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Renewing Accessible Heptazine Chemistry

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Renewing Accessible Heptazine Chemistry View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Archive Ouverte en Sciences de l'Information et de la Communication Renewing accessible heptazine chemistry: 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, a new highly soluble heptazine derivative with exchangeable groups, and examples of newly derived heptazines and their physical chemistry Laurent Galmiche, Clémence Allain, Tuan Le, Régis Guillot, Pierre Audebert To cite this version: Laurent Galmiche, Clémence Allain, Tuan Le, Régis Guillot, Pierre Audebert. Renewing accessible heptazine chemistry: 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, a new highly soluble heptazine deriva- tive with exchangeable groups, and examples of newly derived heptazines and their physical chemistry. Chemical Science , The Royal Society of Chemistry, 2019, 10 (21), pp.5513-5518. 10.1039/c9sc00665f. hal-02276787 HAL Id: hal-02276787 https://hal.archives-ouvertes.fr/hal-02276787 Submitted on 3 Sep 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution - NonCommercial| 4.0 International License Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Renewing accessible heptazine chemistry: 2,5,8- tris(3,5-diethyl-pyrazolyl)-heptazine, a new highly Cite this: Chem. Sci.,2019,10,5513 All publication charges for this article soluble heptazine derivative with exchangeable have been paid for by the Royal Society of Chemistry groups, and examples of newly derived heptazines and their physical chemistry† a a a b Laurent Galmiche, Clemence´ Allain, Tuan Le, Regis´ Guillot and Pierre Audebert *a We have prepared 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, the first highly soluble heptazine derivative possessing easily exchangeable leaving groups. We present its original synthesis employing mechanochemistry, along with a few examples of its versatile reactivity. It is, in particular, demonstrated Received 7th February 2019 that the pyrazolyl leaving groups can be replaced by several secondary or primary amino substituents or Accepted 23rd April 2019 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. by three aryl- or benzyl-thiol substituents. In addition to being a synthetic platform, 2,5,8-tris(3,5- DOI: 10.1039/c9sc00665f diethyl-pyrazolyl)-heptazine is fluorescent and electroactive, and its attractive properties, as well as those rsc.li/chemical-science of the derived heptazines, are briefly presented. Introduction a high nitrogen to carbon ratio, making them strongly electron- decient rings. Because of this reason, heptazines can be 6 Heptazines (Fig. 1) are a fascinating family of high nitrogen reversibly reduced at relatively high potentials, which makes aromatic compounds that have already aroused the special them interesting compounds in view of applications for 1 7 This article is licensed under a interest of Linus Pauling 80 years ago. Indeed, there are still photovoltaic devices. They are also uorescent, and heptazines 8,9 few heptazines fully described in the literature,2 but those displaying delayed uorescence have been described. Most of 2 known to date already present desirable properties.3–5 They have them display a very high thermal stability, which is highly desirable for the development of light-emitting or photovoltaic Open Access Article. Published on 25 April 2019. Downloaded 7/17/2019 11:27:41 AM. devices. A recent review on their synthesis highlights the interest in these molecules.10 Finally, even more attractively, heptazine polymers have demonstrated outstanding photocatalytic properties11–13 for 14,15 16 hydrogen evolution and for CO2 reduction and might be one of the rare molecular platforms which could give rise to molecular catalysts able to perform water splitting. They now constitute an extremely interesting class of materials.17,18 However, a major drawback in heptazine synthetic chemistry lies in the fact that almost all derivatives that have already been reported are prepared starting from the only heptazine known to date to have exchangeable side groups, 2,5,8-trichloro- heptazine, (sometimes designated as cyameluric chloride). Fig. 1 General chemical structure of heptazines. The Kroke group and other groups have indeed shown that the chlorides could be efficiently replaced by various nucleophiles, like amines,6,7 thiols,19 selenols19 and phosphines.20 This very aPPSM, ENS Cachan, CNRS, Universit´e Paris-Saclay, 94235 Cachan, France. E-mail: useful synthon has, however, two major drawbacks. (1) It is only [email protected] sparingly soluble in most common solvents and (2) more bICMMO, Universit´e Paris Sud, CNRS, Universit´e Paris-Saclay, 91405 Orsay, France challenging, the original synthetic route reported by Kroke21 † Electronic supplementary information (ESI) available: Detailed synthetic requires