BORAHETEROHELICENES: SYNTHETIC METHODOLOGIES AND PROPERTIES OF A NOVEL CLASS OF BORON Π-CONJUGATED SYSTEMS

The introduction of a trigonal boron atom into a polyaromatic hydrocarbon (PAH) core is an extremely powerful tool to provide organic scaffolds with optoelectronic properties as well as optimal packing in the solid state.[1] However, boron-doped PAHs (B-PAHs) often display low processability due to their poor solubility. The distortion of the molecular scaffold provides a suitable strategy to enhance the solubility properties of B-PAHs while maintaining good stacking properties and sufficient electronic conjugation. Extreme distortion of the molecular structure can be achieved in helical-shaped PAHs, namely helicenes[2] which are screw-shaped inherently chiral polycycles, formed by ortho-fused aromatic or heteroaromatic rings. The presence of a helical structure in B-PAHs is expected to strongly influence their physico-chemical properties leading to compounds characterized by peculiar features promising for applications in next generation functional materials. Despite the great potential of this class of compounds, only few examples of borahelicenes have been reported in the literature and those mainly consist of carbohelicene-based structures.[3–5] However, the considerable structural diversity achievable by introducing different bora-heterocycles (oxaborine, borole, borepin) and other heteroaromatic rings (thiophene, furan, pyrrole) into the same helical scaffold, suggests that a large variety of compounds with intriguing features could be accessible via currently unexplored synthetic routes. The design, synthesis, and properties investigation of new boraheterohelicenes (BHHs) is therefore a relevant research topic and is the object of this PhD project, aimed to obtain several BHHs with structural diversity, as well as to study their reactivity, electrochemical and photophysical features for better understanding their potential as building blocks for material science. The thesis work was carried out in part at the University of Milan in the laboratories of Prof. Emanuela Licandro and in part at the Goethe Universität Frankfurt am Main under the supervision of Prof. Dr. Matthias Wagner, within a co-tutelle programme. Owing to the long-standing expertise of Prof. Licandro group in the synthesis of tetrathia[7]helicenes (7TH Figure 1) and that of Prof. Dr. Wagner group in the synthesis of boron-doped PAHs (e.g. boron[4]helicene 4BH; Figure 1), I conceived this PhD project designing a series of thiahelicenes containing one or more B-O bond into the helical scaffold. Figure 1. Tetrathia[7]helicene (7TH) and boron[4]helicene (4BH). Tetrathia[7]helicenes, consisting of thiophene and benzene rings fused in an alternating fashion, are configurationally stable heterohelicenes which exist as pair of enantiomers.[6] This class of molecules is particularly interesting since it merges the properties of oligothiophenes with those of helicenes, giving rise to systems with peculiar electronic and chiroptical properties which make them appealing building blocks for applications in manifold fields of science, including optoelectronics,[7] catalysis,[8] and biology.[9] The introduction of trigonal boron atom into a thiahelicene scaffold gives rise to a novel class of unexplored boron π-conjugated molecules with potentially interesting features. The present Ph.D. thesis was therefore intended to provide a meaningful contribution in the development of innovative and versatile syntheses of BO-doped tetrathia[7]helicenes as well as the study of their stereochemical and optoelectronic properties to identify potential applications of these systems in material science. The first selected structures containing one or two oxaborine rings in the helical scaffold are shown in figure 2. The presence of the bulky mesityl group at the boron atom is necessary to ensure stability to the molecule. It is noteworthy that compound 2 is the skeletal isomer of 1, as the direction of the B-O bond is opposite in the two molecules. In the course of the research work, after the evaluation of the photophysical properties of 1, helicene 2 was designed to get information on the structure-property relationship and evaluate how the position of the BO-bond into the helical scaffold can influence the electronic properties of BO-doped thiahelicenes. Figure 2. First selected borathia[7]helicene structures. In detail the thesis work has been focused on the following main goals: 1) study of the synthesis and properties of the doubly BO-doped tetrathia[7]helicene 1; and its “BOisomer”; 2) preliminary study of the synthesis of singly BO-doped tetrathia[7]helicenes 3 and 4. [1] E. von Grotthuss, A. John, T. Kaese, M. Wagner, Asian J. Org. Chem. 2018, 7, 37–53. [2] Y. Shen, C.-F. Chen, Chem. Rev. 2012, 112, 1463–1535. [3] T. Hatakeyama, S. Hashimoto, T. Oba, M. Nakamura, J. Am. Chem. Soc. 2012, 134, 19600–19603. [4] H- Hirai, K. Nakajima, K. Shiren, J. Ni, S. Nomura, T. Ikuta, T. Hatakeyama, Angew. Chem. Int. Ed. 2015, 54, 13581–13585. [5] T. Katayama, S. Nakatsuka, H. Hirai, N. Yasuda, J. Kumar, T. Kawai, T. Hatakeyama, J. Am. Chem. Soc. 2016, 138, 5210–5213. [6] S. Cauteruccio, D. Dova and E. Licandro, Adv. Heterocycl. Chem., 2016, 118, 1. [7] A. Bossi, E. Licandro, S. Maiorana, C. Rigamonti, S. Righetto, G. R. Stephenson, M. Spassova, E. Botek and B. Champagne, J. Phys. Chem. C, 2008, 112, 7900. [8] S. Cauteruccio, A. Loos, A. Bossi, D. Dova, F. Rominger, S. Prager, M. C. Blanco Jaimes, A. Dreuw, E. Licandro, A. S. K. Hashmi and A. K. Stephen, Inorg. Chem., 2013, 52, 7995; S. Cauteruccio, D. Dova, A. Genoni, M. Orlandi, M. Benaglia and E. Licandro, Eur. J. Org. Chem., 2014, 2694. [9] S. Cauteruccio, C. Bartoli, C. Carrara, D. Dova, C. Errico, G. Ciampi, D. Dinucci, F. Chiellini, E. Licandro, ChemPlusChem, 2015, 80, 490.