Q3(c). Bromination of an octalin; DBr and 1,3-cyclohexadiene

Introduction. (i) The octalin has a trans ring fusion (angular CH₃ up, fusion H down), so the chairs cannot flip and every substituent position is fixed as axial or equatorial. (ii) is electrophilic addition to a conjugated diene: 1,2- versus 1,4-addition through an allylic cation, under kinetic or thermodynamic control.

(i) Kinetic product: D (2β,3α-dibromide, diaxial)

1. Axial positions alternate up and down round a chair. Starting from the axial angular CH₃ (β, up) at C8a: C1 down, C2 up, C3 down, C4 up, C4a down (the axial fusion H). So the diaxial pair is 2β-Br, 3α-Br.
2. Br₂ forms a bromonium ion on the α face (the β face is shielded by the axial methyl).
3. Br⁻ opens it from the β face at C2, along the axial direction (Fürst–Plattner rule): the opening passes through a chair-like transition state and gives the trans-diaxial product.

Figure: Ring that is brominated, axial bonds only: C8a–CH₃ up, C2–Br up, C3–Br down, C4a–H down.

1. In the drawings, the upper Br is on C2 and the lower Br on C3. D (C2-Br wedged = β, C3-Br hashed = α) is the diaxial trans-dibromide: the kinetic product. C (2α,3β) is the diequatorial trans isomer; it would need opening through a twist-boat and appears only when D isomerises on heating (thermodynamic product). A and B are cis (syn addition) and do not form.

Scheme: The diaxial dibromide forms first; it equilibrates to the more stable diequatorial isomer only on heating.

(ii) DBr and 1,3-cyclohexadiene

Scheme: D⁺ adds to C-1, giving a secondary allylic cation over C-2, C-3 and C-4; Br⁻ adds at C-2 (1,2) or C-4 (1,4).

1. Kinetic product. The 1,2-adduct, 3-bromo-4-deuteriocyclohexene (Br and D on adjacent carbons). Br⁻ is released next to C-2 when D⁺ is transferred to C-1, so it captures C-2 before it can diffuse to C-4 (proximity effect). It dominates at low temperature, when addition is irreversible.
2. Thermodynamic product. The 1,4-adduct, 3-bromo-6-deuteriocyclohexene. At higher temperature C–Br ionises back to the allylic cation, so the products equilibrate, and the 1,4-adduct is formed more readily than at low temperature: thermodynamic control.
3. Which is more stable? Here the 1,2- and 1,4-adducts are the same compound (3-bromocyclohexene), differing only in where D sits. Both have a disubstituted ring C=C and the same allylic C–Br, so they have essentially equal stability. At equilibrium they are present in nearly equal amounts. This is the case where the 1,4-adduct is not more stable than the 1,2-adduct: the deuterium only labels the two pathways (with HBr one product is seen).

Conclusion. (i) D, the 2β,3α (trans-diaxial) dibromide, from anti, axial opening of the α-bromonium ion in the rigid trans-fused ring. (ii) Kinetic: 1,2-adduct 3-bromo-4-deuteriocyclohexene; thermodynamic: 1,4-adduct 3-bromo-6-deuteriocyclohexene, favoured at higher temperature. The two have essentially the same stability (same disubstituted C=C), so under equilibrium both are present in nearly equal amounts.

Application. Trans-diaxial addition and Fürst–Plattner opening control bromination and epoxide opening of steroid rings (for example 5α-cholest-2-ene gives the 2β,3α-dibromide); DCl and DBr labelling is how kinetic 1,2-addition was proved for conjugated dienes.