Q2(a). Hydroboration–oxidation of 1-methylcyclopentene

Introduction. Hydroboration–oxidation converts an alkene into an alcohol with anti-Markovnikov orientation and syn addition of H and OH. Step 1: BH₃ (as BH₃*THF) adds across C=C; step 2: alkaline H₂O₂ replaces boron by OH.

Scheme: Product racemic: (1R,2R) shown, with its (1S,2S) enantiomer.

Mechanism

1. Hydroboration: the empty p orbital of boron accepts the π electrons; B–C and C–H bonds form together through a four-centre transition state (no carbocation, no rearrangement). B goes to C-2 (the CH carbon), H to C-1 (the carbon carrying CH₃).
2. The monoalkylborane adds to two more alkenes to give the trialkylborane R₃B.
3. Oxidation: HOO⁻ adds to boron; an alkyl group migrates from B to O with loss of OH⁻. This 1,2-shift keeps the configuration of the migrating carbon (retention). Repeated three times, then hydrolysis gives three ROH and borate.

Regioselective

Boron goes to the less substituted carbon. (a) Steric: the bulky boron group approaches the less hindered CH end. (b) Electronic: in the transition state C-1 (tertiary) carries a partial positive charge and the hydride is delivered to it; the H is the nucleophilic end of B–H. The OH therefore ends on C-2: 2-methylcyclopentan-1-ol, not 1-methylcyclopentan-1-ol (the Markovnikov product of acid-catalysed hydration).

Stereoselective (stereospecific)

H and B add to the same face (syn addition). The H on C-1 and the B on C-2 are therefore cis, which places the CH₃ (on C-1) trans to B. Oxidation with retention puts OH exactly where B was: OH trans to CH₃. Attack on either face of the planar alkene is equally likely, so both enantiomers form: a racemic mixture of (1R,2R) and (1S,2S)-trans-2-methylcyclopentanol; no cis isomer.

Conclusion. 1-Methylcyclopentene gives trans-2-methylcyclopentan-1-ol (racemic). Regioselective: B, then OH, on the less substituted carbon (anti-Markovnikov). Stereoselective: syn addition of H–B, then replacement of B by OH with retention.

Application. Hydroboration–oxidation is used to make primary alcohols from terminal alkenes (e.g. 1-octanol from 1-octene) without rearrangement; chiral boranes (Ipc₂BH) give optically active alcohols.