Q6(b). NBS (radical and ionic), KMnO₄/NaIO₄, OsO₄/H₂O₂

Introduction. The same reagent can act by different mechanisms: NBS gives a bromine radical with a peroxide initiator but Br₂/Br⁺ in the presence of bromide ion; KMnO₄ and OsO₄ add two OH syn; NaIO₄ cleaves 1,2-diols.

Reading of the question: “φ.OOH” is taken as a peroxide radical initiator (benzoyl peroxide or perbenzoic acid) and “t-C₄H₇OH” as tert-butanol (C₄H₉OH).

(i) NBS on cyclohexene

Scheme: One reagent, two mechanisms.

• X (Wohl–Ziegler bromination). The peroxide starts a chain: Br. abstracts an allylic H (weakest C–H, resonance-stabilised allyl radical); the radical takes Br from the low, steady concentration of Br₂ made from NBS and HBr. Low Br₂ concentration prevents addition to C=C. X = 3-bromocyclohexene.
• Y. Bromide ion reacts with NBS to release Br₂ (or Br₃⁻) in high local concentration. Electrophilic addition goes through a bromonium ion, opened by Br⁻ from the back face: anti addition. Y = trans-1,2-dibromocyclohexane (racemic).

(ii) Methylenecyclobutane

Scheme: Dihydroxylation then glycol cleavage.

Cold, dilute (alkaline) KMnO₄ adds two OH groups through a cyclic manganate ester: X = 1-(hydroxymethyl)cyclobutan-1-ol. Periodate forms a cyclic periodate ester with the 1,2-diol and cleaves the C–C bond between the two carbinol carbons: the ring carbon becomes C=O and the CH₂OH becomes HCHO. Y = cyclobutanone.

(iii) Benzene with H₂O₂/OsO₄ (Milas reagent)

• H₂O₂ with catalytic OsO₄ in tert-butanol (Milas hydroxylation) adds two OH syn to a C=C through an osmate ester; H₂O₂ reoxidises osmium, so only a trace of OsO₄ is needed.
• Benzene resists addition because of its aromatic stabilisation, so it reacts only very slowly; this is not a preparative reaction, and the expected answer is the product of complete hydroxylation. Once the first syn dihydroxylation has occurred, the product, cis-cyclohexa-3,5-diene-1,2-diol, is no longer aromatic, and its two remaining C=C bonds are hydroxylated in the same way.
• X = cyclohexane-1,2,3,4,5,6-hexol (an inositol), C₆H₆(OH)₆, with each pair of OH added together on one face (syn).

Scheme: Three syn dihydroxylations.

Conclusion. (i) X = 3-bromocyclohexene (radical allylic bromination), Y = trans-1,2-dibromocyclohexane (ionic anti addition). (ii) X = 1-(hydroxymethyl)cyclobutan-1-ol, Y = cyclobutanone. (iii) X = cyclohexanehexol (inositol) by repeated syn hydroxylation.

Application. Allylic bromination with NBS is the standard route to allylic functionalisation in steroid and terpene chemistry; dihydroxylation followed by periodate cleavage (Lemieux–von Rudloff with KMnO₄, Lemieux–Johnson with OsO₄) converts an exocyclic methylene group into a ring ketone.