4 ’-( N-( propan-1 , 2-dienyl ) pyrrol-2-yl )-2 , 2 ’ : 6 ’ , 2 ’ ’-terpyridine

A new pyrrole-substituted terpyridine derivative that possesses an allene moiety was obtained as an “unexpected” sole product during an attempt to alkylate the N-atom of pyrrole with propargyl bromide in order to obtain an alkyne-functionalized terpyridine.


Introduction
Terpyridine ligands (terpy) and their complexes have been widely studied [1]. This can be easily explained by the huge number of terpyridine derivatives that can be obtained by varying the substitution pattern of the ligand as well as the nature of the complexed metal. In particular, terpyridines that contain a five membered heterocycle, such as furan [2] or thiophene [3], have attracted a lot of attention. In fact, they can be used as intermediates in the preparation of materials for solar cells [4] or nanoparticles [5], as biological probes [6], as ligands in catalysis [7], as antimicrobial agents [8], as electrochromic materials [9] or as chromophores [10], to name just a few applications. Although a little less studied, terpyridines that include a pyrrole ring have also been a subject of interest. For example, such terpyridines have been used for the preparation of cytotoxic molecules [11], for application in OLED (organic light emitting diodes) [12] or sensor devices [13] or for the preparation of catalytic materials [14]. Thus, the preparation of terpyridine derivatives that contain a functionalized pyrrole is of interest in the fields of both organic synthesis and material science. This article describes how pyrrole-containing terpyridine 1 was obtained as an unexpected product during attempts to prepare compound 2 (Figure 1), which features an alkyne chain for possible future functionalization [15].

Introduction
Terpyridine ligands (terpy) and their complexes have been widely studied [1]. This can be easily explained by the huge number of terpyridine derivatives that can be obtained by varying the substitution pattern of the ligand as well as the nature of the complexed metal. In particular, terpyridines that contain a five membered heterocycle, such as furan [2] or thiophene [3], have attracted a lot of attention. In fact, they can be used as intermediates in the preparation of materials for solar cells [4] or nanoparticles [5], as biological probes [6], as ligands in catalysis [7], as antimicrobial agents [8], as electrochromic materials [9] or as chromophores [10], to name just a few applications. Although a little less studied, terpyridines that include a pyrrole ring have also been a subject of interest. For example, such terpyridines have been used for the preparation of cytotoxic molecules [11], for application in OLED (organic light emitting diodes) [12] or sensor devices [13] or for the preparation of catalytic materials [14]. Thus, the preparation of terpyridine derivatives that contain a functionalized pyrrole is of interest in the fields of both organic synthesis and material science. This article describes how pyrrole-containing terpyridine 1 was obtained as an unexpected product during attempts to prepare compound 2 ( Figure 1), which features an alkyne chain for possible future functionalization [15].

Results and Discussion
The synthetic approach towards molecule 2 relies on the N-alkylation of the pyrrole moiety of 4 -(pyrrol-2-yl)-2,2 :6 ,2"-terpyridine (3) with propargyl bromide (Figure 2), applying a protocol that has been described for the preparation of N-alkyl terpyridine pyrroles [16]. At the end of the reaction, a single product was noticed by TLC. This product was easily separated from the starting material by flash chromatography, but 1 H NMR did not agree with the structure of 2.
Molbank 2020, 2020, M9xx 2 of 5 a single product was noticed by TLC. This product was easily separated from the starting material by flash chromatography, but 1 H NMR did not agree with the structure of 2. Instead, structure 1 was coherent with 1 H NMR. In particular, the spectrum exhibits the signals for the allenic protons. In fact, a triplet is observed at 7.14 ppm (J= 6.4 Hz). This signal accounts for the allenic proton e (Figure 3). In addition, a doublet is observed at 5.50 ppm (J= 6.4 Hz) for the allenic protons g. These multiplicities and chemical shifts are in accordance with those reported for other allene-functionalized pyrroles [17][18][19]. All other signals arising from the terpyridine and the pyrrole parts of the molecule are present. Additionally, the structure of 1 was further confirmed by 13 C NMR, as well as by HR-MS. For instance, the 13 C NMR spectrum features 15 signals due to the symmetry of the molecule, while mass spectra exhibit the molecular ion peak at 337.14466 (calc. for [C22H16N4+H] + : 337.14477).
As pointed out in the above-mentioned literature, the formation of compound 1 is not so "unexpected". Nevertheless, no trace of alkyne 2 was noticed, while N-propargylation of pyrrole and indole derivatives under similar reaction conditions are described in the literature [20,21]. Therefore, the obtention of 1 as the sole product is "unexpected". Instead, structure 1 was coherent with 1 H NMR. In particular, the spectrum exhibits the signals for the allenic protons. In fact, a triplet is observed at 7.14 ppm (J = 6.4 Hz). This signal accounts for the allenic proton e (Figure 3). In addition, a doublet is observed at 5.50 ppm (J = 6.4 Hz) for the allenic protons g. These multiplicities and chemical shifts are in accordance with those reported for other allene-functionalized pyrroles [17][18][19].

