Synthesis and X-ray crystal structures of silacalix[n]phosphinines: The first sp(2)-based phosphorus macrocycles

The synthesis of silacalix[n]phosphinines (n=3, 4) is presented. Three stra tegies have been investigated. Firstly, the thermal condensation of one equ ivalent of a 1,3,2-diazaphosphinine 1 with one equivalent of the diyne (PhC C)(2)SiMe2: this leads to a mixture of oligophosphinines. A second strategy involves the reaction of a bis(phosphinine) 6 with one equivalent of 1 and with trimethysilylacetylene to give the tetrakis(phosphinine) 10. More con vincing results were obtained from the third approach that first requires t he preparation of a precursor 8, which contains a phosphinine as the centra l unit and two 1,2-azaphosphinine subunits. The reaction of 8 with one equi valent of diyne, under high dilution conditions, leads to the formation of the dimethylsilacalix[3]phosphinine 3, which was isolated and characterized by single-crystal X-rap crystallography. The macrocycle 3 is fluxional in solution and adopts a partial cone conformation in the solid state. The sam e strategy was extended to the synthesis of the dimethylsilacalix [4]phosph inine 4, which shows fluxional behavior in solution and adopts an opened-ou t partial cone conformation in the solid state. In a similar manner, the sy nthesis of these macrocycles was extended to that of mixed derivatives such as dimethylsilacalix[4]-1,3-phosphinines-2,4-thiophenes 15 and furans 16. X-ray structure analyses reveal that, like 4, these macrocycles adopt a ope ned-out partial cone conformation in the solid state.