= Solution
An unsaturated Rh(I)–<BINAP> complex undergoes <oxidative addition> of the aldehydic C–H bond, giving a Rh(III) \b[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 \b[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.
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