Cobalt Schiff-base catalysts

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. ... 

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). ... 

The cobalt(II) Schiff base catalyst forms a superoxocobalt(III) complex which converts the phenol into an aryloxy radical, itself being transformed into free or coordinated HO radical ... 

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. 

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." ... 

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. 

The third class of metal catalysts includes nickel and cobalt complexes of Schiff bases and nitrogen macrocyclic ligands, which can form on electroreduction cobalt(I) and nickel(I) reactive intermediates for the activation of organic halides. 

One of the most useful classes of metal and phase transfer catalyzed reactions are carbonylation reactions. Cobalt carbonyl is a valuable catalyst for such processes(1 ). When used in conjunction with methyl iodide, acetylcobalt carbonyl [CH3C0Co(C0) ] is generated and can undergo addition to various unsaturated substrates including alkynes and Schiff bases. In addition, one can add this species to styrene oxides to give the enol... 

The Schiff bases from substituted benzaldehydes and anilines will carbonylate in the presence of cobalt carbonyl, as catalyst at 225°C. producing phthalimidine derivatives, XVI, in good yield (70, 52). This reaction may be explained as an... 

Cobalt complexes derived from Schiff bases 388 catalyzed the hydroxyacylation of electron-deficient alkenes (Fig. 90) [431, 432]. Thus, methyl acrylate 387 reacted with aliphatic aldehydes 386 in the presence of 5 mol% of the in situ generated catalyst, molecular oxygen, and acetic anhydride to 2-acyloxy-4-oxoesters 389 in 56-77% yield. When acetic anhydride was omitted, the yields of products were lower and mixtures of the free hydroxy compounds and acylated compounds resulting from Tishchenko reactions were obtained. Electron-rich alkenes did not undergo the transformation, since the addition of the acyl radical is much slower. The acylcobalt species inserts oxygen instead and acts as an epoxidation catalyst. 

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]. 

Cobalt chloride in diglyme is a useful catalyst for benzylic [22a] and allylic [22b] oxidation under mild conditions. The addition of sodium azide to oxidations catalyzed by transition metal acetylacetonates, heteropolyacids, phtha-locyanines, bis-(pyridylimino)isoindolines, porphyrins and Schiff bases significantly enhances the rates of the low-temperature catalytic oxidation of alkanes [22c]. Ethylbenzene is slowly oxidized by air in MeCN in the presence of catalytic amount of chromium trioxide [23]. Complexes of Fe(III) and Co(II)... 

Novel platinum(ii) Schiff base complexes were found to be efficient photosensitizers for H2 evolution from mixed solvents (e.g. DMF/H2O) under visible light irradiation, with a cobalt complex as catalyst and TEA as sacrificial electron donor. Catal5Aic activity is influenced by the presence of substituents on the ligand electron-withdrawing groups, in particular, have a positive effect. ... 

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... 

There are cobalt(II) Schiff-base type catalysts which convert olefins to ketones and alcohols. Their behavior has been interpreted in terms of radical processes in which the central role is played by the superoxocobalt(III) derivative. This species abstracts an H-atom from the alcohol solvent ... 

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