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Incoming links: Anti-Markovnikov addition
Past exam of the chemistry B course of the University of Cambridge 2022 Paper 2 B8 c ii Solution 2026-10-20
oxidative addition of an intact Si–H bond initially places H and SiR cis in the metal coordination sphere; this is the local arrangement generated by addition of a sigma bond. hydride and alkene must also attain adjacent sites for intramolecular insertion, and the eventual alkyl and silyl groups must be cis for direct reductive elimination. Subsequent rearrangements should not be confused with the stereochemistry of the bond-addition event itself.
For an alkyl-substituted terminal alkene, insertion puts Co on the terminal carbon and H on the substituted carbon, producing . It avoids the more crowded secondary metal–alkyl arrangement. Replacement of that Co–C bond by C–Si therefore gives the linear product , an Anti-Markovnikov addition outcome. Hydrometallation is syn at the coordinated alkene, although this simple terminal substrate generally does not retain two substituents whose relative stereochemistry could diagnose the complete sequence.
Build the phenethyl grignard reagent, use one portion to make a phenethyl-substituted ester, and add two further equivalents to that ester. Write ; the target is , so all three carbon substituents must be phenethyl, not benzyl.
hydroboration followed by oxidation of styrene gives : use in a suitable ether, followed by alkaline hydrogen peroxide. The Anti-Markovnikov addition regiochemistry is essential; acid-catalysed Markovnikov addition of water would give the wrong alcohol. Convert the primary alcohol to with phosphorus tribromide, then form with magnesium in dry ether. The organomagnesium reagent must be prepared and used without a protic solvent.
carboxylate one portion with carbon dioxide and acidify to obtain , then esterify with ethanol under acid catalysis to give . Treat the isolated ester with at least two equivalents of another portion of and then perform an aqueous acidic work-up:The first addition to the ester gives a ketone after ethoxide elimination; the second addition gives the tertiary alkoxide. Work-up supplies its OH proton. The extra central carbon comes from carbon dioxide, and each original styrene contributes one intact eight-carbon phenethyl unit. The resulting carbon count is , matching the target skeleton.