Atropisomerisation in sterically hindered α,β-disubstituted cyclopentenones derived from an intermolecular cobalt(0)-mediated Pauson-Khand reaction

4-(2-Phenylethynyl)-2H-chromen-2-one reacts with norbornene and Co ₂(CO)₈ in an intermolecular Pauson-Khand reaction by focused microwave dielectric heating. Two regioisomeric products are formed; the electron-deficient coumarin moiety preferentially occupies the β-position of the cyclopentenone ring system, whereas the phenyl occupies the α-position. The sterically hindered α,β-(2,3)-disubstituted cyclopentenone regioisomeric products exhibit pronounced atropisomerisation, and the magnitude of the energetic barrier to interconversion between these atropisomers is dependent on the relative position of the coumarin moieties. Interconversion is slow when the coumarin is found in the α-position, whereas interconversion is relatively fast when found in the β-position. © 2010 The Royal Society of Chemistry