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

[Background]

Newswise – Since the benzene ring is a representative constituent of organic compounds, a structure consisting of only one 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 beginnings of organic chemistry. For example, the structure in which four benzene rings are linked to a representative element (boron, carbon, aluminum, silicon or phosphorus) of groups 13 to 15 of the periodic table (Fig. 1) was synthesized more than 70 years ago. , and the oldest synthetic report is 137 years old. These backbones are collectively referred to as “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, NH4+, is considered the parent ion. Such a compound is called tetraphenylammonium. This compound, actually an ion, has a very simple chemical structure that even a beginner in organic chemistry can easily imagine. Nevertheless, it has proven very difficult to artificially create this structure, and no synthetic report with a clear identification of the structure has been published. Also, since it has not been discovered in nature, it has not been clear until now if tetraphenylammonium can exist. Publications have appeared that assume its existence and only mention its use without describing its synthesis or mode of acquisition. Compound databases only contain the chemical structure. Thus, this ion is sometimes referred to as if it were already known. However, in reality, no one has actually observed it, making tetraphenylammonium a “ghost ion”.

[Results]

In this study, a research team from the Faculty of Pharmaceutical Sciences of Kanazawa University enabled the synthesis of tetraphenylammonium by establishing a new synthetic strategy. The critical point in the synthesis of tetraphenylammonium is the addition of the fourth phenyl group to the nitrogen atom which already has three phenyl groups attached. This synthesis was thought to be difficult to achieve with conventional techniques. In the present study, the research team therefore applied a technique called radical coupling*2) and used a reaction strategy of the radical cation 1 prepared from a triphenylamine derivative with the phenyl radical 2 (Fig. 2). As a result, although the yield was as low as 0.1%, the research team was able to achieve the desired chemical conversion. In such radical couplings, the highly reactive radicals form bonds between themselves, which has the advantage of allowing the formation of bonds which could not be achieved by other methods. On the other hand, it has the drawback of being difficult to control in terms of selectivity because the reactivity is too high, leading to various side reactions. Therefore, in this synthesis, in order to suppress the side reaction of bond formation on the carbon of the radical cation as much as possible 1, the research team also imagined the introduction of protective groups*3) which cause steric hindrance. Finally, a total of five chemical conversion steps of a known triphenylamine derivative, the starting material for the synthesis, were carried out through the introduction of the protecting groups, the radical coupling and the subsequent removal of the protecting groups, leading with 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 carbon atom of the phenyl group contained in this ion is only 1.529 Å (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 obvious that the nitrogen atom of tetraphenylammonium is in an environment more spatially crowded than the other elements. This three-dimensional discomfort is considered to be one of the factors that make it difficult to build this skeleton. Moreover, our results also revealed that tetraphenylammonium has high stability to withstand strong acidic and basic conditions.

[Future prospects]

This study demonstrated that tetraphenylammonium does exist and can be synthesized chemically. If the large-scale synthesis of this ion and its derivatives is carried out in the future, it could potentially be applied in various research fields as an organic cation with high chemical stability. Moreover, the radical coupling strategy used in this study may be applicable to the synthesis of other related ammoniums that could not be made so far.

[Glossary]

*1) Phenyl
A phenyl group is the name of an atomic group/substituent formed from benzene (C6H6) by removal of a hydrogen atom. This represents the simplest and most basic structure of an aromatic carbon group. When a hydrogen atom contained in an ion or another molecule is replaced by a phenyl group, the prefix “phenyl” is used.

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

*3) Protection group
Substituents that are temporarily introduced to suppress unwanted reactions that may take place at other functional groups or positions in the molecule during a particular chemical transformation. Protecting groups allow selective chemical conversions. However, the protecting group must be removed at a later stage.

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


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