Investigation of the C–N bond rotation of spirophosphorane carbamates by dynamic NMR and DFT calculation

Spirophosphorane carbamates are a new type of pentacoordinate phosphorus compounds. In order to explore internal rotation, the dynamic 1H NMR, 31P NMR and molecular theory calculations were employed to investigate the rotation barriers of the N–C(O) bond of symmetrical and asymmetrical spirophosphorane carbamates. The Gibbs free activation, ΔG≠, was calculated by Eyring equation. The results showed that the rotation barriers of spirophosphorane carbamates were about 16–18 kcal/mol, and the rotation isomers coexisted at room temperature. Moreover, it was found that the benzyl group attached to amide nitrogen increased the C–N bond rotation barriers of spirophosphorane carbamates. Furthermore, the preferred conformation of spirophosphorane carbamates was presumed by density functional theory (DFT), and the conformation of solid state were also confirmed by X-ray diffraction analysis.

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