Oxidative cleavage cobalt catalysts

Copper complexes were used as efficient catalysts for selective autoxidations of flavonols (HFLA) to the corresponding o-benzoyl salicylic acid (o-BSH) and CO in non-aqueous solvents and at elevated temperatures (124-128). The oxidative cleavage of the pyrazone ring is also catalyzed by some cobalt complexes (129-131). 

Oxidation of cobalt(ll) to cobalt(lll) by oxygen in the presence of N-hydroxyethylethylenediamine and carbon produces large amounts of ethylenediamine. Other products are formaldehyde, formic acid, and ammonia. The sum of the moles of ethylenediamine and ammonia produced is equal to the total number of moles of cobalt(ll) oxidized. A steady-state concentration of Co(ll)-Co(lll) is established in which the ratio Co(lll)/ Co(ll) = 1.207. Thus cobalt ion behaves as a true catalyst for cleavage of the N-hydroxyethyl-ethylenediamine. The total amount of cobalt(ll) oxidized per unit time, X, was calculated from the derived equation X = 3.8 + 7.0 k2 T — 3.8e-2-2k 1, where k2 = 0.65 hr.—1 The observed rate of formation of ethylenediamine plus ammonia also follows this equation. It is proposed that the cobalt ion serves as a center where a superoxide ion [derived from oxidation of cobalt-(II) by oxygen] and the ligand are brought together for reaction. 

Oxidative cleavage of open-chain ketones or alcohols is seldom a useful preparative procedure, not because these compounds do not undergo oxidation (they do, except for diaryl ketones), but because the result is generally a hopeless mixture. Aryl methyl ketones, such as acetophenone, however, are readily oxidized to aryl carboxylic acids with Re207 and 70% aqueous tert-butyl hydroperoxide. Oxygen with a mixture of manganese and cobalt catalysts give similar oxidative... 

The series of sterically crowded cobalt(II) Schiff base complexes Co(Busalen) have been investigated as catalysts for the oxidative cleavage of 3-methylindole [213,214]. The major product is... 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

The cobalt(III) initiation and catalysis pathways are very effective in many oxidations but suffer some limitations, e.g., Co " is strongly inhibited by cobalt(II) ions, which seem to form dimers with Co ". Such dimers are only weak catalysts in arene oxidations. As a result the rate of oxidations is inversely dependent on the concentration of Co " in the reaction mixture thus the cleavage of such dimers by addition of small amounts of co-catalysts will attain the reaction rate [11c, 12]. Additionally in the case of deactivated, electron-poor systems such as toluic acid or p-nitrotoluene, cobalt(III) alone is not an efficient catalyst - synergistic co-catalysts are necessary to achieve good results. 

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