Electroreduction alkyl halide

Electroreduction of the cationic Rh(IlI) complex [Rh(Por)(MeNFl2)2l in CH2CI2 followed by reaction with alkyl halides has been utilized to form a-alkyl products. The reaction scheme proposed for this reaction was one-electron reduction of Rh(lll) to form Rh(Por)-. This can either dimerize or attack the carbon atom of the alkyl halide RCH2X, the latter step involving elimination of either X- or 2t7.2.ix reactions of Co(ll) and Fe(II) porphyrins M(Por) with... 

The electrolysis of alkyl halides on platinum cathode and tin anode has been mentioned above. A completely different mechanism is associated with alkylation on tin cathodes. Electroreduction of allyl bromide on tin electrodes yields tetraallylstannane (Ca 90%). This is done in acetonitrile solutions with LiClC>4, Et4NBr or BujNBr as electrolyte and followed by CV with Ag/AgBr reference. Yields decrease to 78% in DMF. The proposed mechanism67 in this case is ... 

The same electrochemical process was also used for the coupling between aldehydes or ketones and activated alkyl halides such as a-chloroesters, -nitriles, and -ketones as well as aya-dichloroesters.334 Electroanalytical studies have shown initial electroreduction of Fe(n) to Fe(i) and subsequent formation of an iron organometallic intermediate (e.g., a 7t-allyliron complex in Equation (27)) before reaction with the corresponding carbonyl compounds.335... 

The electroreduction of iminium salts in the presence of alkyl halides has been applied to the synthesis of alkaloid-type compounds as depicted in Scheme 87 [127,128]. 

Tabk 1. Ni- or Co-catalyzed electroreductive coupling of alkyl halides... 

Apart from the electrocarbonylation reactions of organic halides described in Sect. 6, other Ni-catalyzed reactions leading to ketones have been reported. Thus the electroreductive coupling between acylchlorides and alkyl halides, catalyzed by NiBr2bpy leads to unsymmetrical ketones [129]. Recently acylchlorides have been converted to symmetrical ketones in an undivided cell fitted with a nickel or stainless steel anode. In this reaction the active metallic species... 

Notably, Zn2+ ions are more easily reduced than naphthalene. This indicates that the electroreduction of the latter is very likely achieved on a recovered electrolytic zinc cathode. What happens under these conditions So far, no data allow us to answer this question. The only information is the experimental observation of a characteristic transient color of the naphthalene anion radical indicating the reduction of this hydrocarbon, simultaneously with the reduction of Zn2+, owing likely to a too high current density set at the cathode11. Moreover, the presence of an alkyl halide under such conditions would lead to its reduction on the zinc deposit, and this reduction would occur more easily than the reduction of naphthalene according to complex processes (equation 25). 

Electroreduction of chlorophosphines in DMF followed by treatment with alkyl halides gives excellent yields of trialkylphosphines397"399. Yields are in most cases good to excellent in a similar process starting with diaryl phosphites400,401. 

ET-initiated alkylation of azines in the presence of alkyl halides has been found to occur under electrochemical conditions according to Scheme 43. Either alkylated dihydroheterocycles or the rearomatized products are obtained. As an example, electroreduction of quinoline in the presence of 1-bromoadamantane gives alkylated quinolines (10 % in position 2, 5 % in position 7) and 2-methyl- and 2-methoxyquinoline give the 7-adamantyl derivative in 20 and 23 % yield, respectively. 1,10-Phenanthroline gives bis(adamantyl) derivatives, whereas isoquinoline and phenanthridine give the 6-adamantyl-5,6-dihydro and the 9-adamantyl-9,10-dihydro derivatives, respectively [132]. Reaction with terr-butyl chloride also gives alkylated dihydroheterocycles [133]. Arylation has also been performed, e.g. in the synthesis of pyrazolophenanthridines (62, Scheme 44) [134]. 

The tributyltin hydride-mediated carbon-carbon bond formation via radical addition and cyclization of alkyl halides with alkenes has often been a choice for construction of various organic molecules [1], However, the requirement for high-temperature initiators or photo initiation and the difficulties associated with purification of the products from tributyltin halides tend to limit the widespread use of these methods, despite the efforts to make the methods easier [Ic, 2], Recently, nickel-mediated radical additions and cyclizations have been introduced as promising alternatives to the tributyltin hydride methods. These are the nickel powder-acetic acid method for cyclization of haloamides to y-lactams, y -lactams and in-dolones, the borohydride exchange resin-nickel boride method for radical addition, nickel-catalyzed electroreductive cyclization and nickel-catalyzed Kharasch addition of polyhalo compounds. 

Alkyliron porphyrin complexes can also be formed by the attack on alkyl halides by Fe(I) systems, made by electroreduction as in, e.g., (48). 

The electroreduction in DMF of a mixture of activated alkyl halide (a-chloro- or a-bromoester or a-chloronitrile) and a carbonyl compound (aldehyde or ketone) in the presence of the complex FeBr2(Bpy) associated with an iron sacrificial anode leads to the cross[Pg.640]

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

The synthesis of organozinc compounds by electrochemical processes from either low reactive halogenated substrates (alkyl chlorides) or pseudo-halogenated substrates (phenol derivatives, mesylates, triflates etc.) remains an important challenge. Indeed, as mentioned above, the use of electrolytic zinc prepared from the reduction of a metal halide or from zinc(II) ions does not appear to be a convenient method. However, recent work reported by Tokuda and coworkers would suggest that the electroreduction of a zinc(II) species in the presence of naphthalene leads to the formation of a very active zinc capable of reacting even with low reactive substrates (equation 23)11. 

However most organic halides are less easily reduced than Zn(II). So such a process can only be applied if one is able to catalyse the electroreduction of RX. This can be achieved by electrochemically formed zerovalent nickel complexes. Thus Ni(II)-2,2 -bipyridine (Ni-bpy) compounds can catalyse the electroreduction of alkyl and aryl halides1 at potential more positive than... 

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