Cyclazines‐Structure and Aromaticity or Antiaromaticity on the Magnetic Criterion. Issue 8 (22nd February 2022)

• Record Type: Journal Article
• Authors: Kleinpeter, Erich; Koch, Andreas
• Is Part Of: European journal of organic chemistry. Volume 2022:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-22
• Subjects: Aromaticity -- Cyclazines -- NMR spectroscopy -- Peripheral ring current; Chemistry, Organic -- Periodicals; Organic compounds -- Synthesis -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1002/ejoc.202101362
• Languages: English
• ISSNs: 1434-193X

Abstract

Abstract: Structure and spatial magnetic properties, through‐space NMR shieldings (TSNMRSs), of all ten cycl[2.2.2]azine to cycl[4.4.4]azine, hetero‐analogues and the corresponding hydrocarbons have been calculated at the B3LYP/6‐311G(d, p) theory level using the GIAO perturbation method and employing the nucleus independent chemical shift (NICS) concept. The TSNMRS values (actually, the ring current effect as measurable in 1 H NMR spectroscopy) are visualized as iso‐chemical‐shielding surfaces (ICSS) of various size and direction, and employed to readily qualify and quantify the degree of (anti)aromaticity. Results are confirmed by NMR [δ( 1 H)/ppm, δ( 15 N)/ppm] and geometry (planar, twisted, bow‐shaped) data. The cyclazines N[2.2.2] − up to N[2.4.4] − are planar or at most slightly bowl‐shaped and, due to coherent peripheral ring currents (except in N[2.3.3] −, N[2.3.4], N[3.3.4] + and N[2.4.4] + ), develop aromaticity or anti‐aromaticity of the whole molecules dependent on the number of peripheral conjugated π electrons. The cyclazines N[2.3.3] −, N[2.3.4], N[3.3.4] + and N[2.4.4] + develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (in N[3.3.4] + diatropic) including the nodal N pz lone pair into the conjugation. The residual cyclazines N[3.4.4], N[4.4.4] − and N[4.4.4] + are heavily twisted and, therefore, are not developing peripheral or diverse ring currents. The TSNMRS information about cyclazines and the parent tricyclic annulene analogues is congruent subject to structure and number of peripheral or internal conjugated π electrons, the corresponding (anti)aromaticity is in unequivocal accordance with Hückel's rule. Abstract : The spatial magnetic properties of cyclazines, e. g. of cycl[2.2.3]azine N[2.2.3] and cyclo[3.3.3]azine N[3.3.3], actually, the ring current effect as measurable in 1 H NMR spectroscopy, can be calculated, visualized as iso‐chemical‐shielding surfaces (ICSS) of various size and direction [ e. g . ICSS(+0.1 ppm) shielding (yellow) and ICSS(−0.1 ppm) deshielding (red)] and employed to readily qualify and quantify the degree of aromaticity of cyclazines or, occasionally, parts of it.