Synthesis, structure, dynamics and first atropisomer-selective cleavage of the chromium tricarbonyl complex of a lactone-bridged biaryl

The regioselective complexation of the unsymmetric lactone-type bridged biaryl 1 with a chromium tricarbon- ylfragment is reported. By temperature dependent NMR of the resulting complex 3, the activation barrier of the helix inversion was determined. This isomerization process is of fundamental importance for the atroposelective opening of the lactone bridge. In the crystal, by contrast, only one diastereomer exists, as determined by X-ray diffraction study. A first experiment on the stereoselective ring opening using the achiral H-nucleophile sodium borohydride leads to only one detectable atropo-diastereomer and demonstrates the great potential utility of chromium tricarbonyl complexes in directed biaryl synthesis and for mechanistic studies of the stereochemically intriguing lactone opening step.