A facile and efficient route to one-pot synthesis of new cyclophanes using vinamidinium salts

In this study, an efficient method for the synthesis of new cyclophanes (5a–f, 6a–g) through the condensation of 1,4-phenylenedimethanamine (3) or 2,3,5,6-tetramethylbenzene-1,4-diamine (4) with 2-substituted vinamidiniums (2a–g) is described. The cyclophane derivatives are obtained in good to excellent yields in the presence of acetic acid in refluxing acetonitrile after 15 h. The structure of new compounds was validated based on their spectral data (1H NMR, 13C NMR, IR) and elemental analysis.


Introduction
The structure of a vast range of macromolecules 1 conrmed that the design and construction of macrocyclic compounds has been one of the most important reasons for improvement in supramolecular science. A well-known group of macrocyclic compounds are cyclophanes and their particular chemistry has attracted the attention of researchers, recently 2 and has been broadly discussed within the eld of modern supra-molecular chemistry. 3,4 Cyclophanes 5-42 are constrained organic molecules consisting of aromatic ring(s) as well as aliphatic unit(s). The aromatic rings support the rigidity of their structure, while the aliphatic unit(s) create bridge(s) between the aromatic rings and cause the exibility of the whole structure.
Due to their special structure, they are considered as an important class of compounds in "host-guest" chemistry [43][44][45][46][47] and supramolecular assembly. [48][49][50][51] On the other hand, the clear structure and high strain of cyclophanes has contributed in a large number of applications such as pharmaceuticals, 52,53 asymmetric catalysis, 54 insulating plastics, 55 organic electronics, 56 metal capture 57,58 and supramolecular chemistry. 59 Besides, since cyclophane structure is the main foundation unit in many biologically active natural products ( Fig. 1), 60-62 their design and application is a special interest of groups working in elds including biological, medicinal and organic chemistry. Moreover, cyclophanes are attractive in the area of theoretical chemistry due to their particular topology and intra-molecular interactions.
So far, several methods have been reported for the synthesis of cyclophanes compounds 63-66 (Scheme 1). In some instances, cyclophanes are formed in low yields and also with side products. Some procedures require many steps or harsh reaction conditions such as high-pressure, and difficult separation methods. Therefore, designing better routes and improving conditions to achieve cyclophanes is still needed.
Vinamidinium salts are examples of stabilized alkenes by "push-pull" inuences between the electron-donating amino group and the electron-withdrawing ammonium group. They can easily undergo condensation reaction with bifunctional nucleophiles to form heterocycles. During past years, our group has been investigated the utilization of vinamidinium salts for the synthesis of heterocyclic compounds 67-76 (Scheme 2).

Results and discussion
In continuation of these studies and due to the importance of cyclophanes in different branches of sciences, we decided to investigate the one-pot synthesis of cyclophanes from vinamidinium salts (Scheme 3). To the best of our knowledge, vinamidinium salts have not yet been applied for preparing cyclophanes.
In the rst step, vinamidiniums were isolated as the perchlorate salts and used directly without additional purication for the synthesis of cyclophanes.
As illustrated in Scheme 3, the symmetrical vinamidinium salts (2a-g) were able to react with 1,4-phenylenedimethanamine (3) or 2,3,5,6-tetramethylbenzene-1,4-diamine (4) in reuxing acetonitrile in the presence of acetic acid for 15 h to manage the cyclophane derivatives (5a-f, 6ag). To provide the best reaction conditions in second step, the reaction of vinamidinium salt 2f with 1,4-phenylenedimethanamine (3) was chosen as model reaction and the impacts of solvents and catalysts were investigated. The obtained results are summarized in Table 1. When EtOH and MeOH were applied as the solvent and the mixture was subjected to reux in the presence of AcOH, the desired product, 5f, was achieved in low yields (43% and 36%, respectively) aer 24 h (Table 1, entries 1 and 2). In a modied protocol, the reaction was carried out in reuxing acetonitrile. In this case, remarkable  entries 4-7). Therefore, the subsequent reactions were carried out in CH 3 CN. Aer choosing the solvent, the model reaction was conducted under neutral and basic conditions. As shown in entries 8 and 9 of Table 1, in the presence of basic catalysts such as triethylamine and ethyldiisopropylamine, the desired product 5f, was not achieved. In neutral conditions also, no product was formed (Table 1, entry 10). Increasing the amount of AcOH did not affect the reaction appreciably, while, decreasing the amount of AcOH, resulted the product in lower yield (Table 1, entries [11][12][13]. So, acidic media is critical to the success of the reaction. We then applied the obtained optimized conditions for the reaction of different vinamidinium salts with 1,4-phenylenedimethanamine (3). The results are listed in Table 2.
The results demonstrated that the variety of vinamidinium salts were successfully employed in this process, affording novel cyclophanes in high yields in suitable reaction times.
According to our previous works and above results, a reasonable mechanism for the synthesis of cyclophane compounds (5a-f, 6a-g) in the presence of AcOH is illustrated in Scheme 4.
The reaction proposed by the initial attack of the amino group in 1,4-phenylenedimethanamine (3) or 2,3,5,6tetramethylbenzene-1,4-diamine (4) to protonated vinamidinium salt. Then, removal of dimethylamine occurs, followed by the nucleophilic attack of second molecule of amine on the obtained iminium salt to produce intermediate A. The loss of the second dimethylamine molecule produces intermediate B.
The reaction of this intermediate with the second molecule of vinamidinium salt, followed by the loss of two dimethylamine molecules and intramolecular nucleophilic cyclization, yields the desired cyclophane.

Conclusion
In this study, an efficient and applicable protocol has been developed for the synthesis of novel cyclophanes by the reaction of 2-substituted vinamidinium salts with 1,4-phenylenedimethanamine or 2,3,5,6-tetramethylbenzene-1,4-diamine in the presence of acetic acid.
This protocol has several advantages such as: simple and one-step procedure, absence of by-products, inexpensive catalyst, normal atmospheric conditions, high to excellent yields and easy purication of the products. Furthermore, the products are well-known, stable solids and have a long shelf-life when stored in an anhydrous environment.

Conflicts of interest
There are no conicts of interest to declare.

Scheme 4
The proposed mechanism for the synthesis of cyclophanes in the presence of AcOH.