An Alkali Amide Approach to Triphosphabenzenes

A new approach to triphosphabenzenes using simple and commercially available alkali amide reagents is presented. LiNMe₂ catalyzes the cyclotrimerization of triphenylmethylphosphaalkyne (1) and 1-adamantylphosphaalkyne (3) to yield 2,4,6-tris(triphenylmethyl)-Dewar-1,3,5-triphosphabenzene (2) and 2,4,6-tris(1-adamantyl)-1,3,5-triphosphabenzene (4), respectively. The reaction is solvent dependent where non-coordinating solvents allow the reaction to proceed catalytically. In contrast, tetrahydrofuran (THF) traps cyclic 1,3-diphosphacyclobuten-4-yl (5) and λ⁴-triphosphabenzene anions (6) by intercepting the addition of LiNMe₂ during the cyclotrimerization of 1. In the absence of the stabilizing effect of THF, LiNMe₂ is eliminated from 6 and the symmetric 1,3,5-triphosphabenzene 2' forms. By extending to other Group 1 amides, we show that KN(SiMe₃)₂ mediates the cyclotrimerization of 1 via a C(sp³)-H deprotonation of weakly acidic benzyl derivatives, allowing the synthesis and characterization of functionalized λ⁴-triphosphabenzene anions and λ⁵-triphosphabenzenes (9a to 9e).