Reduction mechanism, organotin hydrides

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 dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

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. 

Because of their high tolerance for numerous functionalities, tin hydrides have proved to be invaluable reagents in such simple chemical reactions as the reduction of functional groupsand in many other reactions which proceed via free radical mechanisms. Such has been the reliance on using organotin reagents in free radical chemistry that when efficient homolytic reactions are described without the use of tin reagents, the work is frequently published with phrases such as tin-free radical reactions in the title ... 

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