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 (ΔH^≠=22 kJ mol^?1, ΔS^≠=2 J K^?1 mol^?1) and very low‐temperature rotation of the tBu₂P 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.