Cobalt complexes hydroxy acids

For more recent examples involving induced electron transfer linked to cobalt(III) complexes of a-hydroxy acids and hydroquinone esters see Refs. 110, 111. 

With cobalt complex catalysts, in polar, aprotic solvents like DME it is often possible to get a-keto acids by controlled double carbonylation874-877. Alternatively, a-hydroxy acids are formed when benzyl halides are carbonylated in the presence of calcium hydroxide, in aqueous media878. Presumably the initially formed a-keto acid is reduced in the Meerwein-Ponndorf fashion to give the a-hydroxy group878. 

Acid dyes include metal-complexed azo structures, where the metals used are cobalt, chromium, and iron.10 Examples are 1 1 and 2 3 chromium complexes and 1 2 cobalt complexes, where the numbers employed represent the ratio of metal atoms to dye molecules. Metal-complexed dyes can be formed inside textile fibers by treating suitably dyed fibers with a solution containing metal ions.11 In this case, the metal-free forms of these azo dyes are known as mordant dyes and contain mainly ortho, ortho -bis-hydroxy or ortho-carboxy, ortho -hydroxy groups (e.g., C.I. Mordant Black 11, Mordant Yellow 8, and Mordant Orange 6). When the metal complexes are formed prior to the dye application process, the resultant dyes are known as... 

The tris(Ar-hydroxethyIethylenediamine)cobalt(III) chloride that was used in these reactions was reported to be an orange crystalline solid (111). In a subsequent investigation (69), attempts to prepare this compound by the air oxidation of a mixture of cobalt (II) and the amine failed. The compound was, however, prepared by the displacement of ammonia from [Co(NH3)e]Cl3 by iV-hydroxyethylethylenediamine and a dark red compound was obtained. Attempts to react the hydroxy groups in this red complex with a variety of reagents (nitric acid, thionyl chloride, benzoyl chloride, and acetyl chloride) were as unsuccessful as the previously reported attempts to react the hydroxy groups in the orange cobalt complex. 

Despite being studied several decades ago, cobaloxime -n-cations have seen modest use in organic synthesis. The cations can be generated in situ by treatment of p-hydroxy or p-acetoxy cobaloxime complexes 6349 with acid (Scheme 6.129). Although these reactions seem to resemble those of the Fp complexes, structurally, these cobalt complexes are not straightforward coordinated alkenes. Low-temperature C NMR studies have... 

Kinetic parameters k, often also and AS, occasionally AV ) for formation and dissociation of several pentacyanoferrate(II) complexes [Fe(CN)5L]" have been established. Ligands L include several S- and A-donor heterocycles,4-methyl- and 4-amino-pyridines, a series of alkylamines, 3- and 4-hydroxy- and 3- and 4-methoxy-pyridines, several amino acids, nicotinamide, " 4-pyridine aldoxime, 3-Me and 3-Ph sydnones, several bis-pyridine ligands,neutral, protonated, and methylated 4,4 -bipyridyl, 1,2-bis(4-pyridyl)ethane and traTO-l,2-bis0-pyridyl)ethene, pyrazine- 4,4 -bipyridyl- and bis(4-pyridyl)ethyne-pentaammine-cobalt(III), edta-ruthenium(III), and pentaammineruthenium-(II)and-(III) complexes of... 

Metal Complexation. Azo dyes containing hydroxy or carboxylic acid gronp substituents adjacent to the azo gronp react with transition metal ions, e.g. chromium, cobalt and copper to produce complexes, e.g. Cl Acid Violet 78 (2.15)7 These metal complex dyes are more stable to light than their unmetallised precursors and have been widely nsed as dyes for polyamide and wool fibres. However, there is now a move away from chrominm complexes due to toxicity concerns (see section 2.3.2.). 

Quinaldohydroxamic acid as metal precipitant, 506 Quinodimethane, tetracyano-metal complexes, 263 Quinoline, 8-(alkylthio)-metal complexes, 801 Quinoline, 8-hydroxy-metal complexes, 795 Quinoline, 8-hydroxy-2-methyl-cobalt(II) complexes, 795 Quinoline, 8-mercapto-metal complexes, 800 Quinoline, 8-mercapto-2-methyl-metal complexes, 800 Quinoline, 2-methyl-8-(methylthio)-metal complexes, 801 Quinoline, 8-methylthio-metal complexes, 801 8-Quinolineselenol metal complexes, 807 Quinones... 

Two mechanisms of cobalt(III)-mediated peptide-bond cleavage have been investigated. The first one involves hydrolysis of a directly activated amino acid ester, or peptide (equation 4). The other mechanism involves the intramolecular attack of an amino acid ester or peptide by a cis coordinated hydroxide or water molecule (equation 5). In both cases, the cobalt(III) complex must have two open coordination sites cis to each other. For the directly activated mechanism, these sites are needed to bind the amino acid ester or peptide. The intramolecular reaction requires one site for coordination of the ester or peptide, and one site for the coordination of the hydroxy or water molecnle. One of the initial cobalt(III) complexes to be investigated was... 

Finally, mention may be made of those articles in which this method is utilized in the synthesis of optically active oxiranes for example, the simple synthesis of monosubstituted (S)-oxiranes and the asymmetric cyclization of some chlorohydrins catalyzed by optically active cobalt (salen)-type complexes, or in the enantiomeric selection of racemic oxiranes via halohydrins and /3-hydroxy sulfides. A useful three-step synthesis has been worked out from (S)-amino acids to (R)-alkyloxiranes as well as enantiomer resolution for chiral oxiranes by complexation gas chromatography. ... 

Copper complexes (152) of o-hydroxy-o -(p-aminoethylamino)diarylazo compounds have been prepared ° by the reaction of the corresponding o-chloro-o -hydroxydiarylazo compounds and ethylenediamine in the presence of copper(II) ions (c/. Section 58.2.3. l(iii)(c)). Dyestuffs of this type have been evaluated on nylon but are reported to have very poor fastness properties. Copper complexes such as (153) have been preparedby similar methods. Other, related tetradentate diarylazo compounds, e.g. (154), (155) and (156), are obtained by the reaction of a suitable diazonium salt with the appropriate diarylamine. Chromium(III) and cobalt(III) complexes of dyes of this type, in which the coordination sphere of the metal is completed by a colourless bidentate ligand such as ethylenediamine, salicylic acid or 8-hydroxyquinoline, are reported to have dyeing properties on wool similar to those of the comparably charged 2 1 complexes derived from tri-... 

This is, in fact, a particular example of a general class of reactions in which organocobalt complexes with )8-oxy substituents (including 2-hydroxy-, 2-alkoxy-and 2-alkyloxyalkyl-cobalts) [52,91-93] are decomposed in acid via elimination of aquocobalt complexes (Eqn. 55). 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

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. 

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