Synthesis of StableN′,N′,N′′,N′′-Tetramethylguanidine-Substituted σ4(P)- and σ3(P)-Organophosphorus Compounds withN-Protonated P–N Bonds - The First σ3-Phosphorus-Substituted Ammonium Salts⋆

Abstract

The dependence of the protolytic decomposition rate of phosphorus amides on the substituents at phosphorus is decribed. On treatment with HCl, the N′,N′,N′′,N′′-tetramethylguanidine(= TMG)-substituted σ4(P) compounds 5 and 9 formed salts that were protonated at the imino nitrogen atoms and were completely stable in the solid state, and surprisingly stable in solution. Even with a large excess of HCl, only the imino nitrogen atom underwent protonation, with formation of HCl2– as a counter anion, without cleavage of the P–N bond. The basicity of the imino nitrogen atom also protected σ3(P) species from electrophilic attack, and the ionic compounds 12, 14, and 20c were formed, providing examples of the first stable σ3(P) amides with one or two protonated P–N bonds. Such unusual stability is associated with steric protection by the Ph3C (= trityl) group and charge delocalization over the TMG-moiety. In contrast, MeP(TMG)2 was unstable towards HCl, whereas treatment with HSbF6 led to the phosphonium salt 24, where protonation had occurred at phosphorus. The same result was obtained when triphenylmethylphosphonous dichloride 1 was allowed to react with 3 equivalents of HTMG, forming the stable phosphonium salt 19, soluble in water, which was converted to the bicationic species 20c upon treatment with HCl. – The basicity of the tritylated phosphorus compounds was found to increase in the order 5 < 2 < 19 < 13 HTMG < 21; the basic centre is the phosphorus atom in 21 and the imino nitrogen atom in all other compounds. The fluorinating agent Et3N · 3 HF caused rapid conversion of compound 6to 25, the nucleophilic attack of fluoride ion at phosphorus could not be prevented by the stabilizing effects mentioned. – All compounds were investigated by 1H- and 31P-NMR spectroscopy. The crystal structures of compounds 5, Ph3CP(=O)-(H)TMG, and 6, [5 · 2 HCl], were determined; 6 is protonated at the imino nitrogen, leading to a longer P–N bond [5: 161.6(2), 6: 168.7(2) pm], and the counterion is HCl2–. In the structure of 20c, [Ph3CP(TMG)2] · 3 HCl, the cation is protonated at both imino N atoms and the counterions are Cl– and HCl2– (one of each).