Synthesis and characterization of a 2-chloro-1,3,2-diaza-phospholidine-4,5-diimine

Georgios Mourgas,Dietrich Gudat,Daniela Förster,Martin Nieger,Zoltan Benkö,Girolamo Cirrincione

doi:10.3998/ark.5550190.0013.206

Abstract

An oxalamidine 10a was used as precursor for the synthesis of 2-chloro-1,3,2-diaza- phospholidine-4,5-diimine 7a . Both products were characterized by single-crystal X-ray diffraction studies which revealed for 7a the presence of an unusually short P-Cl bond distance. In contrast to other known 2-chloro-1,3,2-diazaphospholidines, 7a fails to react with gallium trichloride, trimethylsilyl trilate, or silver triflate via heterolytic P-Cl bond fission and formation of an N-heterocyclic phosphenium ion. A computational study suggests that this unusual behavior is due to the fact that the exocyclic imino-groups stabilize the P-Cl bond in 7a by diminishing the hyperconjugation between the lone-pairs of the ring nitrogen atoms and the σ*(P-Cl) orbital and destabilize at the same time the hypothetical phosphenium cation.

Highlights

Five-membered phosphorus-nitrogen heterocycles like 1,3,2-diazaphospholidines 1 1 and 1,3,2diazaphospholenes 2 2,3,4 (Scheme 1) are of central importance for the synthesis of stable phosphenium ions 3, 4 with a cationic, two-coordinate phosphorus atom 5,6,7 which are isoelectronic analogues to neutral diaminocarbenes 5, 6, respectively
Whereas cations 3 with CC-saturated heterocyclic rings were among the first phosphenium ions ever prepared,[8] the interest in the CC-unsaturated N-heterocyclic phosphenium ions (NHP) 4 2,4 developed only much later in the wake of the rise of the chemistry of stable N-heterocyclic carbenes (NHC).[9]
Cations 4 have alternatively been accessed via a [1+4]-cycloaddition route starting from a diazabutadiene and a transient P(I) fragment generated via disproportionation of PI3, or via reduction of PCl3 with SnCl2, respectively.[10]. In view of this approach we became interested in the synthesis of derivatives 7 featuring both a five-membered phosphorus-nitrogen heterocycle and a diazabutadiene moiety

Summary

Introduction

Five-membered phosphorus-nitrogen heterocycles like 1,3,2-diazaphospholidines 1 1 and 1,3,2diazaphospholenes 2 2,3,4 (Scheme 1) are of central importance for the synthesis of stable phosphenium ions 3, 4 with a cationic, two-coordinate phosphorus atom 5,6,7 which are isoelectronic analogues to neutral diaminocarbenes 5, 6, respectively. (1.284(3) Å) and C2–N3 distances (1.361(2) Å) represent typical double and single bonds, and their large difference indicates a quite low degree of conjugation within the amidine NCNmoiety.[14] The observation of broadened resonances in room temperature 1H NMR spectra suggests that in solution, as in the case of other oxalamidines,[11] a mixture of several rapidly equilibrating prototropic and conformational isomers is present.

Results

Conclusion