Evidence for the Coexistence of Two Bond‐Stretch Isomers in Solution

Abstract

The concept of "bond-stretch isomerism" has been introduced by Stohrer and Hoffmann by using strained tricyclic hydrocarbons (Scheme 1): "In the 2,2,2-system the optimum alignment for through-bond coupling of radical lobes creates the conditions for a new type of isomerism-two stable conformations related by a simple bond stretching. These are Scheme 1. Some key compounds in the debate on bond-stretch isomerism.

Catenation of Two Singlet Diradicals: Synthesis of a Stable Tetraradical (Tetraradicaloid)

“…All these geometric parameters are very comparable to those found in the previously reported diradicals of type A. [9,14] In marked contrast, the PBPB cores of m-4 strongly deviate from planarity (interflap angle between the two PBB units 120.7 and 123.68), and the B-B interatomic distances (1.875 and 1.906 ) are shortened by more than 25 % compared to p-4 ( Figure 3). Moreover, the phenyl ring is perpendicular to the PBPB skeletons.…”

“…However, despite the weakness of the phenomenon, the consequences are of importance since in the solid-state p-4 has an open-form structure whereas m-4 features a bis(bicyclic) structure. In fact, as shown in the preceding paper [14] and in contrast with the parent compounds, the energy difference between the planar form and the bicyclic isomer of substituted PBPB derivatives is only a matter of a very few kcal mol À1 . Indeed, as in the case of the "monomeric" derivative 6, the planar structure of p-4 exists in the solid state but is in equilibrium in solution with the bicyclic structure p-4 B (Scheme 3).…”

The Two Structures of the Hexafluorobenzene Radical Cation C6F6.+

“…[15] Experimentally confirmed cases are rare. [16,17] Other cases for bond-stretch isomerism could be isomers of Jahn-Teller expanded and compressed octahedral metal complexes. But the original findings of unusual compressed MnF 6 3À octahedra turned out to be crystallographical artifacts caused by twinning.…”

Fluctional molecules

“…The number of cases is rather small and two examples here will suffice. The first involves R 4 P 2 B 2 R′ 2 (Figure 4) where one isomer has a BB bond and the other is a stabilized biradical 39. The free energy barrier, determined by temperature‐dependent NMR at 143 K, was about 30 kJ/mol, which gives (assuming Δ G and Δ H are the same) a timescale of about 10 −2 s (see Table 1).…”