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 608 kJ mol À1 lower in energy than its singlet diradical valence isomer, the anti-tricyclohexylene, but a tetraphosphorus analogue of the latter has been isolated. According to calculations, the parent amidinium salt is 541 kJ mol À1 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 kJ mol À1 ; provided the right substituents are used, the cyclic form can be isolated. Lastly, among butadiene valence isomers, singlet cyclobutane-1,3-diyls were predicted only as transition states for the inversion of bicyclo[1.1.0]butane. In marked contrast, 1,3-dibora-2,4-diphosphacyclobutane-1,3-diyls can be prepared by valence isomerization of the corresponding butadiene derivatives and isolated under conventional experimental conditions.

2,3‐Dihydro‐1H‐naphtho[1,8‐cd]borinine as a Potent Precursor for Open‐Shell Singlet B‐Heterocycles

“…A biradical bearing two radical centers is usually in the triplet ground state, and the open‐shell character results in high reactivity, which hampers isolation. However, decreasing the open‐shell character is a promising approach towards the construction of isolable biradicals in the singlet ground state . The quest for stable cyclic singlet biradicals has attracted great interest in molecular science and functional materials.…”

Diverse reactions of sterically-protected 1,3-diphosphacyclobutane-2,4-diyls with hydride

“…[14][15][16][17][18] Besides P,C-based 4-membered heterocyclic biradicals, other structurallyrelated cyclic biradicals have also attracted interest. [19][20][21][22][23] In the meantime, we developed a convenient and versatile synthetic method for 1,3-diphosphacyclobutane-2,4-diyls 1. Our synthetic approach utilizes a phosphaalkyne (-C P) bearing a bulky substituent on the carbon atom, such as Mes* (2,4,6-tritert-butylphenyl), and proceeds through cyclic phosphaallyl anion 2 (Scheme 2).…”

Properties of Stable Organic Bond-Stretched Non-Lewis Molecules

“…The quest for stable molecules having a singlet diradical structure led in recent years to the production, crystallization, and characterization of several inorganic species in crystalline form – and in solution . As summarized in an extensive recent review, these species turn out to have considerable charge-transfer character, leading to their designation as diradicaloids. Here, we discuss the properties of some formal organic diradicals that are calculated to be quite stable under ambient conditions.…”