Analytical Study of Superaromaticity in Cycloarenes and Related Coronoid Hydrocarbons

Abstract

Recently synthesized septulene is a unique cycloarene molecule in that no macrocyclic conjugation circuits can be chosen from the π-system. This molecule has essentially no superaromatic stabilization energy (SSE) and can be viewed as an ideal nonsuperaromatic macrocycle. SSEs for kekulene and other cycloarenes are also very small. In these hydrocarbons, a macrocycle formed by fused benzene rings effectively suppresses not only the aromaticity inherent in macrocyclic (4n+2)-site conjugation circuits but also the antiaromaticity inherent in macrocyclic (4n±1)-site circuits. Comparative study of superaromaticity in multilayered coronoid hydrocarbons revealed that not only SSE but also the HOMO contribution to SSE is minimized in odd-layered coronoids.

Graph Theory of Aromatic Stabilization

“…Our method for calculating SSE for macrocyclic π-systems is outlined below. 31,111,113 As an example, we evaluate SSE for kekulene (71). There are two types of circuits in 71: local circuits that do not enclose the inner cavity and macrocyclic ones that enclose the inner cavity.…”

Ring-Current Assessment of the Annulene-Within-an-Annulene Model for some Large Coupled Super-Ring Conjugated-Systems

“…Before moving to the other super-ring 1,30-33 structures (3-7) depicted in Figure 2, we just observe in passing that the structures 1 and 2 (which, so far as we know, are hypothetical) whose current maps have just been presented in Figure 3, are formally related -by the suppression of two (structure 1) or four (structure 2) protons and the concomitant addition of one (structure 1) or two (structure 2) carbon-carbon bonds -to the well-known and characterised 54,55 planar condensed, benzenoid hydrocarbons ovalene 54 (8) and circumanthracene 55 (9), respectively; the HLPM topological current-maps of these structures -which, it should be noted, are not actually 'super-ring' ones 1,[30][31][32][33] -are presented in Figure 4. The topological ring-current intensities in ovalene (8) The peripheral rings in 8 and 9, like their counterparts in 1 and 2, do (as expected) bear avowedly diamagnetic ring-currents of the size routinely encountered in the condensed, benzenoid hydrocarbons.…”

“…11,12 The present authors have applied this technique to a wide variety of extant and hypothetical conjugatedsystems, 1,[13][14][15][16][17] primarily in order to test the so-called 'Annulene-Within-an-Annulene (AWA) Rule' 1,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] for what have been described as 'super-ring' structures. 1,[30][31][32][33] A general super-ring conjugated hydrocarbon is illustrated in Figure 1 (which is taken from Ref. 1.…”

“…32 were all shown not to respect the AWA rule when tested by the HLPM approach. They failed for various different reasons and, indeed, the general conclusion was that, at least when tested against HLPM [5][6][7][8] ring-currents and bond-currents, most superring structures 1,[30][31][32][33] do not 1,13,14 respect the AWA 'rule'; certain carefully 'designed' [23][24][25][26][27][28][29] and occasionally somewhat artificial structures 15,16 do, 1,15,16,23 however. Furthermore, similar overall conclusions have largely been drawn when the AWA rule has been tested against the predictions of much more sophisticated 'ipso-centric' ab-initio calculations [20][21][22][23]35 -please see Ref.…”

Nano‐Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60

“…As ar ing moiety for aC 60 planet, pconjugated macrocycles have been occasionally utilized because of the large attractive interactions between psystems. [5][6][7] Yamago et al reported that [10]CPP (diameter 1.38 nm) selectively formed acomplex [10]CPP'C 60 among aseries of CPPs. [4] Recently,c yclo-p-phenylenes (CPPs) and related cycloarylenes were extensively studied as hoop-shaped p-conjugated molecules.…”