Synthetic routes to five-membered aromatic phosphorus heterocycles

TLDR: Barrans, Malavaud, Majoral and Majoral as mentioned in this paper showed that the two-coordinate phosphorus participates much better in cyclic delocalization than the three-coordinated phosphorus.

Abstract: Only late in the development of heterocycles, phosphorus has been recognized as an effective member of aromatic systems. Here, the two-coordinate phosphorus participates much better in cyclic delocalization than the three-coordinate phosphorus. It contributes one electron to the π-system. To constitute a five-membered 6π phosphorus heterocycle, along with N-, CR-, and eventually more P-members, it thus takes one NR- (or S-) member contributing two electrons. This gives rise to the azaphospholes and thiazaphospholes. Since the first representatives were found in 1970 by Melnikov and Shvetsov-Shilovskii in Moscow, the number of known systems has grown to eight including those with annelated rings. They were prepared primarily in the laboratories of Barrans, Malavaud and Majoral in Toulouse, Issleib in Halle, and our own. We can now present ten more new mono- and bicyclic systems. A review is given of the synthetic routes we employed and investigated. They comprise cyclocondensations and cycloadditi...