Tributyltin hydride (Tributylstannane), Tin hydrides
Organotin hydrides are very good radical reducing agents due to the relatively weak, nonionic bond between tin and hydrogen (Bu3SnH 74 kcal/mol) that can cleave homolytically.
However, these compounds are plagued by their high toxicity and high fat solubility (lipophilicity). Therefore, with few exceptions, the use of tin hydrides should be avoided. The catalytic use of this reagents with a suitable second reducing agent, or the use of radical H-donors such as indium hydrides and silanes [especially tris(trimethylsilyl)silane] are possible alternatives.
Tin is characterized by a pronounced affinity for sulfur, which can be exploited in various reactions:
Additional interesting reactions that employ tin hydrides are dehalogenation and intramolecular radical cyclization.
An organotin hydride-catalyzed, silicon hydride-mediated method for effecting the conjugate reduction of α,β-unsaturated ketones was developed.
D. S. Hays, M. Scholl, G. C.Fu, J. Org. Chem., 1996, 61, 6751-6752.
A stannane, which is simple to prepare, was successfully used in standard radical reactions as replacement of Bu3SnH and Ph3SnH. The tin-containing byproducts are removed by mild hydrolysis and extraction with aqueous NaHCO3. The performance of this new reagent was tested for reactions involving halides, selenides, Barton-McCombie deoxygenation and enyne cyclization.
D. L. J. Clive, J. Wang, J. Org. Chem., 2002, 67, 1192-1198.
Treatment of o-bromonitrobenzenes with various vinyl Grignard reagents gives 7-bromoindoles in good yields, using the o-bromine atom to direct the cyclization. A subsequent reduction using a heteroaryl radical methodology gives 7-unsubstituted indoles in nearly quantitative yields.
A. Dobbs, J. Org. Chem., 2001, 66, 638-641.