Q4(c). SeO₂ allylic oxidation; dihydroxylation and selective oxidation

Introduction. (i) is an allylic oxidation of a trisubstituted alkene: one of the gem-methyl groups becomes CH₂OH and the C=C stays in place. (ii) is a syn-dihydroxylation followed by oxidation of only the secondary OH.

(i) Reagent: SeO₂ (catalytic) with t-BuOOH, CH₂Cl₂

1. Ene reaction. SeO₂ (as O=Se=O) acts as the enophile: the allylic H of the methyl trans to the chain is transferred to O while Se bonds to C-3; the C=C shifts. Product: an allylseleninic acid.
2. [2,3]-Sigmatropic shift. The Se-C₃ bond and the new C=C reorganise through a five-membered cyclic TS: the C=C returns to C2=C3 and oxygen ends on the methyl carbon, as a selenite ester C-O-Se-OH.
3. Hydrolysis gives the allylic alcohol and Se(II), which t-BuOOH reoxidises to SeO₂ (so only a catalytic amount is needed and over-oxidation to the aldehyde is limited).
4. Selectivity: the hydroxyl appears on the (E)-methyl, the one trans to the longer chain, because the ene TS with the chain pointing away is less crowded: product (E)-2-methylhept-2-en-1-ol.

Scheme: Ene reaction, [2,3]-sigmatropic shift and hydrolysis.

(ii) Reagents

1. 1-Methylcyclohexene → cis-diol: OsO₄ (catalytic) with NMO (or cold, dilute alkaline KMnO₄); syn addition through a cyclic osmate ester gives cis-1-methylcyclohexane-1,2-diol.
2. Diol → α-hydroxy ketone: oxidise the secondary OH only, with Swern reagent (DMSO, (COCl)₂, Et₃N, -78°C) or PCC. The tertiary OH has no C–H and is untouched; neutral conditions avoid C–C cleavage of the diol.

Conclusion. (i) SeO₂/t-BuOOH: ene reaction then [2,3]-sigmatropic shift gives (E)-2-methylhept-2-en-1-ol. (ii) OsO₄/NMO, then Swern (or PCC) oxidation: 2-hydroxy-2-methylcyclohexan-1-one.

Application. SeO₂ oxidation of the (E)-methyl of trisubstituted alkenes is used to make terpene allylic alcohols and aldehydes, for example in syntheses of geraniol-derived fragrances.