Alkyl halide, reduction with organotin hydride

The mechanism for reduction of alkyl halides is not always nucleophilic substitution. For example, reductions with organotin hydrides generally " take place by free-radical mechanisms,as do those with Fe(CO)5. Alkyl halides, including fluorides and polyhalides, can be reduced with magnesium and a secondary or ter-... 

A radical mechanism may also be proposed for the other reductions, at least for the alkyl and aryl halides in view of the clarified relations of their reduction with organotin hydrides (756). For carbonyl or other unsaturated compounds (Table IX), a polar mechanism is also possible it could be of Sn2 type. Further experiments are needed for clarification. 

The reduction of alkyl, aryl, and acyl halides by organotin hydrides would suggest that other halides might also be reduced. It has been shown that AT -bromosuccinimide is reduced in 95 % yield with triphenyltin hydride (82). Diphenylchlorophosphine undergoes exothermic reduction with triphenyltin hydride providing diphenylphosphine in good yield (8J). 

The reduction of chlorodeoxy sugars has been achieved [31,32] in high yield by means of tributyltin hydride in the presence of 2,2 -azobis(2-methylpropionitrile). The reaction with methyl 2,3-di-0-acetyl-4,6-dichloro-4,6-dideoxy-a-D-galactopyranoside at 60°C gave [31] methyl 2,3-di-0-acetyl-6-chloro-4,6-dideoxy-ot-D-xylo-hexopyranoside as the main product. A free-radical mechanism has been proposed [33] for the reduction of alkyl halides by organotin hydrides. In accordance with this proposal, the presence of the radical initiator 2,2 -azobis(2-methylpropionitrile) was essential for the reduction of chlorodeoxy sugars moreover, the relative reactivities of the two chlorine atoms in methyl 2,3-di-0-acetyl-4,6-dichloro-4,6-dideoxy-a-D-galactopyranoside follow a free-radical order. 

The reactions are commonly initiated with AIBN at ca. 80 °C in a solvent such as benzene or toluene, but alkyl halides have also been reduced at -60 °C with tributyltin hydride under sonication conditions.133-134 Some reductions of halides have been carried out by generating the tin hydride in situ from a molar equivalent of the reducing agent [lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane)] and a catalytic amount of organotin hydride, halide, or oxide. Reduction of halides has also been carried out under aqueous conditions, using 4,4 -azobis(4-cyanovaleric acid) (ACVA) as a water-soluble radical initiator.60... 

Reactions.—Replacement of halogen atoms in alkyl halides with hydrogen can often be achieved by trialkyltin hydride reduction. Two new work-up procedures" "" are said to simplify removal of the organotin halide by-product residues, either by their partition between hexane and acetonitrile " or by their conversion into insoluble polymeric organotin fluorides." ... 

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