Stannylalkyl 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). 

Free radical reagents usually react with tetraalkyltin compounds at a hydrogen centre in one of the alkyl ligands to give a stannylalkyl radical. P-Stannylalkyl radicals prepared in this way can then undergo P-scission to break the Sn-C bond. Thus the photolysis of di-t-butyl peroxide in the presence of trimethylisobutyltin and ethylene or an alkyl bromide, gives rise to the ESR spectrum of the P-stannylethyl radical or of the alkyl radical respectively, which are formed by the sequence of reactions shown in equations 5-38.50... 

A number of other p-stannylalkyl radicals have been used as sources of stannyl radicals. Thus isobutyltrimethyltin can be used as a photolytic source of trimethylstannyl radicals (equation 20-13),44 and bis(O-trialkylstannyl) derivatives of benzpinacol can be used as a thermal source (equation 20-14).42 45 47... 

Stannylalkyl radicals R SnCV are formed when a radical abstracts hydrogen from an alkyltin compound. The reactivity is enhanced at the a- and p-methylene groups (equation 20-22), and approximate rate constants are given in Table 20-5. 

The high a-reactivity probably results principally from stabilisation of the canonical form 20-11 in the transition state by the electron release from the tin, and the P-reactivity from stabilisation of the resulting P-stannylalkyl radical by hyperconjugation (Section 3.1). 

P-Stannylalkyl radicals, R3SnCC , are also formed by the addition of a stannyl radical to an alkene, or of any radical to an allylstannane. 

Addition to alkenes is reversible, and is accelerated by substituents Y which can stabilise the P-stannylalkyl adduct radical 20-4 by resonance, or the negative charge... 

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