Introducing <i>N</i>-Heterocyclic Iminophosphoranes (NHIPs): Synthesis by [3 + 2] Cycloaddition of Azophosphines with Alkynes and Reactivity Studies

Abstract

Azophosphines (Ar-N═N-PR₂) were prepared from <i>N</i>-aryl-<i>N</i>'-(trimethylsilyl)diazenes (Ar-N═N-SiMe₃) and R₂PCl by Me₃SiCl elimination or oxidation of phosphinohydrazines (Ar-NH-NH-PR₂) by 2,5-dialkyl-1,4-benzoquinones. Azophosphines underwent 1,3-dipolar cycloaddition with cyclooctyne and dimethylacetylene dicarboxylate to give <i>N</i>-heterocyclic iminophosphoranes (NHIPs), which are structurally similar to cyclic (alkyl)(amino)carbenes. The cycloaddition reaction is compatible with various phosphorus atom substituents including phenyl (<b>NHIP-1,4,6</b>), isopropyl (<b>NHIP-2</b>), cyclohexyl (<b>NHIP-3</b>), and dimethylamino (<b>NHIP-5</b>) groups. The p<i>K</i>_BH⁺ values of the NHIPs in acetonitrile range from 13.13 to 23.14. On the basis of the Huynh electronic parameter, <b>NHIP-1</b> and <b>NHIP-2</b> have σ-donor strengths comparable with that of 1,8-diazabicyclo[5.4.0]undec-7-ene. <b>NHIP-1</b> underwent facile 1,2-addition with pentafluoropyridine to form a rare fluorophosphorane. The treatment of <b>NHIP-1</b> with triphenylsilane resulted in P-N bond cleavage, accompanied by the reduction of phosphorus(V) to phosphorus(III). A homoleptic, cationic Cu^I-<b>NHIP-1</b> complex was also prepared. The potential utility of π-donating NHIPs was demonstrated by the stabilization of a reactive iminoborane (Cl-B≡N-SiMe₃). The facile scalable synthesis, tunability of steric demands, and basicity of NHIPs suggest that this new heterocycle class may find a wide range of applications in synthetic chemistry.