Q8(d). SeO₂, NBS and Pb(OAc)₄ reactions

(i) Riley oxidation

Scheme: The CH₃ next to C=O becomes CHO.

1. SeO₂ oxidises a CH₂ or CH₃ group α to a carbonyl (Riley oxidation). The enol of the ketone attacks Se; the resulting α-seleninic ester rearranges and eliminates Se and water, leaving a C=O on the former α-carbon.
2. X = 2-oxo-2-(naphthalen-2-yl)acetaldehyde (2-naphthylglyoxal), ArCOCHO, C₁₂H₈O₂: a 1,2-dicarbonyl compound. The aromatic ring is not attacked.

(ii) Allylic bromination with NBS

Scheme: Br replaces an allylic H; the C=C is kept.

1. Light or heat starts a radical chain. Br• abstracts an allylic H (the weakest C–H, because the allyl radical is resonance-stabilised).
2. NBS reacts with the HBr formed to keep a very low concentration of Br₂; the allyl radical takes Br from Br₂ and regenerates Br•. At this low concentration addition of Br₂ to the C=C cannot compete (Wohl–Ziegler reaction).
3. In cyclopentene the two ends of the allyl radical are equivalent, so there is only one product: X = 3-bromocyclopentene (racemic).

(iii) Glycol cleavage of trans-decalin-4a,8a-diol

Scheme: The bond between the two carbinol carbons (the ring fusion) breaks; two C=O groups form in a ten-membered ring.

1. Pb(OAc)₄ (like HIO₄) breaks the C–C bond of a 1,2-diol, giving two carbonyl groups. Both carbinol carbons are tertiary, so both become ketones.
2. Cutting the shared C4a–C8a bond opens the bicyclic system into one large ring: X = cyclodecane-1,6-dione (C₁₀H₁₆O₂).
3. The two OH groups of the trans-diol are anti-periplanar and cannot form the cyclic lead ester that cis-glycols form; it is still cleaved by Pb(OAc)₄ (more slowly, through an acyclic transition state), whereas HIO₄, which works through a cyclic periodate ester, does not cleave it readily. This is the classic case showing that the cyclic-intermediate mechanism does not explain all lead tetraacetate oxidations.

Conclusion. (i) 2-naphthylglyoxal, ArCOCHO (SeO₂, α-oxidation). (ii) 3-bromocyclopentene (NBS, allylic radical bromination). (iii) cyclodecane-1,6-dione (Pb(OAc)₄ glycol cleavage, which works even on this trans-diol).

Application. Glycol cleavage of bicyclic diols is used to make medium-sized rings (ten-membered rings for macrocyclic musks); NBS allylic bromination and SeO₂ oxidation are standard steps in terpene and steroid synthesis.