cobalt-carbene complexes. 

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