Cyclobutadienes and phosphaalkynes - Tools for the development of heteroaromatic compounds and their valence isomers [1]

The classical valence isomers of benzene, namely Dewar-benzene, benzvalene, and prismane, are well known. Phospha-, azaphospha-, and diphosphabenzenes as well as the Dewar and benzvalene derivatives with the same combinations of heteroatoms are described in this review. Phospha- and azaphosphaprisma nes are now also known and discussed. Starting materials for these compound s are the kinetically stabilized cyclobutadienes, azacyclobutadienes, as we ll as the short-lived 1,3-diphosphacyclobutadienes which, upon reaction wit h phosphaalkynes, provide an access to the mentioned classes of compounds ( Scheme 1).