Phosphininium salts

Phosphinines are Hiickel aromatic systems even though their properties show pronounced differences to those of the homologous pyridines. Thus, for example, phosphininium salts are not stable (in contrast to pyridinium salts), whereas the formation of phosphininyl anions and radicals as reactive intermediates is known (neither species has been described in the case of pyridine). 

TetraphenyIphosphorinium tetrachloroaluininate (38) the first phosphorinium (phosphininium) salt analogous to the pyridinium salts is obtained by treating 1-fluoro-1,2,4,6--tetraphenyl-A -phosphorin (37) with aluminium trichloride in methylene dichloride at -78°. Salt (38) on treatment with MeOH, EtOH, PhLi, or Cl affords the corresponding derivative (37 F=MeO, EtO, Ph, or Cl) (T.N. Dave, Kaletsch, and Dimroth, Angew. Chem., 1984, 96, 984). 

In 2008, protonation of 2,4,6-tri-ter-butylphosphinine 61 (as well as the methylation and silylation of 1,3,5 triphosphabenzene) was achieved by using the toluenium salt of a carborane anion [40], The X-ray crystal structure of the corresponding phosphininium salt 62 has been recorded and it was shown that, upon protonation, the phosphorus atom gains an hypervalent character (Scheme 14). 

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