A Combined Experimental and Theoretical Study on the Stereodynamics of Monoaza[5]helicenes: Solvent-Induced Increase of the Enantiomerization Barrier in 1-Aza-[5]helicene

Helicenes and heterohelicenes are attractive compounds with great potential in materials sciences to be used in optoelectronics as ligand backbones in enantioselective catalysis and as chiral sensors.[1] Synthetic protocols were developed to obtain helicenes with skeletons consisting of ortho-fused benzene rings or analogue structures incorporating a heteroatom, as in thiophene-, furane-, or pyridine-containing helicenes.[2] In recent years, a repertoire of synthetic strategies was developed to access all monoaza[5]helicenes as well as some diaza[ 5]helicenes.[3] The properties of these materials are related to the stereodynamics of these helical chiral compounds. By exploration of the unexpected broad range of physicochemical properties of aza[n]helicenes it was realized that there is an opportunity to modulate a specific property by controlled design of the position of the N atoms in the helical molecular frame. In this contribution, we show a complete stereodynamic characterization of monoaza[5]-helicenes combining enantioselective dynamic HPLC and DFT calculations. At variance with previous theoretical calculations[4], 1-aza[5]helicene shows a surprisingly high enantiomerization barrier, which is triggered by specific solvent interactions. [5] References [1] a) H. A. Staab, M. A. Zirnstein, C. Krieger, Angew. Chem. Int. Ed. Engl. 1989, 28, 86–88; Angew. Chem. 1989, 101, 73– 75;b) T. R. Kelly, Acc. Chem. Res. 2001, 34, 514 –522; c) T. J. Wigglesworth, D. Sud, T. B. Norsten, V. S. Lekhi, N. R. Branda, J. Am. Chem. Soc. 2005, 127, 7272 – 7273; d) L. Vyklicky´, S. H. Eichhorn, T. J. Katz, Chem. Mater. 2003, 15, 3594 –3601; e) M. Gingras, Chem. Soc. Rev. 2013, 42, 1051– 1095. [2] a) M. Gingras, Chem. Soc. Rev. 2013, 42, 968–1006; b) M. Gingras, G. F_lix, R. Peresutti, Chem. Soc. Rev. 2013, 42, 1007 –1050; c) Y. Shen, C.-F. Chen, Chem. Rev. 2012, 112, 1463– 1535. [3] a) C. Bazzini, S. Brovelli, T. Caronna, C. Gambarotti, M. Giannone, P. Macchi, F. Meinardi, A. Mele, W. Panzeri, F. Recupero, A. Sironi, Eur. J. Org. Chem. 2005, 1247 – 1257; b) S. Abbate, C. Bazzini, T. Caronna, F. Fontana, C. Gambarotti, F. Gangemi, G. Longhi, A. Mele, I. Natali Sora, W. Panzeri, Tetrahedron 2006, 62,139 –148; c) T. Caronna, F. Fontana, A. Mele, I. Natali Sora, W. Panzeri, L. Vigan_, Synthesis 2008, 413– 416; d) T. Caronna, S. Gabbiadini, A. Mele, F. Recupero, Helv. Chim. Acta 2002, 85, 1 –8; e) T. Caronna, F. Castiglione, F. Fontana, D. Mendola, I. Natali Sora, Molecules 2012, 17, 463 –479. [4] S. Abbate, C. Bazzini, T. Caronna, F. Fontana, F. Gangemi, F. Lebon, G. Longhi, A. Mele, I. Natali Sora, Inorg. Chim. Acta 2007, 360, 908 –912. [5] T. Caronna, A. Mele, A. Famulari, D. Mendola, F. Fontana, M. Juza, M. Kamuf, K. Zawatzky, and O. Trapp, Chem. Eur. J. 2015, 21, 1–7.