Let's play with valence isomers: The influence of different main group elements on their relative stability

Abstract

Valence isomers are compounds featuring the same fragments connected in different ways. In this personal account, we describe how we first met by serendipity this kind of isomerism, and how later on we used the specific properties of phosphorus and boron to prepare two types of valence isomers that are not accessible with elements exclusively from the first long row. In the carbon series, benzene is calculated to be 608kJmol-1 lower in energy than its singlet diradical valence isomer, the anrf-tricyclohexylene, but a tetraphosphorus analogue of the latter has been isolated. According to calculations, the parent amidinium salt is 541 kJmol-1 more stable than its three-membered heterocyclic valence isomer, but in the phosphorus series, the difference in energy between the open and closed forms is only a few kJmol-1; provided the right substituents are used, the cyclic form can be isolated. Lastly, among butadiene valence isomers, singlet cyclobutane-l,3-diyls were predicted only as transition states for the inversion of bicyclo[1.1.0]butane. In marked contrast, l,3-dibora-2,4- diphosphacyclobutane-l,3-diyls can be prepared by valence isomerization of the corresponding butadiene derivatives and isolated under conventional experimental conditions. © 2007 The Chemical Society of Japan.