The free energies of reaction (ΔG) and activation (ΔG‡) were determined for the Curtius-like rearrangement of dimethylphosphinoyl, dimethylphosphinyl, and dimethylphosphoryl azides as well as the corresponding singlet and triplet nitrenes by CBS-QB3 and B3LYP computational methods. From CASSCF calculations, it was established that the closed-shell configuration was the lower energy singlet state for each of these nitrenes. The triplet states of dimethylphosphinyl- and dimethylphosphorylnitrene are the preferred ground states. However, the closed-shell singlet state is the ground state for dimethylphosphinoylnitrene. The CBS-QB3 free energy of activation (ΔG‡) for the Curtius-like rearrangements of dimethylphosphinyl and dimethylphosphoryl azides was 45.4 and 47.0 kcal mol−1, respectively. For the closed-shell singlet dimethylphosphinyl- and dimethylphosphorylnitrene, the CBS-QB3 ΔG‡ for the rate-limiting step of the Curtius-like rearrangement was 22.9 and 18.0 kcal mol−1, respectively. It is unlikely that the nitrenes will undergo a Curtius-like rearrangement because of competing bimolecular reactions that have lower activation barriers. The pharmacology of weaponized organophorphorus compounds can be investigated using phosphorylnitrenes as photoaffinity labels. Dominant bimolecular reactivity is a desirable quality for a photoaffinity label to possesses, and thus, the resistance of phosphorylnitrenes to intramolecular Curtius-like rearrangements increases their usefulness as photoaffinity labels.
Keywords: Azide, Phosphoryl, Phosphinyl, Phosphinoyl, Photoaffinity label, Nitrene, Curtius