Halides catalytic reduction

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Bromoadamantane and 1-bromoadamantane are reduced to adamantane in yields of 84% and 79%, respectively, when treated with triethylsilane and catalytic amounts of aluminum chloride.186 Similar treatment of benzhydryl chloride and exo-2-bromonorbomane gives the related hydrocarbons in yields of 100% and 96%, respectively.186 In contrast, 2-bromo-l-phenylpropane gives only a 43% yield of 1-phenylpropane the remainder consists of Friedel-Crafts alkylation products.186 Some alkyl halides resist reduction by this method, even when forcing conditions are employed. These include p-nitrobenzyl bromide, 3-bromopropanenitrile, and 5-bromopentanenitrile.186... 

The use of electrochemical methods for the destruction of aromatic organo-chlorine wastes has been reviewed [157]. Rusling, Zhang and associates [166, 167] have examined a stable, conductive, bicontinuous surfactant/soil/water microemulsion as a medium for the catalytic reduction of different pollutants. In soils contaminated with Arochlor 1260, 94% dechlorination was achieved by [Zn(pc)] (H2pc=phthalocyanine) as a mediator with a current efficiency of 50% during a 12-h electrolysis. Conductive microemulsions have also been employed for the destruction of aliphatic halides and DDT in the presence of [Co(bpy)3]2+ (bpy=2,2 -bipyridine) [168] or metal phthalocyanine tetrasulfonates [169]. 

As a last point, it should be mentioned that cobalt(II) salen (26) and cobalt(II) salophen (27) are known to interact with molecular oxygen in solution. Dioxygen forms a peroxo bridge between two cobalt centers, resulting in an often-undesired dicobalt(III) species that can attenuate the efficiency of electrogenerated cobalt(I) salen or cobalt(I) salophen for other reactions of interest, for example, the catalytic reduction of alkyl halides discussed in the following text. 

Some of the earlier work pertaining to the cobalt(I) salen-catalyzed reduction of organic halides is mentioned in a recent review [127]. Fry and coworkers have examined in detail the catalytic reductions by electrogenerated cobalt(I) salen of both... 

In a series of investigations [140-146], Gomes etal. utilized cobaltous bromide (although some work was done with cobaltous chloride) as a precursor for the catalytic reduction of aryl halides in the presence of activated olefins to afford... 

Dehalogenation. Barton et at. (1, 148) effected dehalogenation of steroidal /i-hydroxy halides with chromium(II) acetate and butancthiol as the proton donor in DMSO. The method is only useful with tertiary halides. A recent improvement that permits reduction of halides of all types uses the ethylenediamine complex of CrtCIOzh and the tetrahydropyranyl ethers of the /J-hydroxy halide. Catalytic amounts of the reducing agent can be used in "indirect electrolysis." The reaction is convenient for preparation of deoxynucleosides.1... 

Catalytic reduction of RX.2 This silane can be used in catalytic amounts in the radical reduction of alkyl halides if NaBH4 is added to regenerate the silane from the [(CH3)3Si]3SiX formed on reduction of RX by 1. Yields in the catalytic hydrodehalo-genation process are only slightly lower than those obtained by use of an equimolar amount of the silane. 

Catalytic hydrogenation, especially the use of active Raney nickel with or without gaseous hydrogen, has been used as a method for removing halide substituents an example is the debromination of ibjS-bromo-iya-hydroxypregnan-ao-ones [g8]. No detailed mechanistic description of such reactions has been given. It is probable, in common with catalytic reduction of C=C and C=0 double bonds, that the reaction involves bond dissociation and combination with hydrogen atoms attached to the catalyst surface, represented schematically by ... 

The yield of the more active RRR-a-tocopherol can be improved by selective methylation of the other tocopherol isomers or by hydrogenation of a-tocotrienol (25,26). Methylation can be accomplished by several processes, such as simultaneous haloalkylation and reduction with an aldehyde and a hydrogen halide in the presence of stannous chloride (27), aminoalkjdation with ammonia or amines and an aldehyde such as paraformaldehyde followed by catalytic reduction (28), or via formylation with formaldehyde followed by catalytic reduction (29). 

In contrast to the behavior of the foregoing nickel(I) complexes as catalysts, the catalytic reactions of alkyl halides with cobalt(I) species such as vitamin Bi2s, cobaloximes(I), and cobalt(I) salen exhibit a significant difference. Cobalt(I) species, acting as potent nucleophiles in Sn2 reactions with alkyl halides, give stable alkylcobalt(III) intermediates. Lexa and coworkers [318] have discussed this mechanistic scheme for the catalytic reduction of l-bromobutane by the electrogenerated cobalt(I) tetraphenylpor-phin complex, where TPP denotes the ligand. Reversible one-electron reduction of the parent cobalt(II) species... 

Stolzenberg and coworkers have used electrogenerated nickel(I) tetrapyrrole complexes for the catalytic reduction of dichloromethane and methyl iodide [364], alkyl halides [365-367], and aryl halides [367], and Lexa and coworkers [368] have discussed the catalytic reduction of frm75 -l,2-dibromocyclohexane to cyclohexene by electrogenerated nickel(I), cobalt(I), and iron(I) porphyrin complexes. 

Allyl halides have been reduced with electrogenerated tris(bipyridine)cobalt(I) to afford 1,5-hexadiene [369,370]. Some of the earliest work with cobalt(I) salen involved its use for the catalytic reduction of bromoethane [371], bromobenzene [371], and /er/-butyl bromide and chloride [372]. More recently. Fry and coworkers examined the cobalt(I) salen-cata-lyzed reductions of benzal chloride [373-375] and of benzotrichloride [376], and the catalytic reductions of 1-bromobutane [377,378], 1-iodobutane [378], 1,2-dibromobutane [378], benzyl and 4-(trifluoromethyl)benzyl chlorides [379], iodoethane [380], diphenyl disulfide [381], 1,8-diiodooctane [382], and 3-chloro-2,4-pentanedione [383] have been investigated. 

The alkylation of 4-amino-3-pyrazolin-5-ones has already been discussed to a considerable extent in connection with the preparation of such compounds having two substituents on the extranuclear nitrogen atom (see p. 76). The most straightforward alkylation is that with alkyl halides.101,431 Alkylation by means of formation of the Schiff base with aldehydes or ketones followed by catalytic reduction has frequently been used.111,131,1303 Treatment of 4-amino-2,3-dimethyl-l-phenyl-3-pyrazolin-5-one with formaldehyde and sodium bisulfite in 5 ... 

In an unusual twist to the Negishi cross-coupling reaction, p-substituted benzyl halides undergo reduction to p-substituted toluenes in the presence of Et2Zn (one or two equivalents) and catalytic amounts of Pd(0).222 Consider the reaction below, and propose a catalytic cycle that explains how it might occur. Assume that a L Pd-H species forms and the by-products of the reaction are ethene and EtZnCl. 

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