Catalytic Hydrostannation

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

Tris[(2-perfluorohexyl)ethyl]tin hydride has three perfluorinated segments with ethylene spacers and it partitions primarily (> 98%) into the fluorous phase in a liquid-liquid extraction. This feature not only facilitates the purification of the product from the tin residue but also recovers toxic tin residue for further reuse. Stoichiometric reductive radical reactions with the fluorous tin hydride 3 have been previously reported and a catalytic procedure is also well established. The reduction of adamantyl bromide in BTF (benzotrifluoride) " using 1.2 equiv of the fluorous tin hydride and a catalytic amount of azobisisobutyronitrile (AIBN) was complete in 3 hr (Scheme 1). After the simple liquid-liquid extraction, adamantane was obtained in 90% yield in the organic layer and the fluorous tin bromide was separated from the fluorous phase. The recovered fluorous tin bromide was reduced and reused to give the same results. Phenylselenides, tertiary nitro compounds, and xanthates were also successfully reduced by the fluorous fin hydride. Standard radical additions and cyclizations can also be conducted as shown by the examples in Scheme 1. Hydrostannation reactions are also possible, and these are useful in the techniques of fluorous phase switching. Carbonylations are also possible. Rate constants for the reaction of the fluorous tin hydride with primary radicals and acyl radicals have been measured it is marginally more reactive than tributlytin hydrides. ... 

Reaction of isoprene with tributyltin hydride in the presence of catalytic amounts of Pd(PPh3)4 gave 1,4-hydrostannation product 8 in high stereo- and regioselectivity [Eq.(3)] [19]. The Z-stereochemistry can be explained as for the hydrosilylation [cf. Eq.(2)]. 

Some examples which illustrate the scope and conditions of the homolytic hydrostannation reactions of alkenes are shown in equations 4-56 to 4-59,99 4-60,100 4-61,94 4-62,101 4.63,88 and 4-64.102 The stannane can be formed in situ from the reduction of BusSnCl with I hSiH in the presence of a Lewis acid catalyst (equation 4-65).103 In the hydrostannation of an enone, the tin enolate which is formed can be reduced by PhSiH3 to regenerate the tin hydride (equation 4-66), and only a catalytic amount of the tin hydride is required.104... 

This type of reaction is also observed in Main Group chemistry e.g. hydroboration (p. 66), hydroalumination (p. 80), hydrostannation (p. 108). It and its reversal are central to catalytic cycles such as those involved in the hydrogenation (p. 182) and hydroformylation (p. 387) of alkenes. Hydrozirconation (p. 290) provides another example of its use in stoichiometric organic synthesis. Its reverse... 

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