RaSn- Radicals

New sources of RaSn- radicals that have been developed include the reversible thermal dissociation of bis(trialkylstannyl)pinacols (290-292), the )3-scission of /3-stannylalkyl radicals (293), and the photolysis of cyclopentadienyltin compounds (294). 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

Very much more persistent radicals, with a half-life of up to a year, and with the structures [(Me3Si)2CH]3Sn- and [(MejSiljCHljRSn-(R = Pr , Bu , Me, Et, Bu, or cyclopentadienyl) have been prepared by the photolysis of [(Me3Si)2CH]2Sn(II), or by photolyzing a mixture of the appropriate halide [ Me3Si)2CH]2RSnX with an electron rich olefin (299, 300). 

The reaction of tributyltin hydride with ring-substituted benzyl chlorides gives a Hammett p-factor of -1-0.81, confirming the nucleophilic character of the BusSn- radical (303). 

Like the Sn(III) radicals, the Sn(II) stannylenes are familiar both as persistent species of unlimited life and as transient, highly reactive intermediates. Many of the compounds that, in the older literature, were 

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After initiation, the RaSn radical adds to alkenes and alkynes (Fig. 19). Reactions run at different temperatures indicate that this addition occurs in the bulk solution. 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

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