Cobalt-SchifFs base complex

Bozell, J. J., Hames, B. R., and Dimmel, D. R., Cobalt-Schiff base complex-catalyzed oxidation of parasubsti-tuted phenolics - Preparation of benzoquinones. J Organic Chem 1995, 60 (8), 2398-2404. 

The precatalyst used in these water-based kinetic resolution reactions is the cobalt Schiff-base complex 9.40. Its structural similarity to the asymmetric epoxidation catalysts 9.38A and 9.38B is to be noted. In the actual catalytic system 9.40 is activated with small amounts of acetic acid and air to give a cobalt(III) complex where CH3C02 and H20 are additional ligands. The mechanistic details of this reaction are as yet unknown. 

Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. These systems demonstrate high activity. For example, Backvall and coworkers [146] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiffs base complex. Optimization of the electron-rich quinone, combined with the so-called Shvo Ru-cata-lyst, led to one of the fastest catalytic systems reported for the oxidation of secondary alcohols (Fig. 4.59). 

The aerobic oxidation of phenols in the presence of cobalt-Schiffs base complexes as catalysts is facilitated by (electron-donating) alkyl substituents in the ring and affords the corresponding p-quinones, e.g. the Vitamin E intermediate drawn in Fig. 4.87. When the para-position is occupied the reaction may be directed to the ortho-position [252, 253]. Copper compounds also mediate this type of oxidation, e.g. the Mitsubishi Gas process for the Vitamin E intermediate... 

PdCl2 in the presence of a base, and cobalt-Schiff base complexes have also been proposed as catalysts. 

An alternative oxidation using O2 and a RuCls catalyst converted pyridine to pyridine A-oxide. Bromamine-T and RuCls in aq. acetonitrile also oxidizes pyridine to the A-oxide. Tertiary amines are oxidized to the A-oxide with O2 and Fe203 in the presence of an aliphatic aldehyde. " Oxygen and a cobalt-Schiff base complex also oxidzes tertiary amines, including pyridine. ... 

E. Tsushida, H. Nishide, M. Ohyanagi, H. Kawakami, Facilitate transport of molecule oxygen in the membranes of polymer — coordinated cobalt Schiff-base complexes, Macromolecules 20 (1987) 1907-1912. 

Cobalt-Schiff base complexes catalyze the selective oxidation of phenols by dioxygen into quinols (equation 245 ) or quinones (equations and 247 ) under mild conditions. 

Cobalt -Schiff base complexes, e.g. Co(salen), Co(acacen) and cobalt(II) porphyrins, e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21). This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177). Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251). ... 

Vinck E, Murphy DM, Fallis lA et al (2010) A pulsed EPR and DET investigation of the stabilization of coordinated phenoxyl radicals in a series of cobalt Schiff-base complexes. Appl Magn Reson 37 289-303... 

Chen D, Martell AE, Sun YZ (1989) New synthetic cobalt Schiff-base complexes as oxygen... 

Bozell JJ, Hames BR, Dimmel DR (1995) Cobalt-Schiff base complex-catalyzed oxidation of ptira-substituted phenolics - preparation of benzoquinones. J Org Chem 60 2398... 

Cedeno D, Bozell JJ (2012) Catalytic oxidation of para-substituted phenols with cobalt-Schiff base complexes/02-selective conversion of syringyl and guaiacyl lignin models to... 

USE OF ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY TO STUDY THE INTERACTION BETWEEN COBALT SCHIFF BASE COMPLEXES AND PHOSPHINES OR PHOSPHITES IN SOLUTION+... 

COBALT SCHIFF BASE COMPLEXES AND PHOSPHINES IN SOLUTION... 

An aerobic oxidation of tertiary amines catalyzed by Cobalt Schiff-base complexes afforded N-oxides in high yields [120]. The reaction was run at room temperature with 0.5 mol% of the cobalt catalyst 35 (Eq. (8.28)). The presence of molecular sieves (5 enhanced the rate of the reaction. With this procedure, various pyridines were oxidized to their corresponding N-oxides in yields ranging from 50 to 85%. Electron-deficient pyridines such as 4-cyanopyridine gave a slow reaction with only 50% yield. 

More recently, the same group described a further improvement in which a zeolite-encapsulated cobalt-Schiffs base complex was used as the cocatalyst. ... 

Homogeneous catalyst systems include transition metals, e.g. the cobalt Schiff base complex salcomine." "" " However, the quite expensive Schiff base cannot be recovered and recycled, which is a drawback of this method. Copper salts, e.g. CuCl2 and CuBr2, are widely used for the oxidation of TMP with molecular oxygen at larger scales." In addition, applications of the halides of Cr, Mn, Fe, Ni and Zn as catalysts have been described. Copper halides have been combined with earth metal halides, e.g. magnesium chloride." ... 

Cobalt Schiff Base Complexes as Simple Dioxygenase Models... 

Co (salen) and Co (salpr), typical cobalt Schiff base complexes [Co SB)], and their derivatives are investigated most extensively as dioxygenase type catalysts. The dioxygenase-type catalytic activity of Co (SB) should be related to their redox formal potential (Co VCo ). Therefore, it is important to determine the values of Co HSB) as basic data. 

Scheme 2. Mechanism for oxygenolysis of 4-methylindole by cobalt schiff base complexes [23].

The highest selectivity seen in the cobalt Schiff base complex promoted dioxygenation reactions results from the direct nonradical dioxygen incorporation into oxygen-sensitive substrate anions. The substrate anions are produced under neutral conditions by the acid-base reaction between the substrates and hydroxocobalt(III) Schiff base complexes [Co (SB)(OH)] formed in situ. 

Cobalt Schiff base complexes as simple dioxygenase models 158... 

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