Insights into the Staudinger reaction: Experimental and theoretical studies on the stabilization of cis-phosphazides

The Staudinger-model reaction H3P (1) + HN3 (2) --> H3P=NH (5) + N-2 (6) ha s been investigated at the CCSD(T)/6-31G** // MP2(Full)/6-31G* level. Prima ry products formed in this reaction are the phosphazides H3P=N-N=NH (3) whi ch exist as trans and cis isomers. In contrast to some previous assumptions , cis-3 is 8.2 kcal mol(-1) more stable than trans-3 but decomposes rather easily into the expected products H3P=NH and N-2. This decomposition can be effectively hampered by intramolecular donor-acceptor interactions as show n by calculations on model compounds as well as by experiments. Thus the re action of a methylene-sigma(3),lambda(3)-phosphanyl-sigma(5),lambda(5)-phos phorane with PhN3 led to a new four-membered heterocycle containing a therm ally remarkable stable cis-phosphazide moiety.