Methyl groups at the \(\beta\) position of pyridine behave much like those on toluene because they lack resonance stabilization from the ring nitrogen. Always target the \(\alpha\) or \(\gamma\) positions for regioselective functionalization.
Step 1: Understanding the Concept:
The acidity of alkyl groups on a pyridine ring depends on the position relative to the nitrogen atom. Benzylic-type carbanions at the 2- (\(\alpha\)) and 4- (\(\gamma\)) positions are stabilized by resonance involving the electronegative nitrogen. Step 2: Key Formula or Approach:
Acidity order of methyl groups in pyridines: \(\gamma>\alpha \gg \beta\).
LDA (Lithium diisopropylamide) is a strong, hindered base used for kinetic deprotonation. Step 3: Detailed Explanation:
1. Deprotonation: In 3,4-dimethylpyridine, the methyl group at the 4-position (\(\gamma\)) is significantly more acidic than the one at the 3-position (\(\beta\)). This is because the negative charge on the 4-picolyl carbanion can be delocalized directly onto the nitrogen atom, whereas the 3-picolyl carbanion cannot.
2. Reaction with LDA: LDA selectively removes a proton from the more acidic C4-methyl group to form a stabilized organolithium intermediate.
3. Alkylation: This nucleophilic carbanion then attacks allyl bromide (\(CH_2=CHCH_2Br\)) via an \(S_N2\) mechanism, attaching the allyl group to the C4 position. Step 4: Final Answer:
The product P is 3-methyl-4-(but-3-enyl)pyridine.
