The CH by N replacement effects on the aromaticity and reactivity of phosphinines

Geometries, aromatic character, Mulliken charge distribution, and MO diagra ms of 1,2-aza-, 1,3,2-diaza-, 1,3-aza-, and 1,3,5-diazaphosphinines have be en calculated and compared to those of phosphinine and pyridine. This study reveals that the introduction of nitrogen atoms at the position adjacent t o phosphorus significantly reduces the aromatic delocalization and induces a [1,4] dipolar character through an increase of the positive charge on the P atom. This phenomenon does not occur in 1,3-aza- and 1,3,5-diazaphosphin ines, which exhibit a poor dipolar character. This comparison confirms the high reactivity of 1,3,2-diazaphosphinines toward alkynes. A [4 + 2] cycloa ddition reaction between these two types of diazaphosphinines and acetylene has been modelized. Calculated geometries of the resultant [4 + 2] diazaba rrelene cycloadducts and that of their respective transition states reveal that, especially with 1,3,2-diaza isomers, the cycloaddition proceeds via a disymmetrical pathway that involves the preliminary formation of the P-C b ond.