Hyperconjugation symmetry-enhanced

Arylstannanes react by the same mechanism, much more rapidly, by ipso substitution,4 and this is exploited for introducing a substituent regiospecifically into an arene or heteroarene under mild conditions (see Section 7.1).5 Whether M = H or R3Sn, the Whe-land intermediate is stabilised by symmetry-enhanced hyperconjugation (see below). 

The aryl-Sn bond is cleaved by electrophiles much more readily than the aryl-H bond, and aryl-tin compounds react by ipso substitution by the classic mechanism for electrophilic substitution.36 The part that symmetry-enhanced hyperconjugation plays in determining the reactivity and regioselectivity is discussed in Section 3.I.2.2.37... 

Davies, A. G. (1999). The Whiffen effect of symmetry-enhanced and symmetry-forbidden hyperconjugation in spin-paired molecules. Journal of the Chemical Society, Perkins Transactions 2, 11, 2461-2467. 

This ability of ESR to quantify hyperconjugation showed that the magnitude of the effect is subject to symmetry control, and can be enhanced or forbidden when the CH group is bonded to both ends of a conjugated system which carries the unpaired electron, in what is known as the Whiffen effect (3-5).10-11 The hyperfine coupling is now given by the equation 3-29, where ca and ca are the coefficients at the two flanking ends of the n system. 

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). 

In the Raman spectra, the symmetric in-phase CH3 deformation band near 1375 cm" is quite weak in alkanes, but when the CH3 group is next to a double or triple bond or an aromatic ring, the Raman intensity is noticably enhanced. Possibly, hyperconjugation (which connects the pi orbitals of the unsaturated group with a CH3 localized orbital with the same symmetry) increases the change in polarizability in these cases, since the CH3 deformation can deform the pi orbitals. In the infrared spectra, when a methyl is next to a carbonyl, the symmetric, in-phase CH3 deformation band appears at 1375-1350 cm" and is intensified, making it stand out in the IR spectra of larger molecules. 

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