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

The Staudinger-model reaction H3P (1) + HN3 (2) → H3P=NH (5) + N2 (6) has been investigated at the CCSD(T)/6-31G**//MP2(Full)/6-31G* level. Primary products formed in this reaction are the phosphazides H3P=N–N=NH (3) which exist as trans and cis isomers. In contrast to some previous assumptions, cis -3is 8.2 kcal mol−1 more stable than trans -3 but decomposes rather easily into the expected products H3P=NH and N2. This decomposition can be effectively hampered by intramolecular donor–acceptor interactions as shown by calculations on model compounds as well as by experiments. Thus the reaction of a methylene-σ3,λ3-phosphanyl-σ5,λ5-phosphorane with PhN3 led to a new four-membered heterocycle containing a thermally remarkable stable cis-phosphazide moiety.