Computational Study of the Curtius-like Rearrangements of Phosphoryl, Phosphinyl, and Phosphinoyl Azides and Their Corresponding Nitrenes

The free energies of reaction (Δ<i>G</i>) and activation (Δ<i>G</i><sup>⧧</sup>) 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 Δ<i>G</i><sup>⧧</sup> values for the Curtius-like rearrangements of dimethylphosphinyl and dimethylphosphoryl azides were 45.4 and 47.0 kcal mol<sup>-1</sup>, respectively. For the closed-shell singlet dimethylphosphinyl- and dimethylphosphorylnitrene, the CBS-QB3 Δ<i>G</i><sup>⧧</sup> values for the rate-limiting step of the Curtius-like rearrangement were 22.9 and 18.0 kcal mol<sup>-1</sup>, 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 organophosphorus compounds can be investigated using phosphorylnitrenes as photoaffinity labels. Dominant bimolecular reactivity is a desirable quality for a photoaffinity label to possess, and thus, the resistance of phosphorylnitrenes to intramolecular Curtius-like rearrangements increases their usefulness as photoaffinity labels.