Further Investigations in the Field of Penta- and Hexaaryl Phosphorus Compounds

4 Further Investigations in the Field of Penta- and Hexaaryl Phosphorus Compounds 

With the accessibility of the fairly stable pentaphenylphosphorane (12) and its even more stable homologues, there arose the intriguing question of the stereochemistry of such species. The first speculations were of course based on a trigonal bipyramidal model, since this structure type had already been proved for phosphorus penta-halides21). It was not possible, however, to differentiate in chemical reactions between the unlike ligands in compounds such as tetraphenyl(triphenylmethyl)-phosphorane (22) and tetraphenyl-p-tolylphosphorane 33) or in pentaphenylphosphorane derivatives appropriately labeled with deuterium or 14C 39). 

Here again a note on nomenclature is needed. If compounds of type 25 are considered as substituted phosphoranes, names such as the one given above are appropriate. If, on the other hand, their heterocyclic nature stays in the foreground, the correct names (as used by Chemical Abstracts) are such as 5-phenyl-5,5 -spirobi[5H-dibenzophosphole]. Both naming procedures are consistent with IUPAC rules and are applicable to similar arsenic, antimony- and bismuth-heterocycles as well (see later chapters). 

Since the earlier methods could not be used for preparing 25 and similar compounds, a more general and also more convenient synthesis for pentaarylphosphoranes was developed. This consisted in the reaction of quatemized triarylphosphine imines with two equivalents of aryllithium 40 42). 

Obviously, in the precursor immonium compounds 26,27 the phosphorus bears enough partial positive charge to allow easy attack by aryl anions. Along similar lines of argument an even simpler synthetic procedure could be found in using analoguous p-toluenesulfonylimine (tosylimine) derivatives 2943), which had the appropriate polarization of the P=N double bond already built in4.  

---