Q1(b). Norbornene and OsO₄: exo-cis diol

Introduction. OsO₄ adds to a C=C in a concerted syn manner through a cyclic osmate(VI) ester; NaHSO₃ then cleaves the Os–O bonds (reductive work-up), so both new C–O bonds come from the same face. In norbornene the two faces of C2=C3 are not alike.

Figure: Attack from the face of the one-carbon bridge C7 (exo); the endo face is screened by the C5–C6 bridge.

1. The exo face of the double bond lies on the side of the single-carbon bridge C7. The endo face lies under the two-carbon bridge C5–C6, whose endo hydrogens screen it. OsO₄ therefore approaches from the exo face.
2. Both OH groups enter from that face: X = cis-exo-bicyclo[2.2.1]heptane-2,3-diol (C₇H₁₂O₂); the two H atoms at C2 and C3 are endo.
3. Stereospecific: syn addition fixes the two OH groups cis. Stereoselective: of the two possible cis diols (exo and endo), the exo diol forms almost exclusively.
4. X has a mirror plane through C7 and the mid-points of C2–C3 and C5–C6: it is a meso compound and optically inactive, even though C1, C2, C3 and C4 are stereocentres.

Conclusion. X = cis-exo-norbornane-2,3-diol, formed by syn addition of OsO₄ from the less hindered exo face; the product is meso.

Application. Exo selectivity of norbornene (epoxidation, hydroboration, dihydroxylation) is the textbook case of steric approach control, used to set stereocentres in prostaglandin syntheses that start from bicyclo[2.2.1] intermediates.