Reducing agents organotin hydrides

Many other groups can be selectively reduced by organotin hydrides. Although these reactions have not been used to date to form labelled compounds, the reactions are included here using tributyltin deuteride as the reducing agent to illustrate the synthetic possibilities for this reagent. 

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... 

Dumartin and associates described the preparation of in situ polymer-supported organ-otin hydrides for use as clean reducing agents (equation 15)47, while Deleuze and coworkers reported the preparation of a novel, macroporous polymer-supported organotin hydride (37), for use in catalytic free-radical reductions (equation 16)48,49. 

Organotin chemistry exploits the weakness of C-Sn bonds to deliver whatever is attached to the tin to another reagent. You have already seen (Chapter 39) tributyltin hydride used as a radical reducing agent because of the ease with which the Sn-H bond can be broken. Carbon substituents can be transferred by a radical mechanism... 

The special characteristics of organotin hydrides as reducing agents are rationalized by the fact that the tin-hydrogen bond is both weaker and less polar than the B—H or A1—H bonds. These characteristics are manifested in reactions that proceed by either a free radical chain or polar mechanism, depending on the substrate, catalyst and reaction conditions. 

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... 

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