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

Azophosphines (Ar-N═N-PR 2 ) were prepared from N -aryl- N '-(trimethylsilyl)diazenes (Ar-N═N-SiMe 3 ) and R 2 PCl by Me 3 SiCl elimination or oxidation of phosphinohydrazines (Ar-NH-NH-PR 2 ) by 2,5-dialkyl-1,4-benzoquinones. Azophosphines underwent 1,3-dipolar cycloaddition with cyclooctyne and dimethylacetylene dicarboxylate to give N -heterocyclic iminophosphoranes (NHIPs), which are structurally similar to cyclic (alkyl)(amino)carbenes. The cycloaddition reaction is compatible with various phosphorus atom substituents including phenyl ( NHIP-1,4,6 ), isopropyl ( NHIP-2 ), cyclohexyl ( NHIP-3 ), and dimethylamino ( NHIP-5 ) groups. The p K BH + values of the NHIPs in acetonitrile range from 13.13 to 23.14. On the basis of the Huynh electronic parameter, NHIP-1 and NHIP-2 have σ-donor strengths comparable with that of 1,8-diazabicyclo[5.4.0]undec-7-ene. NHIP-1 underwent facile 1,2-addition with pentafluoropyridine to form a rare fluorophosphorane. The treatment of NHIP-1 with triphenylsilane resulted in P-N bond cleavage, accompanied by the reduction of phosphorus(V) to phosphorus(III). A homoleptic, cationic Cu I - NHIP-1 complex was also prepared. The potential utility of π-donating NHIPs was demonstrated by the stabilization of a reactive iminoborane (Cl-B≡N-SiMe 3 ). 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.