Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / Section A / 37 / c / Solution

An unsaturated Rh(I)–BINAP complex undergoes oxidative addition of the aldehydic C–H bond, giving a Rh(III) acyl hydride. Coordination of the tethered alkene brings it into the chiral coordination sphere. hydride migration to the internal, tert-butyl-substituted alkene carbon places Rh on the terminal methylene. The resulting acyl–alkyl complex has the connectivity required for a five-membered ring. reductive elimination between its acyl and alkyl carbons closes the C–C bond, releases the cyclopentanone and regenerates Rh(I).

The aldehyde carbon becomes the ketone carbonyl carbon; its hydrogen ultimately reaches the alkene-derived internal carbon. The original tert-butyl substituent remains attached to the internal alkene carbon, which becomes the beta stereocentre of the cyclopentanone. alkene coordination and insertion favour the chiral pocket arrangement with the bulky tert-butyl group in the less crowded region of the BINAP complex. For the stated (S)-BINAP system the observed product is the $R$ beta-tert-butylcyclopentanone, equivalently the product with tert-butyl on the solid-wedge face in the supplied ring orientation. Its priorities are tert-butyl, the shorter carbonyl-side ring path, the other ring path, and hydrogen. The ligand's S label alone is not a general rule that every product stereocentre must be S; the substrate orientation in the catalyst must also be specified. No decarbonylation is needed in the productive hydroacylation cycle.