Keywords: Phosphorus / Iminophosphorane / Staudinger reaction / Heterocycles / Ab initio calculations The Staudinger-model reaction H 3 P (1) + HN 3 (2) Ǟ H 3 P=NH H 3 P=NH and N 2 . This decomposition can be effectively hampered by intramolecular donor-acceptor interactions as (5) + N 2 (6) has been investigated at the CCSD(T)/6-31G**// MP2(Full)/6-31G* level. Primary products formed in this shown by calculations on model compounds as well as by experiments. Thus the reaction of a methylene-σ 3 ,λ 3 -reaction are the phosphazides H 3 P=N-N=NH (3) which exist as trans and cis isomers. In contrast to some previous phosphanyl-σ 5 ,λ 5 -phosphorane with PhN 3 led to a new fourmembered heterocycle containing a thermally remarkable assumptions, cis-3 is 8.2 kcal mol -1 more stable than trans-3 but decomposes rather easily into the expected products stable cis-phosphazide moiety. Since 1919 the reaction between azides and phosphanesThe structures of some phosphazides have been determined experimentally.[3] [4] Most of these compounds con-(Staudinger reaction) has been widely used by phosphorus chemists for the preparation of λ 5 -iminophosphoranes tain a trans-configured phosphazide moiety [3] while the cisconformation [4] has rarely been observed. This led to the (phosphazenes).[1] In the course of the reaction the primary imination products, phosphazides, are formed, which are assumption that the trans conformers are more stable than their corresponding cis isomers. However, at least for the rarely stable under ambient conditions.[2] In a few cases, where R is strongly electron donating, RЈ strongly electron parent model system H 3 PϭNϪNϭNH our calculations show a different picture: cis-3 is more stable than trans-3 by withdrawing, or where both R and RЈ are sterically bulky, remarkable stable phosphazides R 3 PϭNϪNϭNRЈ have 8.2 kcal mol Ϫ1 . The isomerisation of trans-3 to cis-3 proceeds via the transition state TS1 and requires an activation been isolated. [3] [4] Here we wish to report on the first detailed ab initio study of the Staudinger reaction and a new energy of 6.4 kcal mol Ϫ1 . Indeed, the cis isomer, though thermodynamically more stable, decomposes rather easily possibility to stabilize phosphazides. The Staudinger-model reaction H 3 P (1) ϩ HN 3 (2) Ǟ into H 3 PϭNH (5) and N 2 (6). On the reaction coordinate, the transition state TS2 has to be overcome, which is 8.2 H 3 PϭNH (5) ϩ N 2 (6) has been investigated at the CCSD(T)/6-31G**//MP2(Full)/6-31G* level of theory.[5] In kcal mol Ϫ1 higher in energy than cis-3. This reaction can be described as an intramolecular nucleophilic attack of the the first step of this reaction the trans-configured and cisconfigured phosphazides trans-3 and cis-3 are formed, negatively charged nitrogen centre N3 (Ϫ0.55 e) on the positively charged phosphorus centre (ϩ0.95 e). As a result, respectively. Subsequently, decomposition of these intermediates leads to the phosphazene 5 and dinitrogen (6) the PϪN3 distance is shortened from 2.402 Å in cis-3 to 1.880 Å in TS2. The...

SpirophosphorateThe oxidation of 2,4-bis(diethylamino)-1,5-dimethyl-l,5,2,4diazadiphosphorinan-6-one (1 b) with two equivalents of tetrachloro-ortho-benzoquinone (TOB) (2) leads to the cleavage of the original heterocycle and to the formation of a mixture of compounds, including the 1,3,2-diazaphosphetidine 6 ("h6P-") and the spirophosphorane 7. Compound 7 exists as two isomers that undergo slow spontaneous transformation in solution to compound 9, containing a six-coordinate phosphorus atom. When the oxidation of 1 b is carried out in two stages, with the second equivalent of TOB being added after an interval of four days, the cleavage of the original molecule does not occur, and compound 17, containing a seven-membered ring with two phosphorus atoms of opposite formal charge and different coordination number ("h4P+,h6P-") is formed. The structures of 6, ?, 9, and 17 were established by Iow-temperature X-ray analysis. Compound 6 displays crystallographic twofold symmetry; the coordination at phosphorus is octahedral, but distorted by the phosphetidine ring. The phosphorus atom in 7 possesses square pyramidal coordination geometry, the pyramid base being defined by the four oxygen atoms. Compound 9 ("h6P-") crystallized with two molecules of CDC13. Compound 17 crystallized as a dichloromethane solvate. Distorted octahedral and tetrahedral geometries were observed for P1 and P2, respectively. The four-membered ring is planar. The bridging hydroquinone ligand displays much wider 0 -C -C( -0) and C -0 -P angles than the chelating hydroquinone.

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