Organotin Radicals R3Sn

For studies of R3Sn radicals in fluid solution by ESR spectroscopy, the radicals are usually generated by abstraction of hydrogen from an organotin hydride, and this can conveniently be carried out by photolysing a mixture of di-t-butyl peroxide and the hydride within the cavity. The spectral resolution is often better at low temperatures, and this was ascribed to the slowing of the symmetrical exchange reaction between stannyl radicals and the stannane (Section 15.3.5.1).7 8 

Organotin Chemistry, Second Edition. Alwyn G. Davies Copyright 2004 Wiley-VCH Verlag GmbH Co. KGaA. ISBN 3-527-31023-1 

Radiation has to be continuous to maintain an adequate standing concentration of the stannyl radicals. If the t-butoxyl radicals are generated by laser flash photolysis and the formation and decay of the R3Sn radicals is monitored by optical spectroscopy (e.g. BusSn, vmax 400 nm), the rate constant at 295 K for the self-reaction of the BusSn radicals is found to be ca. 1.4 x 109 M 1 s, and that for the reaction of BulO radicals with the stannane is 1.9 x 108 M 1 s 1 

Azoisobutyronitrile, [Me2(NC)CN=NC(CN)Me2 AIBN], is an alternative initiator which can be used under either thermal or photolytic conditions.14 It has a half-life of 10 h in toluene at 65 °C, and is often used in reactions which are carried out in refluxing benzene. 

Ketone triplets react with tin hydrides in the same way as t-butoxyl radicals (equation 20-3), with a rate constant of 2 x 108 M 1 s 1 (presumably at room temperature) the hydroxyalkyl radical abstracts hydrogen from a second molecule of stannane to give isopropyl alcohol in unit quantum yield.15 

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The principal routes by which organotin radicals R3Sn" can be generated are shown in Scheme 20-1. 

In recent years, the organotin hydrides have found extensive applications in organic syntheses which involve radical chain reactions in which the stannyl radical R3Sn is a chain-carrying intermediate. Tributyltin hydride has been used most widely, and is commercially available. 

Studies by cyclic voltammetry of anodic oxidation of organotin compounds at a platinum electrode in acetonitrile show that the primary irreversible process is the outer sphere oxidation of R4Sn to the radical cation, followed by rapid fragmentation into R3Sn+ and R which is then rapidly oxidised further to R+. The rate constants of the reactions correlate with those for the oxidation by Fe(III) complexes. Values for the oxidation potentials and the ionisation energies are given in Table 5-3. 

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