Cobalt complexes, with Subject

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Gibson and co-workers have introduced a well-designed latent cyclobutadiene moiety. Compound 49 reacts with a phenylethyne-cobalt complex to give PKR product 50 as a single diastereomer that resulted from the reaction at the less sterically hindered site, and 50 was subjected to the retro-Diels-Alder reaction at 205 °G under a vacuum of 6 torr to give 51 (Equation (23)). ... 

The cobalt(II) complexes which undergo spin equilibrium are of several different types. Octahedral high-spin complexes with a T ground state are subject to Jahn-Teller distortion in the low-spin d1 2E state. This effect is best documented in structures of the Co(terpy)22+ spin-equilibrium complex. The high-spin isomer is nearly octahedral, with a difference in Co N bond lengths between the central and distal nitrogens of only 6 pm. In the Jahn-Teller distorted low-spin state this difference has increased to 21 pm (58). 

In addition, such a method of synthesis can be generated by making use of another destructive means for ligand synthesis. In that sense, the method was verified by achieving some polycondensation products of polyethylene terephthalate and ethylenediamine and their complexation with different metals by irradiating the system with cobalt-60 (gamma rays). These results also make up the subject of other studies. 

In the synthesis of the 1 1 cobalt complex 17 by the nitrite method, a mixture of CoS04-7 H20 and NaN02 in water and the dye formed by acid coupling of diazotized 2-amino-4,6-dinitrophenol with 2-phenylaminonaphthalene-4 -sulfonic acid is subjected to dropwise addition of dimethylformamide. Metallization commences immediately and is complete within 2 h. The 1 1 cobalt complex 17 is precipitated by addition of NaCl and isolated. It is a suitable intermediate for producing unsymmetrical 1 2 cobalt complex dyes such as 18. 

Very recently Sutherland and coworkers merged the Pd-catalyzed Overman rearrangement with RCM and ATRC (Fig. 44) [256]. Treatment of dienols 179 with trichloroacetonitrile in the presence of 10 mol% Pd(MeCN)2Cl2 afforded the non-isolated trichloroacetimidate 180, which underwent the [3,3] rearrangement to /V-dienyltrichloroacetamides 174. They were immediately subjected to the RCM/ ATRC sequence using 10 mol% of 145. When chiral cobalt complex (S)- or (R)-181 was used as the catalyst for the Overman rearrangement, indolinones 177 were isolated in 51-70% yields and 89-94% ee. 

Picolinic acid (pyridine 2-carboxylic acid) complexes of chromium(III) have been the subject of a number of studies. Complexation by picolinic acid in water/ethanol (30% v/v) follows an ion-pairing, Eigen-Wilkins type mechanism.Activation parameters suggest an associative character for the reaction of the aqua complex. Chelated complexes of chromiuni(ni) and picolinic acid are the products of the rapid, inner-sphere reduction of [Co (pico)(NH3)5p with chromium(II). The reaction of the related 4-carboxylic acid complex of cobalt(III) with chromium(II) is also rapid in contrast, pyridine-3-carboxylic acid (nicotinic acid) complexes undergo slower reactions. A -hydroxy-bridged dimeric complex [Cr2(pico)4(OH)2] has also been prepared. A study of magnetic properties in the temperature range 16-300 K leads to J - —6 cm and g = 2, typical for such complexes. 

Werner concentrated on the complexes of cobalt, chromium and platinum, with coordination numbers four and six. More recent studies have involved most metals of the periodic table, and complexes with coordination numbers from two to twelve or even higher have been investigated. An additional spur to the recent rapid development of coordination chemistry has been the realisation of the important role played by complexes in catalysis and in biological processes. All the metals in the periodic table from vanadium to zinc, as well as several others such as molybdenum and magnesium, are involved in life processes. A thriving new branch of the subject, bioinorganic chemistry, has developed to study these complexes. 

The [Co(CN)5] -catalysed hydrogenation of butadiene is thought to proceed via a butenyl cobalt complex the structure of which depends on the concentration of CN . At high values of [CN ] equal amounts of cis- and trans-forms (50) and (51) are present but on reduction of [CN ] the jy -7i-complex (52) is favoured. These results correlate with the observations that but-l-ene is the principal hydrogenated product at high values of [CN ] but at low values /ra s-but-2-ene is obtained preferentially. The mechanism of formation of [Co(CN)6(H)] , the precursor to butenyl complexes (50), (51), and (52), is the subject of computational studies. ... 

Oxirans.—A simple four-stage preparation of (5)-propylene oxide from ethyl L-( —)-maleate has been described (Scheme 2). This work is of importance for the synthesis of nonactin carboxylic acid. Another synthesis of optically-active propylene oxide involves the cyclization of OL-propylene chlorohydrin with a variety of bases in the presence of a cobalt complex the highest optical purity was 27%. Wynberg and co-workers have shown that the base-catalysed epoxidation of electron-poor alkenes is subject to catalytic asymmetric induction hydrogen peroxide and t-butyl hydroperoxide were used as oxidants in the presence of quaternary... 

Remarkably, the dianion 136 has been known for longer than the electroneutral 135, and some metallocene like complexes such as 139 and 140 have been prepared from 136 by reaction with nickel(ll) or cobalt(ll) chlorides [157, 158]. Complexes of this kind have recently been the subject of extensive theoretical calculations [159-161]. There are also complexes with the metal atoms on either side of the pentalene hgand, for example, the heterobimetalhc complex 141 (Scheme 10.49) [162]. Furthermore, such bimetallic and related pentalene... 

Gibson et al. have used polymer-bound cobalt complexes for immobilizing alkynes [27]. Scheme 5 demonstrates the indirect loading approach, meaning that the alkyne-cobalt complex is formed prior to the attachment onto soUd phase. The alcohol moiety of 7a/7b, formed as a mixture, can then be subjected to a munber of transformations, and the alkyne is subsequently released from the polymer by aerial oxidation. The direct loading approach is also described, where the cobalt complex is formed on soUd phase before the alkyne complexation step. This approach gives a somewhat lower substrate loading, but the subsequent steps proceed with almost the same efficiency in both cases. 

Perfluoroalkyl or -aryl halides undergo oxidative addition with metal vapors to form nonsolvated fluonnated organometallic halides and this topic has been die subject of a review [289] Pentafluorophenyl halides react with Rieke nickel, cobalt, and iron to give bispentafluorophenylmetal compounds, which can be isolated in good yields as liquid complexes [290] Rieke nickel can also be used to promote the reaction of pentafluorophenyl halides with acid halides [297] (equation 193)... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

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