Recent Advances in the Chemistry of Anellated Azaphospholes

Azaphospholes are 6π aromatic heterocycles incorporating at least one σ2, λ3-phosphorus atom and one σ3, λ3- 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 1H, 13C and 31P 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 σ2, λ3-P atom have been obtained. A variety of coordination compounds formed by σ- coordination of the σ2, λ3-P atom have been prepared.