Synthesis of tetraphenylammonium, a phantom ion whose existence had not been confirmed

[Background]

Newswise – Since the benzene ring is a representative constituent of organic compounds, a structure consisting only of a common element plus the benzene ring is considered one of the most fundamental chemical skeletons. Due to their importance, the chemical synthesis of such molecules has been studied since the early days of organic chemistry. For example, the structure in which four benzene rings are bonded to a representative element (boron, carbon, aluminum, silicon or phosphorus) from groups 13 to 15 in the periodic table (Fig. 1) was synthesized more than 70 years ago, and the oldest synthetic report dates from 137 years old. These backbones are collectively called “tetraphenyl” structures by adding “tetra”, meaning “4”, to “phenyl”*1), meaning that the structure contains 4 benzene rings. When the central element is nitrogen, ammonium, NH . is4, is considered the mother ion. Such a compound is called tetraphenylammonium. This compound, basically an ion, has a very simple chemical structure that even a novice in organic chemistry can easily imagine. Nevertheless, it has proved very difficult to artificially create this structure and no synthetic reports with clear structure identification have been published. In addition, since it has not been discovered in nature, it has not been clear until now whether tetraphenylammonium can exist at all. There have been publications that presume its existence and only mention its use without describing its synthesis or acquisition method. Composite databases contain only the chemical structure. So this ion is sometimes referred to as if it were already known. In reality, however, no one has observed it, turning tetraphenylammonium into a “ghost ion”.

[Results]

In this study, a research team from the Faculty of Pharmaceutical Sciences, Kanazawa University enabled the synthesis of tetraphenylammonium by establishing a novel synthetic strategy. The critical point in the synthesis of tetraphenylammonium is the addition of the fourth phenyl group to the nitrogen atom to which already three phenyl groups are attached. It was thought to be difficult to achieve this synthesis using conventional techniques. In the present study, therefore, the research team applied a technique called radical coupling *2) and used a strategy to make the radical cation react 1 prepared from a triphenylamine derivative containing the phenyl radical 2 (Figure 2). As a result, although the yield was only 0.1%, the research team managed to achieve the desired chemical conversion. In such radical linkages, highly reactive radicals form bonds with each other, which has the advantage of allowing bond formation that could not be achieved by other methods. On the other hand, it has the disadvantage that it is difficult to control the selectivity because the reactivity is too high, leading to various side reactions. Therefore, in this synthesis, to suppress the side reaction of bond formation on the carbon of radical cation as much as possible 1, the research team also devised the introduction of protective groups*3) that cause steric nuisance. Finally, a total of five steps of chemical conversion of a known triphenylamine derivative, the starting material for the synthesis, were carried out through the introduction of the protecting groups, radical coupling and subsequent removal of the protecting groups, resulting in tetraphenylammonium.

Based on the data obtained from various instrumental analyses, the structure of tetraphenylammonium was confirmed. X-ray crystallography*4) revealed that the bond length between the nitrogen atom and the phenyl group carbon atom in this ion is only 1.529 (Fig. 3). Since this bond length is shorter than that of a tetraphenyl structure containing another element (boron, carbon, aluminum, silicon or phosphorus), it is clear that the nitrogen atom of tetraphenylammonium is in a more spatially constrained environment than other elements. This three-dimensional obstruction is considered to be one of the factors that complicate the construction of this skeleton. Furthermore, our results also revealed that tetraphenylammonium has high stability to withstand strongly acidic and basic conditions.

[Future prospects]

The present study has shown that tetraphenylammonium does indeed exist and can be chemically synthesized. If large-scale synthesis of this ion and its derivatives is realized in the future, it could potentially be applied in various fields of research as an organic cation with high chemical stability. In addition, the radical coupling strategy used in this study may be applicable to the synthesis of other related ammonium compounds that could not be made until now.

[Glossary]

*1) Phenyl
A phenyl group is the name of an atomic group/substituent formed from benzene (C6huh6) by removing one hydrogen atom. This represents the simplest and most basic structure of an aromatic carbon group. When a hydrogen atom in an ion or other molecule is replaced with a phenyl group, the prefix “phenyl” is used.

*2) Radical clutch
A reaction in which two radicals form a bond. Electrons in a molecule generally form pairs while radicals have highly reactive unpaired electrons. Thus, a radical has a high propensity to undergo a reaction to form a chemical bond with another radical to create a shared electron pair. For the synthesis in this study, radical cations, positively charged radicals, were used in the reaction.

*3) Secure group
Substituents introduced temporarily to suppress unwanted reactions that may occur at other functional groups or positions in the molecule during a given chemical transformation. Protecting groups allow selective chemical conversions. However, the protecting group must be removed in a later step.

*4) X-ray crystallography
An analytical method that uses the diffraction of X-rays to determine the structure of molecules in the crystalline state.


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