Ambident PCN heterocycles: N- and P-phosphanylation of lithium 1,3-benzazaphospholides.

Synthetic and structural aspects of the phosphanylation of 1,3-benzazaphospholides 1(Li), ambident benzofused azaphosphacyclopentadienides, are presented. The unusual properties of phospholyl-1,3,2-diazaphospholes inspired us to study the coupling of 1(Li) with chlorodiazaphospholene 2, which led to the N-substituted product 3. Reaction of 1(Li) with chlorodiphenyl- and chlorodicyclohexylphosphane likewise gave N-phosphanylbenzazaphospholes 4 and 5, whereas with the more bulky di-tert-butyl- and di-1-adamantylchlorophosphanes, the diphosphanes 6 and 7 are obtained; in the case of 7 they are isolated as a dimeric LiCl(THF) adduct. Structural information was provided by single-crystal X-ray diffraction and solution NMR spectroscopy experiments. 2D exchange spectroscopy confirmed the existence of two rotamers of the aminophosphane 5 at room temperature; variable-temperature NMR spectroscopy studies of 6 revealed two dynamic processes, low-temperature inversion at ring phosphorus (DeltaH( not equal)=22 kJ mol(-1), DeltaS( not equal)=2 J K(-1) mol(-1)) and very low-temperature rotation of the tBu(2)P group. Quantum chemical studies give evidence that 2-unsubstituted benzazaphospholides prefer N-phosphanylation, even with bulky chlorophosphanes, and that substituents at the 2-position of the heterocycle are crucial for the occurrence of P-N rotamers and for switching to alternative P-substitution, beyond a threshold steric bulk, by both P- and 2-position substituents.