heating the intimate solid mixture of the tris potas- protocols, spectroscopic properties, DFT optimization, and X-ray diffraction analyses CCDC. CCDC 1843875. For ESI and crystallographic data in CIF or sium salt of cyameluric acid and phosphorus pentachloride, other electronic format see DOI: 10.1039/c9sc00665f with cautious multi-step extraction of the gaseous hydrochloric This journal is © The Royal Society of Chemistry 2019 Chem. Sci.,2019,10, 5513–5518 | 5513 View Article Online Chemical Science Edge Article acid formed (along with traces of chlorine gas), followed by Table 1 Recapitulative table of the mechanochemical conditions used sublimation or Soxhlet extraction with dry toluene; variants of for heptazine 1 production, with corresponding yields this synthesis are slightly better but do not really solve the 15,22 Trihydrazinoheptazine Milling time Silica amount problem. This synthetic route is delicate and dangerous to Entry quantity (g) (min) (g) Yield (%) some extent, which probably explains the relatively low number of publications on heptazines except for the discovery (again by 14 8 0 15 Kroke) that amino groups could be exchanged for hydrazino 2 2.5 8 0 25 3 1.25 8 0 31 groups,23 which can be converted to azide, and then imino- 23 4 1.25 16 0 31 phosphorane groups. However, again 2,5,8-triazidoheptazine is 5 1.25 8 1.25 36 an unstable compound. We felt that 2,5,8-trihydrazinoheptazine 64a 80 30 would be a better starting point than cyameluric acid salts, and a In a 250 mL milling bowl. we decided to convert it into another derivative bearing easily exchangeable pyrazolyl groups. crushed for 8 min (2 times, 4 min) at 500 rpm. The resulting Results and discussion paste is extracted three times with 30 mL dichloromethane and Synthesis subjected to chromatography. Although the yields are average, the work-up and purication processes are extremely simple We describe in this article the facile synthesis of a heptazine and scalable, and allow routine preparation of 1–2 g batches of derivative possessing exchangeable diethylpyrazolyl leaving the compound, aer simple ash chromatography. groups (Scheme 1), along with several examples of new hepta- We have investigated to some extent the inuence of a few zines derived by nucleophilic exchange of these groups, to (empirical) parameters for mechanical milling (Table 1), which illustrate the versatility of this new synthetic pathway. leads to 3 tendencies. (1) A time of 8 min is sufficient to reach Creative Commons Attribution-NonCommercial 3.0 Unported Licence. As previously mentioned, we preferred trihydrazinoheptazine the appropriate yield; increasing the milling time to 16 min to the weakly reactive potassium salt of cyameluric acid, as does not lead to better efficiency (entry 4). However, we did not a starting point for new heptazines. We were able to prepare it try shorter times. (2) Addition of silica30 slightly improves the 23 according to Kroke's method. A er careful drying in a desic- yields, though it is not obvious if this is a mechanical effect, or if cator in the presence of phosphoric anhydride, the trihy- it works as a co-catalyst (entry 5). (3) It is better if the product drazinoheptazine is processed without further puri cation. We load is not in excess (e.g. over 1.5–2 g, for an 80 mL milling found out that, although elegant, the puri cation method pub- bowl). Otherwise the yields progressively decrease (entry 3). We Caution lished by Kroke et al. was tedious ( : Kroke's procedure checked that the reaction can be scaled-up using a 250 mL uses hydrazine hydrate, a compound which is stable but toxic. This article is licensed under a milling bowl (entry 6) and probably further. We recommend the use of a cautiously tightly sealed Te on- We have been able to obtain an X-ray structure of heptazine 1 lined pressure reaction vessel inserted into a very resistant (Fig. 2). Details of renement data and crystal packing are shielded stainless steel tted mantle). provided in the ESI section.† In the crystal, the heptazine core is Open Access Article. Published on 25 April 2019. Downloaded 7/17/2019 11:27:41 AM. Therefore, we found it more convenient to use the carefully almost coplanar with the pyrazole substituents. The molecules dried crude trihydrazinoheptazine. Since mechanochemistry are organized in shied columnar packing, with stacking has been recently demonstrated to improve the facileness and interactions between pyrazole substituents. 24–27 the yields in organic synthesis, especially in heterocyclic We have tested the facileness of exchanging the pyrazolyl 28,29 synthesis, we developed a mechanochemical route for groups on heptazine 1. Actually, we found out that exchange 1 preparing . The trihydrazinoheptazine is reacted with 3,5- with mild nucleophiles like amines and thiols was straightfor- heptanedione to provide the desired 2,5,8-tris(3,5-diethyl- ward.
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