Phenyl halides, reduction

It is well known104 that organotin hydrides are very efficient reagents for organic halide reduction. Not surprisingly, therefore, 1-stannaindene (94) in CDCI3 solution gives rise to 1-chloro-l-phenyl-1-stannaindene (95) (equation 38). 

Alkyl Phenyl Telluriums3 (Reduction with Lithium) A 250-ml, three-necked flask is fitted with a reflux condenser, a high-speed wire stirrer, and a nitrogen inlet tube. The flask is charged with 4.1 g (10 mmol) of diphenyl ditellurium and 100 ml of dry THF and the apparatus is flushed with nitrogen. 1.4 g (200 mmol) of freshly cut, small pieces of lithium are added and the mixture is stirred at 20° for 6 h. Unreacted lithium is then removed using a spatula and 20 mmol of the alkyl halide (neat or in THF solution) are added dropwise to the stirred, yellowish-brown solution of lithium benzenetellurolate. The mixture is stirred at 20° for 0.5 h and under reflux for 0.5 h. The solvent is distilled off, 10 ml of water and 25 ml of diethyl ether are added to the residue, and the phases are mixed thoroughly. The diethyl ether layer is separated, the solvent is distilled off, and the residue is fractionally distilled under vacuum yield 62-79%. 

The synthesis of Cp 3U Cp = p -CsMes carved a new path for the researchers to go one step ahead in the electrochemical studies of organoac-tinide complexes. The reduction reaction involving more than two electrons are not common for metal complexes containing just one metal. However, the synthesis of complexes of the type Cp 3M led to the development of sterically induced reduction (SIR) chemistry in which sterically crowded complexes of redox inactive metals act as reductants [87,88]. Evans et al. showed that the sterically induced reduction couple, U(III)/U(IV), can act as a multielectron reductant [89]. As an example, Cp 3U reacts as a three-electron reductant with 1,3,5,7-C8H8, (Eq. 12) in which one electron arises from U(III) (Eq. 13) and two result from two CsMes /CsMes half reactions (Eq. 14) presumably via SIR. This phenomenon was further corroborated by the stepwise reduction of phenyl halide with Cp 3lJ (Eq. 15) ... 

However in aprotic media the reactions of R are more varied. Thus in the presence of carbon dioxide, benzyl halide reduction in aprotic solvents produces some phenyl acetate. In aqueous solutions only toluene is formed at negative potentials. Rifi has reported the synthesis of bicyclobutane derivatives from the corresponding 1,3-dihalo-cyclo butanes at a mercury pool cathode in DMF containing LiBr e.g. 

Clocking and Kingston [25], and later Rabinovitz [26], showed that the reduction of phenyl halides (to obtain biphenyl) with CgK afforded 85-99% yield, in contrast... 

Three substituted 5-phenyl unsymmetrical disulfides have been prepared, i, ii, and iii —compounds i and ii by reaction of a thiol with a sulfenyl halide, compound iii from a thiol and an aiyl thiosulfonate (ArS02SAr). The disulfides are cleaved by reduction (NaBH4) or by treatment with excess thiol (HSCH2CH2OH). 

Another group of Japanese workers91 found that the sulphoxonium salt, 7, was reducible to sulphoxides with either alkyllithiums or lithium dialkylcuprates, the exact reaction pathway being complicated by halide ions originating from the preparation of the metal alkyls. However, good yields of methyl phenyl sulphoxide were obtained by reduction of 7 with sulphur dioxide or a thiol in pyridine (equation 37). 

The dehalogenases (EC 3.8.1), a subclass of the hydrolases that act on the halide bonds in C-halide compounds, catalyze reactions of hydrolytic de-halogenation (Fig. 11.3,a), i.e., the replacement of a halide atom at a sp3 C-atom with a OH group. Exceptions include thyroxine deiodinase (EC 3.8.1.4), which catalyzes reductive deiodination on phenyl rings, and the bacterial 4-chlorobenzoate dehalogenase (EC 3.8.1.6), which forms 4-hydroxy-benzoate. 

Several publications deal with acetylenic halides that can undergo reductive intramolecular cyclization [37]. For example, electrolysis of 6-iodo-l-phenyl-l-hexyne at a carbon cathode gives benzylidenecy-clopentane in 36% yield. 

The addition of aryl radicals, generated by chemical reduction of aryldiazonium salts, onto arenes in the Gomberg-Hey reaction is well established [163]. The addition of these radicals to alkenes in the Meerwein reaction is also well known [164], Aryl o-radicals generated by electrochemical reduction of aryl halides take part in similar reactions. Good yields of the products are obtained when the intermediate phenyl radical can react in an intramolecular manner. The addition step is then fast and competes successfully with further electron transfer to form the phenyl car-banion, followed by protonation. 

Phenyl o-radicals generated by reduction of aryl halides can also interact with an intramolecular alkene bond. Ihe method has been developed for the formation of dihydroindoles by reductive cyclization of N-allyl-2-chloroacetanilides. The results indicate the importance of a time interval between electron addition to give a radical-anion and the fragmentation of this species to give the active a-radical, The time interval allows the radical-anion to diffuse away from the electrode surface so that when the a-radical is foimed, it has time to cyclize before it can be reduced at the surface. 

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