Beyond the Limits of Atropochirality: Design of Highly Conformationally Restrained Biaryls with Bridgehead Phosphine Oxide.

In the last three decades, reacting sterically congested ortho-substituted arenes to form atropochiral biaryls is an appealing venture and a challenging subject that has garnered significant attention. Therefore, there is interest in developing methods to prepare these compounds. In this study, we presebt an efficient approach to produce a new class 2,2' disubstituted biaryls bridgehead phosphine oxides with an unusual topology and exceptional conformational stability. Our methodology demonstrates that, depending on the substitution pattern on the aryl moieties, the methanophosphocine backbone could be rigid enough to observe a double atropochirality, resulting in an under covered class of molecules. Notably, our studies revealed that replacing only one hydrogen at the ortho position by a fluorine atom led to sufficiently restricted rotation at temperatures below 80 °C, extending "far away" the limits of atropostability. Finally, our investigations which employed variable-temperature NMR spectroscopy and DFT calculations, yielded unique insights into the isomerisation mechanism, indicating that the two biaryl motifs are fully independent in spite of their proximity.