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

Abstract

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.

Difluorophosphoryl Nitrene F2P(O)N: Matrix Isolation and Unexpected Rearrangement to F2PNO

“…level of theory ( Table S2 in the Supporting Information and Figure 5), which is slightly higher than those reported for (CH 3 ) 2 P(O)N (65.7 kJ mol À1 ) and (CH 3 O) 2 P(O)N (57.7 kJ mol À1 ) at the CBS-QB3 level of theory. [9] For F 2 P(O)N, both the closed-shell singlet and ground-state triplet exhibit C s symmetry, but, due to an intramolecular OÀN interaction in the former (Scheme 2), very different structures were predicted. The main differences are the P À N bond length and the O-P-N bond angle.…”

“…1 40.1 1664 (261) 29.7 45.5 n 1 , A' 860.0 (22) 0.0 0.0 835 (142) 0.0 0.0 n 2 672 (7) 12.9 5.0 n 3 396.9 (4) 1.4 375 (7) 1.9 1.0 n 4 286 (5) 2.3 6.5 n 5 221 ( www.chemeurj.org the O-P-N bond angle is reduced by 32.98 compared to those of the triplet species (Table 3). Substitution of R in R 2 P(O)N by F changes the relative stabilities of the singlet nitrene R 2 P(O)N < RP(O)NR (R = H, CH 3 , CH 3 O), [9] and the rearranged isomer FP(O)=NF is higher in energy than singlet F 2 P(O)N by 48.0 kJ mol À1 , and separated from the latter by an activation barrier of 140.4 kJ mol À1 (TS1 in Figure 5). According to the DFT calculations the barrier for the 1,3-fluorine shift to the electron-deficient N atom [Eq.…”

“…[8] These observations have been rationalized by a recent high-level computational study on the decomposition of phosphoryl ((RO) 2 P(O)N 3 ), phosphinyl (R 2 P(O)N 3 ), and phosphinoyl (R 2 PN 3 ) azides and their corresponding nitrenes. [9] First UV and EPR spectroscopic evidence for triplet diphenoxyphosphoryl nitrene was obtained by laser flash photolysis of the Abstract: Triplet difluorophosphoryl nitrene F 2 P(O)N (X 3 A'') was generated on ArF excimer laser irradiation (l = 193 nm) of F 2 P(O)N 3 in solid argon matrix at 16 K, and characterized by its matrix IR, UV/Vis, and EPR spectra, in combination with DFT and CBS-QB3 calculations. On visible light irradiation (l > 420 nm) at 16 K F 2 P(O)N reacts with molecular nitrogen and some of the azide is regenerated.…”

Kinetic studies of the reaction of phenacyl bromide derivatives with sulfur nucleophiles

“…On the basis of the hard/soft acid base principle [51] phenacyl bromide is considered as ambident electrophile with hard electrophilic carbonyl carbon (calculated charge density=0.266 by using ab initio calculations, GAUSSIAN 2003) and soft electrophilic methylene carbon (CH 2 , calculated charge density=0.010 by using ab initio calculations, GAUSSIAN 2003 (M. J. Frisch et al, Gaussian, Inc., Wallingford, Connecticut, USA)). [21,22,26] According to the rule which states that hard acids (electrophiles) prefer to bond to hard bases (nucleophiles), and soft acids (electrophiles) prefer to bond to soft bases (nucleophiles), there is an extra stabilization if both the acid and base are hard, or if both the acid and base are soft. Therefore, the soft nucleophiles thioglycolic acid and thiophenol will preferably attack the methylene carbon atom rather than the carbonyl carbon ( Figure 3).…”

Carbenes and Nitrenes

“…122 Phosphinyl-and phosphorylnitrenes have triplet ground states, whereas phosphinoylnitrenes have a singlet ground state. The Curtius-type rearrangement is unlikely to occur since competing bimolecular reactions have lower activation barriers.…”