Reduction with 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-... 

Triphenyltin hydride added to a soln. of 3/>-acetoxypregna-5,14,16-trien-20-one in xylene, and refluxed 2 hrs. under a Ng-stream, more triphenyltin hydride added, and refluxing continued 2 hrs. -> 3)5-acetoxypregna-5,14-dien-20-one. Y 60%. F. e. s. T. Nambara, K. Shimada, and S. Goya, Chem. Pharm. Bull. 18, 453 (1970) reductions with organotin hydrides, review, s. H. G. Kuivila, Synthesis 1970, 499. 

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. 

Tri-n-butyltin hydride/azodiisobutyronitrile Radical reductions with organotin hydrides Carboxylic acid amides from carboxylic acid azides... 

Although less well developed, the reduction of aldehydes and ketones is also possible with organotin hydrides supported on polystyrene maffix or on silica." " ... 

Monohalides afforded by reduction with tin hydrides are potentially difficult to isolate and purify by distillation. The problem is circumvented by isolating the product by column chromatography and by adding potassium fluoride which results in precipitation of the organotin fluoride, e.g. isolation of 11. ... 

Reductive dechlorination with organotin hydrides must usually be performed at elevated temperatures to take place. As a result 1-bromo-l-chlorocyclopropanes can be selectively... 

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). 

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

Several less general methods for preparing organotin hydrides are also used, though often with less satisfactory yields. However, reduction of N, A-diethylaminostannanes by dire -butylaluminum hydride or diborane occurs with high yields224 ... 

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 iodo and bromo derivatives of monosaccharides can be reduced by a variety of reducing agents to afford the corresponding deoxy sugar. Many such examples have been presented. The more stable chloro derivatives can be reduced with Raney nickel (Scheme 1). Selective reduction of a secondary chloride with respect to a primary chloride may be achieved if the reduction is performed in the presence of triethylamine.11,12 Selective reduction of a secondary chloride has been also achieved by using organotin hydrides.13 The radical initiator... 

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