Past exam of the chemistry B course of the University of Cambridge / 2024 / Paper 1 / B4 / c / Solution

The reacting alkene pi bond and two Os=O units undergo a concerted cycloaddition to give a five-membered cyclic osmate ester. Both new carbon–oxygen bonds form on the same face. Hydrolysis and proton transfer cleave the Os–O attachments, releasing the vicinal diol and a reduced osmium-containing species.

This is syn dihydroxylation of the terminal alkene. The terminal carbon becomes $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$, and the adjacent carbon becomes $\mathrm{CH}(\mathrm{OH})$; the remote vinyl chloride remains as drawn. The protected region denoted by $R$ is a spectator for this local mechanism. No explanation of alkene chemoselectivity is required here. Since no terminal reoxidant is specified, do not invent a catalytic regeneration step for Os(VIII); osmium reduction and product release suffice for the stated transformation.