Insights into the Staudinger Reaction: Experimental and Theoretical Studies on the Stabilization ofcis-Phosphazides

Stabilised phosphazides

“…The stability of s-cis 1 is thought to arise from a favourable electrostatic interaction between the positively charged P atom and the negatively charged N ␥ atom (Scheme 3), as substantiated by the rather short P···N ␥ distance (2.40 Å), and the NBO atomic charges (P: 0.95, N ␥ : −0.55) [6]. The second step of the Staudinger reaction is the cyclisation of the phosphazide s-cis 1 to afford the four-membered cyclic intermediate 2 that subsequently extrudes dinitrogen with almost no energy barrier [8].…”

“…NBO partial charge analysis confirmed a stabilising electrostatic interaction between the P and N ␥ atoms in the transition state for the formation of the s-cis phosphazide s-cis 1, as proposed by Rzepa et al [5] and Grützmacher et al [6]. Interestingly, the reaction via initial nucleophilic attack on N ␥ (TS4) was predicted to proceed along a lower energy pathway than that via formation of the s-trans intermediate.…”

“…The first computational mechanistic studies were carried out simultaneously and independently by Rzepa [5] and Grützmacher [6]. Their work predicts that the Staudinger reaction proceeds according to the mechanism shown in Scheme 2.…”

“…Furthermore, the group of Grützmacher hypothesised that the necessary change in hybridisation at the N atoms required for extrusion of dinitrogen could be hampered kinetically if the N-lone pair was involved in a donor-acceptor interaction, in this case with a suitably positioned electron-deficient P atom [6]. This was demonstrated experimentally and this aspect of the work will be detailed in Section 4.2 of this review.…”

New Trends in Hexacoordinated Phosphorus Chemistry

“…Later, Schevchenko extended the protocol to the reaction of 52 with isocyanates to give 5-membered heterocycles 54a and 54b (R=Me and Et, respectively) [68]. In 1999, he and Grützmacher further demonstrated the reactivity of 52 with azides [69]. The organic azide moiety undergoes a Staudinger reaction with the P(III) atom and the distal nitrogen then takes a bridging position between the two phosphorus atoms to give 55.…”