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

Azophosphines (Ar-N=N-PR2) were prepared from N-aryl-N'-(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 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 pKBH+ 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 sigma-donor strengths comparable with that of 1,8-diazabicyclo[5.4.0]undec-7ene. 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 CuI-NHIP-1 complex was also prepared. The potential utility of pi-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.

Automated summary

This study presents the preparation and cycloaddition reactions of Azophosphines. The results show that these compounds have good adaptability to phosphorus atom substituents and can generate structures similar to cyclic (alkyl)(amino)carbenes. The NHIPs exhibit strong sigma-donor strengths and basicity, and display unique reactivity in certain reactions. The tunability of steric demands and scalability of these compounds suggest that they may have a wide range of applications in synthetic chemistry.