= 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.
\b[This is syn <dihydroxylation> of the terminal <alkene>.] The terminal <carbon> becomes $\mathrm{CH_2OH}$, and the adjacent <carbon> becomes $\mathrm{CH(OH)}$; 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.
Back to article page
