Recent advances in the chemistry of anellated azaphospholes

Azaphospholes are 6 pi aromatic heterocycles incorporating at least one sigma(2),lambda(3)-phosphorus atom and one sigma(3),lambda(3)-nitrogen atom. This review covers the synthesis, structural characterization, reactions and theoretical calculations of azaphospholes anellated to a six- or five-membered heterocyclic ring, reported during the last 10-12 years. Three synthetic methods, namely [4+1] cyclocondensation, [3+2] cyclocondensation and 1,5-electrocyclization make a variety of these phosphorus heterocycles available. The aromatic character of these compounds has been confirmed by carrying out density functional theory (DFT) calculations. Mainly H-1, C-13 and P-31 NMR studies have been employed for the structural characterization. Almost all the functionalities present in the azaphosphole ring have been investigated and electrophilic substitution reactions of the 1-unsubstituted compounds and Diels-Alder reactions as well as 1,2-addition reactions to the > C=P-moiety have been accomplished successfully. The stereo- and regioselectivities observed in the Diels-Alder reactions have been explained on the basis of DFT calculations. Cheletropic cycloadducts formed from [4+1] cycloaddition of o-quinones on the sigma(2),lambda(3)-P atom have been obtained. A variety of coordination compounds formed by a coordination of the sigma(2),lambda(3)-P atom have been prepared.