Organophosphorus zwitterions engaged in a conjugated macrocycle on fullerene

Abstract

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 C₆₀ 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, β-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_N2 reaction as a key step instead of the hitherto considered carbene pathway.

Fig. 1. Synthesis of 1-phosphonium-5-oxabetaine.

a Reaction of 1 with PMe₃. b ¹H (500 MHz) and ³¹P (202 MHz) NMR spectra (ODCB-d₄).

Fig. 2. Single-crystal X-ray structures.

a Structure of ^fA-2. b Packing mode of 2. c Structure of 3d. d Selected bond lengths (units in Å). Thermal ellipsoids are shown in 50% probability. The encapsulated molecules, solvent molecules, aryl groups, and another independent molecule (^fC-2) are omitted for clarity. The bond lengths shown with gray color belong to ^fC-2.

Fig. 3. Plausible mechanism.

The ΔG (kcal/mol at 298 K) values were calculated at the M06-2X/6-31 G(d,p) level of theory.

Fig. 4. Electrochemical and absorption properties.

a Cyclic voltammograms of 1 and 3c (1 mM in ODCB, 0.10 M n-Bu₄N ∙ BF₄, 100 mV/s). b UV–vis-NIR absorption spectra of 1, 2, and 3c (50 μM in toluene). The inset shows an expanded view of long wavelength region with calculated oscillator strengths, which were plotted at corresponding transition energies calibrated by a factor of 0.72 (TD CAM-B3LYP/6-31G(d)//B3LYP/6-31G(d)).