Racemization cobalt complexes

Jacobi and co-workers have applied the above Schreiber/Evans chiral boron enolate methodology to afford stereoselective routes to precursors of biologically important tetrapyr-roles [187], pyrromethanenones (114) (Scheme 4-59) [188], phycocyanin and phytochrome precursors, and P-amino acids [189], versatile intermediates for P-lactams of the carbapenem class. Generally, reaction of achiral or matched enolates with racemic cobalt complexes gave excellent selectivity. With a careful choice of mis-matched chiral enolate, moderate to good anti selectivity could also be achieved, leading to a formal total synthesis of thienamycin [190]. 

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. 

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). 

A salen-cobalt complex has been appended to the PASSflow monolith system to form catalyst 42 and used for the dynamic kinetic resolution of epibromohydrin, 43. Because 43 undergoes rapid racemization under the conditions used, all the starting materials can theoretically be converted to the desired diol 44 (Scheme 4.75). The... 

The first report regarding quenching of DNA-bound metal complexes was of enantiomeric ruthenium(II)tris-diammine complexes by racemic cobalt(III)tris-... 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

All the cobalt complexes of this ligand show bands near 3200 cm in their infrared spectra assignable to the N—H modes. A wide variety of co-balt(III) complexes have been prepared and characterized with this saturated ligand with the analogous racemic ligand. ... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization of potassium diaquodioxalatochromium(III) and the racemization of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

In 1995, Morgan et al. synthesized a layered aluminophosphate compound by using a chiral cobaltammine complex as the template for the first time.[61] Recently, the Jilin group has synthesized a number of 2-D layered and 3-D open-framework metal phosphates by using a racemic mixture or an optically pure chiral metal complex as the template, and has systematically studied the chirality transfer from the guest chiral complex templates to the host inorganic open frameworks.1901 Table 7.15 lists some metal phosphates and oxides with open-framework structures templated by optically pure or racemic cobalt ammine complexes. 

In the early belief that these reactions were completely stereospecific, we felt that the reaction of [Co(1-pn)2CI2]+ with carbonate could not give an optical inversion under any experimental conditions. This is, of course, not true, as was soon shown by the work of McReynolds (28) and Sister Mary Martinette Hagan (29). In a cobalt complex containing asymmetric ligands, one form is preferred, but both forms can and do exist. In the same belief, we hoped that stereospecific effects could be used in the resolution of racemic potential ligands. This does give partial resolution, as shown by the work of Johnson (30),... 

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

A series of papers have also reported the coupling of alkyl and aryl electrophiles with aryl Grignard reagents catalyzed by iron (Equation 19.15) and cobalt complexes. These reactions build upon Kochi s and Molander s early results on coupling reactions catalyzed by complexes of these metals. The recent reactions have been conducted with simple metal salts in many cases and with discrete metal complexes as catalyst precursors in others. Although little recent mechanistic data is available on these reactions, they have earlier been shown to involve radical intermediates. Kochi concluded that the catalytic process occurs by an Fe(I)-Fe(III) couple reactions of optically active alkyl halides generate racemic coupled products and reactions of diastereomerically pure aUcyl halides generate equal ratios of diastereomeric products, as depicted in Equations 19.16 and 19.17. ... 

The Salen motif has been widely utilized as a ligand for transition metals. Jacobsen et al. reported that chiral salen-cobalt complex (Co-salen) could be utilized as a Lewis acid catalyst for hydrolytic kinetic optical resolution of racemic... 

Cobalt(iii) Complexes.—Two kinetic studies of cis-trans isomerization of cobalt(iii) complexes of simple ligands have been described, for the complexes c/j -[Co(en)2(glycinenitrile)Cl] + and [Co(en)2(OH2)(N3)] +. Possible mechanisms of isomerization and racemization for complexes containing flexible quadridentate ligands have been discussed in general terms, for amino-acid complexes [Co(trien)(LL)] +, and for the specific case of the complex [Co(trien)(N-methyl-S -alaninato)] +. The rearrangement of the a -R- to the aj8-5-form of [Co(tetren)(NCS)] + follows the rate law... 

In 2005, Peretti and co-workers reported a highly active and isoselective (TOF = 220h mm >99%) cobalt complex, [(salph- Bu)CoOAc] 19 (Scheme 18(a)) for the polymerization of racemic propylene oxide. This is the first example of highly isotactic poly(propylene oxide) generation from racemic propylene oxide without concomitant atactic byproduct. 1-Butene... 

The phosphitylation of D-mannose with phosphorous triazolides has afforded primarily the mannofuranose 2,3,6-O-phosphite 12. The complexes formed separately between o-fructose and L-sorbose with 1,10-phenanthroline and Co(III) ions have been characterised, while composites prepared by the reaction of simple sugars with tetraethoxysilane and water have been shown to be capable of resolving racemic cobalt and chromium ion complexes. Sodium alkoxide salts of free sugars have been treated with ferric chloride to give iron-sugar complexes and five-fold deprotonated D-mannose forms dinuclear metalates [X2(P-D-Man-/)2] of trivalent (X = Fe, V, Cr and Al) with O-1,2,3,5 and 6 involved in complexation. ... 

In 2010, a cooperative dual-catalyst system was reported to promote the highly enantioselective fluoride ring opening of various meso epoxides having alkene, ester, and protected amine functionalities. The reactions were conducted with a chiral (Salen)cobalt complex, (-)-tetramisole, benzoyl fluoride as a latent source of fluoride in the presence of HFIP. The efficient catalytic enantioselective reaction is explained by the generation of a (Salen)Co(III) fluoride under the cocatalytic conditions that occurred in good yields with up to 95% ee (Scheme 44.32). Racemic terminal epoxides, such as styrene oxide, were also studied, but they almost exclusively lead to the fluorine in the primary position therefore, the fluorine atom was not introduced on a stereogenic center. 

A discovery that extends the scope of asymmetric epoxidation reactions is the hydrolysis of a racemic mixture of epoxides, using the cobalt complex 8.36 as a chiral precatalyst. Under the catalytic conditions, in the presence of air and small amounts of acetic acid, 8.36 is converted to a Co -containing active catalytic intermediate with acetate and water present as additional ligands. 

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