Carbon Cations the P-Tin Effect

The hyperconjugative effect between a C-Sn bond and a P-positive charge is greater in a cation than in a radical, and there are many examples of the enhancement of the rate of both nucleophilic and electrophilic reactions resulting from the introduction of a stannyl 

The stabilisation of a carbocation by hyperconjugation by a P-C-Sn bond is sometimes referred to simply as the P-tin effect. It is apparent in the abstraction of hydride by Ph3C+ BF4 from alkyltin compounds.19 Tetramethyltin is unreactive, but ethyltin compounds undergo elimination. 

The P-effect similarly enhances the reactivity of alkynyl- (Section 8.2.2), alkenyl-(Section 8.1.2), allyl- (Section 9.1.3.2), and aryl- (Section 7.1) stannanes in their reactions with electrophiles (equations 3-33-3-36), and its effect can be recognised in other contexts such as the ene reactions of allylstannanes (Section 9.1.3.4), and the charge-transfer reactions and ring-substitution reactions of benzylstannanes.22 

The Wheland intermediates that are formed in reaction 3-36 are important as their pentadienyl cation LUMO (3-10) has the appropriate symmetry for (Whiffen) enhanced hyperconjugation, which is in large part responsible for the high ipso reactivity, and the relative reactivities of compounds C6H5MEt3, compared with benzene, towards electrophiles, as M varies, are approximately (benzene 1), Si, 5 x 104, Ge 106, Sn 5 x 1011, Pb 1020.25 The 4-stannylbenzyltin compounds should further benefit from both the Whiffen effect at the ipso position and the simple C-Sn hyperconjugation at the 4-position (3-11). 

The P-tin effect is also active in promoting 1,5-hydrogen shift in cyclooctyl and cyc-lodecyl systems (equation 3-37).26 

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