Thermodynamic Investigations of the Staudinger Reaction of Trialkylphosphines with 1-Adamantyl Azide and the Isolation of an Unusual s-cis Phosphazide

The reaction of PR3 (R = Cy, iPr) with 1-adamantyl azide (N3Ad) in benzene results in an equilibrium of the starting material and the phosphazide R3PN3Ad. Thermodynamic and kinetic measurements were taken of the reaction of PiPr3 with N3Ad and yielded ΔH = −18.7 ± 1.0 kcal mol−1, ΔS = −52.5 ± 2.0 cal mol−1 K−1, ΔH⧧ = +12.0 ± 1.0 kcal mol−1, and ΔS⧧ = −25.3 ± 1.2 cal mol−1 K−1. The phosphazides, R3PN3Ad, do not readily lose N2 for R = Cy and iPr; however, the reaction of PMe3 with N3Ad results in a smooth formation of the phosphazene Me3PNAd with N2 loss. Calorimetric investigations of this reaction led to an estimation of ΔH = −40 ± 3 kcal mol−1 for the loss of N2 from the intermediate phosphazide R3PN3Ad and also yield an estimation of 72 ± 5 kcal mol−1 for the bond dissociation energy of the PN bond in R3PNAd. The X-ray crystal structure of Cy3PN3Ad is reported.