Stannane radical cations

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

Two examples belonging to this class of reaction are shown in Scheme 14.8. In the first transformation, o-dicyanobenzene was benzylated to form o-benzylbenzo-nitriles when irradiated in the presence of either benzyltrimethylsilane (75% yield) [43] or benzyltributylstannane (81%) [44] via the benzyl radical formed by fragmentation of the organometallic radical cations. The reaction led to a clean monoben-zylation and the formation of disubstituted products could be prevented, because the reduction potential of the monobenzylated benzonitrile in the excited state is not sufficient to allow any further oxidation of either the silane or the stannane (Scheme 14.8a). 

Radiation of stannanes gives radical cations which show strong sp... 

Hyperconjugation by a C-Sn o bond (and indeed by most carbon-metal a bonds) is much more effective than C-H hyperconjugation, and it is an important factor in determining the structure and stability of not only radicals and cations, but also of compounds with filled n systems such as allyl-, benzyl-, and cyclopentadienyl-stannanes. The importance of vinyl-, allyl-, and aryl-stannanes in organic synthesis owes much to the stabilisation of radical and cation intermediates by a stannyl substituent, and under suitable conditions this can accelerate a reaction by a factor of more than 1014. 

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