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

Abstract

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...

Photoisomerizable Heterodienes Derived from a Phosphine Borane

“…4 More recently, computational studies have provided more insight into the whole process 5. In particular, combined experimental and theoretical studies by Grützmacher and co‐workers suggested that phosphazides can be significantly stabilized when the nitrogen atom in the α position to the phosphorus is coordinated intramolecularly to a Lewis acid 5b. This prompted us to investigate the reaction of azides with phosphine borane derivatives ortho ‐(R 2 P)C 6 H 4 (BR′ 2 ) that we have recently introduced as ambiphilic ligands for transition metals 6.…”

Mechanism of Redox‐Active Ligand‐Assisted Nitrene‐Group Transfer in a ZrIV Complex: Direct Ligand‐to‐Ligand Charge Transfer Preferred

“…[50,51] Most relevantly, the classical Staudinger reaction [52][53][54][55][56] between phosphines and azides is thought to involve a transition state that consists of a fourmembered ring. [57][58][59] By using 15 N-labeled azides, Bergman et al showed that the nitrogen atom that is directly bound to the organic fragment becomes the imido nitrogen that remains bound to the metal upon extrusion of dinitrogen, [22] which is a direct consequence of the metallacyclic transition state, as illustrated in Scheme 5. The fundamentally different electronic structures of the Ta and Zr complexes translate to significantly different mechanisms for dinitrogen extrusion in these two seemingly related complexes.…”

Staudinger Ligation as a Method for Bioconjugation

“…The mechanism of the classical Staudinger reaction has been thoroughly investigated by various experimental [15] as well as computational methods, [16,17] and is known to proceed via several intermediates. [15a, 18] The first step involves a nucleophilic attack of the phosphorus atom of 6 onto the azide moiety of 5 to form the phosphazide intermediate I (Scheme 3).…”