Organophosphorus zwitterions engaged in a conjugated macrocycle on fullerene

Organophosphorus zwitterions are one of the most important but elusive intermediates for carbon-carbon bond formation in synthetic chemistry and biology. However, a lack of isolated examples due to their lability has hampered in-depth understanding of structures and their reaction mechanisms. In this study, we crystallographically reveal the solid-state structure of a phosha-Michael adduct engaged in a cage-opened C(60)skeleton, which is formed as a kinetic product. This compound exhibits dark brown colour in solution with an intense absorption band that extends to 1000 nm, reflecting intramolecular charge transfer transitions. From the 1,2-dicarbonyl moiety on the conjugated orifice, beta-oxo-phosphorus ylide is formed as a thermodynamic product. The reaction mechanism that has long been disputed is examined by experimental and theoretical studies, showing a pathway which includes an S(N)2 reaction as a key step instead of the hitherto considered carbene pathway. Organophosphorous zwitterions are important intermediates in organic synthesis but can be unstable and difficult to characterise. Here, two analogues of such intermediates stabilised by conjugation to an open-cage fullerene derivative allow for characterisation by single-crystal X-ray diffraction, with mechanistic implications.