Question

The major product formed in the following reaction is p

Hint:

This sequence is a classic "protection-deprotection" strategy used to temporarily mask an alkene or to purify it through a crystalline diol/acetal intermediate. The final step is known as the Hines fragmentation.

Answer 1

Step 1: Understanding the Concept:
The reaction sequence involves the syn-dihydroxylation of an alkene to form a cis-1,2-diol, the protection of this diol as a cyclic benzylidene acetal, and a final fragmentation step triggered by a strong base (\(n\)-BuLi).
Step 2: Key Formula or Approach:
1. Alkene + \(KMnO_4/OH^-\) (cold) $\rightarrow$ \textit{cis}-diol.
2. Diol + Aldehyde + \(H^+\) $\rightarrow$ Cyclic Acetal.
3. Cyclic Acetal of \textit{cis}-diol + \(n\)-BuLi $\rightarrow$ Regenerated Alkene (Hines deoxygenation).
Step 3: Detailed Explanation:
1. Step 1: Cyclohexene reacts with cold, alkaline \(KMnO_4\) to undergo syn-hydroxylation, yielding \textit{cis}-cyclohexane-1,2-diol.
2. Step 2: This \textit{cis}-diol reacts with benzaldehyde (\(PhCHO\)) in the presence of an acid catalyst (\(H^+\)) to form a cyclic acetal (2-phenyl-hexahydro-1,3-benzodioxole).
3. Step 3: Treatment of the cyclic acetal with \(n\)-butyllithium leads to the deprotonation of the benzylic carbon. The resulting carbanion undergoes a concerted fragmentation (cyclofragmentation), eliminating lithium benzoate to regenerate the double bond. This overall process is a method for the stereospecific deoxygenation of diols back to alkenes.
Step 4: Final Answer:
The final product (P) is cyclohexene.