Triphenyltin radical

Reduction of triphenyltin piperidyldithiocarbamate in acetone was shown by polarography and voltammetry to consist of two diffusion-controlled peaks and two peaks which seem to reflect adsorption142. Apparently, a dithiocarbamate group dissociates and triphenyltin radical forms by reduction. The latter partly dimerizes and partly reduces to triphenyltin anion. 

CV studies indicate that Ph3SnY, where Y = Cl, OTf, OCHO, SnPh3, do not serve as a source of the triphenyltin radical because in these instances the radical is generated at potentials where it is either oxidized or reduced [19]. Reaction of the triphenyltin radical with an alkyl halide or a disulfide produces a carbon- or a sulfur-centered radical, respectively. Use of the latter transformation in conjunction with a subsequent intramolecular cyclization onto an alkene is nicely illustrated by the conversion of /6-lactam 62 to the cephalosporin analog 63. 

Addition of the triphenyltin radical to 1-alkynylcyclopropenes 20 leads to reaction at either the cyclopropene or the alkyne, followed in each case by cyclization. ... 

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

The few cases in which thermodynamic control is not effective appear to be dominated by steric effects39. Thus, trapping of radical 25 with triphenyltin hydride regioselec-tively yields product 26, in which the double bond is separated from the ester through a methylene bridge (equation 12). 

Haloarenes have been found to undergo nucleopilic substitution when irradiated with the triphenyl stannyl anion46, reacting via a radical S l mechanism. In many cases the reaction will only occur under photochemical conditions. The reaction is found to proceed with chloro- and bromo-substituted arenes, but not iodo-compounds. The anion is produced either by treatment of triphenyltin chloride or hexaphenylditin with sodium metal in liquid ammonia, and will react with a wide variety of arenes (reaction 30). 

In addition, Kozuka and Lewis measured the tritium isotope effect for the reaction between the n -hexyl, the 2-hexyl and the 2-methy 1-2-pentyl radicals with triphenyltin hydride and triphenyltin hydride-t see the last three entries in Table 11. The isotope effect of 2.55 found for the triphenyltin hydride-w-hexyl radical reaction was slightly smaller... 

Phenyl aUcyl tellurides are reduced by triphenyltin hydride under mild conditions, giving the corresponding hydrocarbons in high yields. An excess of the reagent is necessary (2-2.8 equiv), but a radical initiator need not be employed. 

Although tributyltin and triphenyltin hydrides are excellent reagents for radical chemistry, they have the disadvantage that they are costly and of relatively high molecular weight. For small operations, in the laboratory, this is not serious. However, there is a major additional problem, particularly for industrial use, because of the toxicity of these tin compounds and because of the difficulty of removing tin dimers that are formed as by-products [13]. Similar comments apply to germanium hydrides, which are even more expensive [21]. 

Recent studies on radical-induced cyclisation reactions have led to a simple, one step method for the preparation of hydroazulenes from appropriately substituted cyclopentanone precursors. Treatment of 1, for example, with triphenyltin hydride and AH3N in refluxing toluene gave 2 in 79% yield. 

The development of new radical initiators is an area of active investigation. Tada et at. have described a (triphenyltin)cobaloxime complex as a reagent for radical generation from bromides [95JOC6635]. This is an alternative to reductive processes involving tin hydride and allows for the introduction of an additional functional group in the product. 

However, radical allylations mainly proceed through pathway (eq. 4.31b). For example, treatment of adamantyl iodide and ally 1-3,3-<72-triphenyltin in the presence of AIBN generates mainly 3-adamantyl-l-propene-3,3-d2, together with a minor amount of 3-adamantyl-1 -propene-1,1 -d2. 

For the compounds that we overview here, an attractive conversion of thionoesters to ethers [191] has used the radical addition of an excess of triphenyltin hydride, instead of tri-n-butyltin hydride, which avoided the deoxygenation reaction. The process involves in a first step the very favourable formation of a sulfur-tin bond and the subsequent desulfurisation of the mixed thioketal. 

There is no doubt that free-radical chemistry has benefited enormously through the invention of tin-based chain-carrying reagents " . Of these, tributyltin hydride and, to a lesser extent, triphenyltin hydride have been the reagents of choice. Their ready availability and favourable rate constants for attack of the corresponding tin-centred radicals at a variety of radical precursors , coupled with useful rate constants for hydrogen transfer to aUtyl and other radicals, provide for reagents superior to their silicon counterparts only tris(trimethylsilyl)silane rivals tributyltin hydride in its synthetic utility . 

While the large bulk of free-radical chemistry has been carried out using reagents such as tributyl, trimethyl and triphenyltin hydride, there are several examples of the use of designer stannanes that satisfy desired niche... 

Selenides are also nucleophilic and produce isolable selenonium salts (9) when treated with alkyl halides. They are easily oxidized to selenoxides (10) and further to selenones (11) under more forcing conditions (see Section 4). Reduction of selenides to the corresponding hydrocarbons is most conveniently achieved with nickel boride,or with tri-n-butyl- or triphenyltin hydride under radical conditions. " Other reagents for reductive deselenization include Raney nickel, lithium triethylborohydride, and lithium in ethylamine (Scheme 4). Benzylic selenides undergo radical extrusion reactions under thermal or photolytic conditions to produce... 

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