Reduction of organic halides

The most celebrated example of this process is the Fenton reaction which is discussed at some length to illustrate the general characteristics of such reductions. An interesting recent example is the reduction of organic halides, viz. 

RATE COEFFICIENTS FOR THE REDUCTION OF ORGANIC HALIDES BY PENTACY-... 

Organolithium compounds are often prepared by the reduction of organic halide with lithium metal. 

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 main classes of metal complex that act as electrocatalysts for reduction of organic halides are polypyridyl, macrocyclic, and phosphine complexes. 

One-electron reduction of organic halides is convenient for generating radicals. gem-Dibromocyclopropancs are reduced by such a system [77]. Using an excess of MoH2Cp2, stereoselective debromination is successful, possibly because of... 

Association of sodium hydride and alkali alkoxides with nickel salts has been reported to be effective in many hydrogenations and selective reductions of organic halides (211, 221). 

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

Quite often the resulting anionic species undergoes a fast follow-up reaction. Thus, homogeneous ET becomes a crucial step in many electrode reactions a well-known example is the cathodic reduction of organic halides. In mercury, heterogeneous ET to most halides is slow. This kinetic... 

The reduction of organic halides in the presence of aromatic hydrocarbons, the subject of detailed kinetic studies, provide rate constants for the homogeneous ET [147-150] and the follow-up reaction [151]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers during the years 1978-80 [152-157]. Homogeneous ET also plays an important role in the protonation of anion radicals [158]. 

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... 

J. Dsroszewski, J. Lusztyk, M. Degueil, C. Navarro, and B. Maillard, FleptamelhyItrisiIane-2-thiol-mediated free-radical-chain reduction of organic halides, J. Chem. Soc. Chem. Commun. p. 586 (1991). 

Since a carbon-halogen bond is more easily reduced than a silicon-halogen bond, cathodic reduction of organic halides such as allyl, benzyl, aryl and vinyl halides in the... 

A third catalytic route consists of using cathode materials with catalytic properties. The electrocatalytic activity of electrode materials towards the reduction of organic halides has been the object of many studies during the past few years [82], with silver having been shown to possess powerful electrocatalytic activities... 

Hydrodehalogenation. Triethylborane (10 mole %) can initiate reduction of organic halides, particularly iodides or bromides, by Bu3SnH even at —78°. Alkenyl iodides are also reduced under these conditions. Aryl halides require room temperature or higher. Yields are comparable to those obtained by radical initiators. 

The reduction of organic halides has been well used for organic synthesis. The rate constants for the reduction of alkyl, aryl, and vinyl radicals are shown in Table 1.10. 

Reduction of organic halides and chalcogenides with Bu3SnH has been used frequently in organic synthesis. Rate constants for the reaction of organic halides and chalcogenides with Bu3Sn are shown in Table 1.13. 

Me3Si)3SiH is a less stable oil and is oxidized under aerobic conditions, while 1,1,2,2-tetraphenyldisilane (TPDS) (Q) is stable crystal under aerobic conditions, and remains stable even when left on a table for 6 months. TPDS can be used for the reduction of organic halides, chalcogenides, and xanthates, and can be used for C-C bond formations... 

The reductions of organic halides, phenyl selenides, isocyanides and thionocarbonates with (TMS)2Si(H)Me119 and (RS)3SiH120 are achieved under normal conditions. Two examples are shown in equations 62 and 63. (TMS)2Si(H)Me is an effective reducing agent which allows the formation of the desired product to be favoured due to a slower hydrogen transfer (Table 4). 

Baizer and Chruma reported electrolytic reductive coupling, in a broad study in which reduction of organic halides was conducted in the presence of electrophiles as seen in equation 1034. Controlled potential electroreduction of reactive halides at a mercury cathode in the presence of olefinic substrates with electron-withdrawing groups (Michael receptors) gave moderate yields (50-75% in many cases) of carbanion addition products35. Current yields in excess of 100% in the case where chloroform was used as a cosolvent with carbon tetrachloride indicated the intervention of electrocatalytic reactions (equations 10 and 11). 

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