Cobalt carboxylate complexes

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

When the cobalt salt of carboxylic acid and bromide ion are dissolved in acetic acid, a cobalt bromide complex is formed instantaneously. For cobalt dibromide a pronounced induction period was observed, but adding sodium acetate eliminates entirely the induction period, suggesting that cobalt monobromide is responsible for the catalysis. 

Intramolecular lactonization has been studied on cobalt(III) complexes (Scheme 44).144 The reaction is catalyzed by general acid and the attack of coordinated water occurs at a greater rate than that of coordinated hydroxide ion. Presumably, the relatively non-nucleophilic water is assisted by hydrogen bonding to the carboxyl group to become a pseudo-hydroxide ion. 

The mechanism of the reaction of the alcohol (or water) with the acyl complex to produce ester (or acid) and regenerate the cobalt hydride complex is not known. Because the reaction of the analogous manganese complex with alcohols is known to proceed through a hemiacetal-like complex, this mechanism has been written for the carboxylation reaction (equation 42). 

The formation of hydrated cobalt(n) complexes of pyridine carboxylic acids and the subsequent thermal decomposition to lower hydrates has been documented.82,83 Cobalt(n) halides react with 6-methylpicolinic acid (6-mpaH), picolinic acid (paH), nicotinic acid (naH), and pyridine-2,6-dicarboxylic acid (2,6-py) to form Co(6-mpa) (6-mpaH)X (X = Cl, Br, or NCS), Co(naH)nX2 (n = 2, X = Cl, Br n = 3, X = NCS), and Co(pa)(paH)X, EtOH (X = Cl, Br, or NCS) which are all probably octahedral.83 6-Methylpicolinic acid also formed Co(6-mpaH)4X2,2HX (X = Cl or Br) which were formulated [(6-mpaH)2H]2[CoX4], since the electronic spectra show absorptions characteristic of tetrahalogenocobaltate(n) ions.83... 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

Table 63 Preparations Recent Quadridentate and Tridentate Carboxylate-Cobalt(III) Complexes... 

TaWe 64 Preparations Some Sexidentate and Quinquedentate Carboxylate Complexes of Cobalt(III) GO O... 

The reactivity of metals in Reactions 11 and 12 can be influenced by the gegenion or by complexing agents (26-29). Generally, coordinated metals are less reactive. Thus, the cobaltic ethylenediaminetetraacetic acid complex is not reduced by hydroperoxides (30), although cobaltic carboxylates are reduced very rapidly (28, 31). Coordination of metallic catalysts has been generally employed for their deactivation (32). On the other hand, researchers (10) have reported that the coordination of tran-... 

Hydrogenation of acrylic acid esters with high enantioselectivity has usually been accomplished with difficulty. The enantioselective reduction of a,p-unsaturated carboxylates with sodium borohydride in the presence of cobalt-semicorrin complexes has been achieved in up to 96% ee (equation 14). The (Eland (Z)-isomers each afford products of opposite configuration, and the isolated double bonds remain un-touched. ... 

The ionic pair of [Co(AMMEsar)] + cation with an anthracene carboxylate anion (A-Co(III)) was used as both a photosensitizer and an ETA in the photodecomposition of water to produce hydrogen [387]. The photoreduction of encapsulated cobalt(III) ion to cobalt(II) ion occurs on excitation of anthracene chromophore (v< 25 000 cm-i). The A-Co(III) complex shows almost no fluorescence (0<2x 10 ), whereas the A-Co(II) complex produces specific violet fluorescence (Fig. 66). The cobalt(II) complex is formed in the presence of EDTA on light irradiation of the A-Co(III) solution (v> 25 641 cm i). The visible band at 21 276 cm- disappeared, and violet fluorescence was observed. The quantum yield of cobalt(II) complex formation was... 

Fig. 66. Absorption (A) and fluorescence (B) spectra of cobalt diaminosarco-phaginates...anthracene carboxylate ion pairs [387]. (1) Cobalt(IIl) complex (2) cobaltfll) complex (3) after cobalt(II) complex oxidation.

FIGURE 14,13. Mode of three-point attachment of prochiral methylaminomalonate to a chiral surface (a cobalt(III) complex), (a) Chemical formula of complex, (b) stereoview of complex, and (c) stereospecific decarboxylation. The carboxyl group that is hydrogen bonded to the complex is lost rather than the metal-bound carboxylate. 

Since a halide free, non-corrosive catalyst for DMC production would be a further process improvement, alternative catalytic systems have been investigated. Cobalt(II) complexes with N,0 ligands, such as carboxylates, acetylacetonates and Schiff bases, have been shown to produce DMC with a good reaction rate and selectivity [74]. 

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