Methyl group charge stabilisation

The /-butyl group provided us with some useful work-horses for developing the scope of the cyclisation, since A-t-butylbenzylamine is cheap and readily available. However, we found it was almost impossible to remove from the products, even on extended treatment with acid. Replacing one of the methyl groups of the /-butyl with a phenyl group, however, stabilised the positive charge formed during El elimination and allowed us to deprotect the products to yield A-unsubstituted... 

Just as a carbonyl group stabilises an adjacent negative charge as an enolate anion, so the anion derived from 2-methyl pyridine is stabilised by delocalisation of the negative charge onto the electronegative nitrogen atom. 

Here the addition of the extra methyl group is of less assistance in the stabilisation of the charge, because the transmission of the charge by the inductive effect becomes less efficient with distance For most purposes, the effect can be ignored after the second a bond from the charged centre. 

Due to the electron donating effect of methyl group, one of the resonance forms obtained from ortho or para attack gets stabilised more. This form has positive charge situated on the carbon to which the methyl group is attached. Similar stabilisation of the intermediate does not occur when the attack occurs at the meta position. It means that a more favourable transition state (intermediate) results when the attack occurs at the o- and /v-position of toluene. This explains the ortho- and para - directing influence of the methyl group on the compound. 

In the case of cyclopropyl methyl ketone, disconnection of either the 1,2- or 1,3-carbon-carbon bond of the cyclopropane ring results in the preferred charge distribution shown in (4), namely, the carbanion site is adjacent to the meso-merically stabilising carbonyl group, and the carbocation site may be viewed as a halide-carrying carbon. The reagent equivalent may therefore be 5-chloro-pentan-2-one. 

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