Analogy of Phosphaalkenes and Azaphospholes with their Respective Non-phosphorus Analogues

Phosphaalkenes and azaphospholes both have C=P- moiety; while the former is stabilized kinetically, the latter attains stability thermodynamically. A close examination of the synthesis, structures and reactivity of these two classes of phospha-organic compounds reveals that these compounds exhibit close analogy with their respective non-phospha analogues, namely alkenes and azoles. In this mini-review, the analogies between phosphaalkenes and alkenes on the one hand and between azaphospholes and azoles on the other hand have been explained and highlighted on the basis of some important examples of syntheses, structures and reactions in each case. Some of the reactions of these two classes of compounds which are not given by their carbon-analogues are attributable to a weaker carbon-phosphorus pi-bond and the presence of a lone pair on the phosphorus atom.