Cobalt, racemization

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-... 

Cobalt, aquachlorobis(l, 2-ethanediamine)-racemization solid state, 1,467 Cobalt, aquahalotctraammine-di halide... 

Cobalt, R-a-cyanoethylbis(dimethylglyoximato)-racemization, 1,470 Cobalt, diacetatobis(methylimidazolyl)-angular parameters, 1, 59 Cobalt, diamminebis(dimethylglyoxime)-decomposition, 1,186 Cobalt, diamminediaquadinitroso-structure, 1,26... 

Cobalt, tris(l,2-ethanediamine)-conformation, 1,25,197 polarography, 1,481 racemization, 1, 466 solid state, 1,466,467 reactions, 1, 27 redox potential, 1, 514 structure, 1, 67... 

Cobalt, tris(oxalato)-racemization solid state, 1, 467 structure, 1, 68... 

The cobalt(II) complex, which is optically unstable, was formed in reaction mixtures by the addition of ethylenediamine (excess) to Co . The rate law obtained from the racemization data was... 

Molecules having only a sulfoxide function and no other acidic or basic site have been resolved through the intermediacy of metal complex formation. In 1934 Backer and Keuning resolved the cobalt complex of sulfoxide 5 using d-camphorsulfonic acid. More recently Cope and Caress applied the same technique to the resolution of ethyl p-tolyl sulfoxide (6). Sulfoxide 6 and optically active 1-phenylethylamine were used to form diastereomeric complexes i.e., (-1-)- and ( —)-trans-dichloro(ethyl p-tolyl sulfoxide) (1-phenylethylamine) platinum(II). Both enantiomers of 6 were obtained in optically pure form. Diastereomeric platinum complexes formed from racemic methyl phenyl (and three para-substituted phenyl) sulfoxides and d-N, N-dimethyl phenylglycine have been separated chromatographically on an analytical column L A nonaromatic example, cyclohexyl methyl sulfoxide, did not resolve. 

Racemic threo 57>-[methylphenyl(2-phenylpropyl)stannyl]tricarbonyl(triphenylphos-phine) cobalt (76) reacts with NaFe(CO)2Cp to give the same 45/55 mixture of [methylphenyl(2-phenyIpropyl)stannyl]dicarbonylcyclopentadienyl irons (77)J(77)B... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

The amidocarbonylation of aldehydes provides highly efficient access to N-acyl a-amino acid derivatives by the reaction of the ubiquitous and cheap starting materials aldehyde, amide, and carbon monoxide under transition metal-catalysis [1,2]. Wakamatsu serendipitously discovered this reaction when observing the formation of amino acid derivatives as by-products in the cobalt-catalyzed oxo reaction of acrylonitrile [3-5]. The reaction was further elaborated to an efficient cobalt- or palladium-catalyzed one-step synthesis of racemic N-acyl a-amino acids [6-8] (Scheme 1). Besides the range of direct applications, such as pharmaceuticals and detergents, racemic N-acetyl a-amino acids are important intermediates in the synthesis of enantiomeri-cally pure a-amino acids via enzymatic hydrolysis [9]. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

The cobalt complex is cleaved by Cl2/PPh3 with complete racemization, whereas the iron complexes may be cleaved with retention, inversion or racemization, depending on the electrophile and the substrate (Table 4). 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R. Ahmad, I. Bhatt, A. Jasra R. V. (2005) Improved catalytic activity of homochiral dimeric cobalt salen complex in hydrolytie kinetic resolution of terminal racemic epoxides.. Chirality, 17 590-594. 

Several other cobalt(III) oxalate photoreductions have been reported. Qualitative observations of both reduction and racemization... 

In the realm of hydrolytic reactions, Jacobsen has applied his work with chiral salen complexes to advantage for the kinetic resolution of racemic epoxides. For example, the cobalt salen catalyst 59 gave the chiral bromohydrin 61 in excellent ee (>99%) and good yield (74%) from the racemic bromo-epoxide 60. The higher than 50% yield, unusual for a kinetic resolution, is attributed to a bromide-induced dynamic equilibrium with the dibromo alcohol 62, which allows for conversion of unused substrate into the active enantiomer <99JA6086>. Even the recalcitrant 2,2-disubstituted epoxides e.g., 64) succumbed to smooth kinetic resolution upon treatment with... 

B) Deviation from the expected 1 1 ratio for a racemate (second peak diminished) upon enantiomer separation of 2-(chloromcthvl)oxirane on cobalt(Il) bis[3-(heplafluorobutanoyl)-(l/ )-camphorate] by complexation gas chromatography at 60 =C. 

Each of the compounds mentioned, therefore, should exist in a racemic form and two optically active forms. After Werner had succeeded in isolating optically active forms of the corresponding cobalt compound, he attempted to isolate, by the same means, isomers of the chromic series. Optically active tartaric acid, chloro- or bromo-tartarie acid, eamphor-sulphonic acid, and brom-eamphor-sulphonic acid proved unsatisfactory, since the aqueous solutions change on evapora-... 

Both the racemic salts and the optically active salts appear to be isomorphous with the corresponding cobalt salts when examined CTystallographically.1... 

It is concluded that mechanism B is unlikely, since no total racemization was found in the reactions of chromium (III) and cobalt (III) acetylacetonates with a variety of electrophiles, and that these reactions occur in the same manner as in an aromatic system. 

The partially resolved cobalt acetylacetonate was found to be optically stable in solution or in the solid state for long periods. However, slow crystallization of this substance always produced racemic crystals (14). Several of the optically active substituted cobalt chelates exhibited the same strange phenomenon. Removal of the solvent from solutions of optically active cobalt acetylacetonate with a slow stream of air yielded a solid which showed little apparent crystalline character under a polarizing microscope but dissolved to form a solution of about the same specific rotation as the starting solution. 

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