Materials and Methods
a single product was noticed by TLC. This product was easily separated from the starting material by flash chromatography, but 1 H NMR did not agree with the structure of 2. Instead, structure 1 was coherent with 1 H NMR. In particular, the spectrum exhibits the signals for the allenic protons. In fact, a triplet is observed at 7.14 ppm (J= 6.4 Hz). This signal accounts for the allenic proton e (Figure 3). In addition, a doublet is observed at 5.50 ppm (J= 6.4 Hz) for the allenic protons g. These multiplicities and chemical shifts are in accordance with those reported for other allene-functionalized pyrroles [17][18][19]. All other signals arising from the terpyridine and the pyrrole parts of the molecule are present. Additionally, the structure of 1 was further confirmed by 13 C NMR, as well as by HR-MS. For instance, the 13 C NMR spectrum features 15 signals due to the symmetry of the molecule, while mass spectra exhibit the molecular ion peak at 337.14466 (calc. for [C22H16N4+H] + : 337.14477).
As pointed out in the above-mentioned literature, the formation of compound 1 is not so "unexpected". Nevertheless, no trace of alkyne 2 was noticed, while N-propargylation of pyrrole and indole derivatives under similar reaction conditions are described in the literature [20,21]. Therefore, the obtention of 1 as the sole product is "unexpected". All other signals arising from the terpyridine and the pyrrole parts of the molecule are present. Additionally, the structure of 1 was further confirmed by Supplementary Materials 13 C NMR, as well as by HR-MS. For instance, the 13 C NMR spectrum features 15 signals due to the symmetry of the molecule, while mass spectra exhibit the molecular ion peak at 337.14466 (calc. for [C 22 H 16 N 4 + H] + : 337.14477).

Materials and Methods
As pointed out in the above-mentioned literature, the formation of compound 1 is not so "unexpected". Nevertheless, no trace of alkyne 2 was noticed, while N-propargylation of pyrrole and indole derivatives under similar reaction conditions are described in the literature [20,21]. Therefore, the obtention of 1 as the sole product is "unexpected".

Materials and Methods
All reagents were purchased from commercial suppliers and used as received. The 4 -(Pyrrol-2-yl)-2,2 :6 ,2"-terpyridine (3) was prepared according to the literature [16]. A volume of 85% Potassium hydroxide pellets (VWR Chemicals, France) was powdered using a mortar and a pestle. Anhydrous DMSO and 80% propargyl bromide solution in toluene were purchased from ACROS Organics (Geel, Belgium) and used as received. Flash chromatography was carried out on a Combiflash Rf+ Lumen (Teledyne ISCO, Lincoln, NE, USA) using a Redisep Rf neutral alumina column (Teledyne ISCO, Lincoln, NE, USA) with a hexane/ethyl acetate mixture (100:0 to 80:20 v:v) as eluent. The 1 H and 13 C NMR spectra were recorded on a Brucker AC 400 (Bruker, Wissembourg, France) at 400 and 100 MHz, respectively, using CDCl 3 as a solvent. The melting point was recorded with a Stuart SMP 10 melting point apparatus (Bibby Sterilin, Stone, UK) and was uncorrected. HR-MS was recorded at Sayence SATT, Dijon, France.

Conclusions
A new pyrrole-containing terpyridine has been prepared and characterized. It features an allenic part that is linked via the N-atom of the pyrrole nucleus. Considering the fact that allenes are valuable intermediates in the preparation of polymer [22] or molecular materials [23], and the impressive metal-coordination properties of terpyridine ligands [1], this new compound could be useful for the fabrication of novel metal-containing functional materials.