Benzotriptycenes (o-benzenobenzanthracenes). A proton magnetic resonance study

Abstract

NR4 7TJ lH N.m.r. spectra at 100 MHz are reported for five different benzotriptycenes (o-benzenobenzanthracenes) (11)-(VI) and three model compounds, measured in CCI, solution. Calculated coupling constants are given, and no transannular resonance or polarization effect between the chromophores is detectable. Experimental T values for the cage bridgehead protons are compared to those of other reported compounds by means of both the Johnson-Bovey and McWeeny ' aromatic ring current ' models. The chromophores in (1)-(VI) are evidently under bond angle strain caused by the cage. This causes partial double bond fixation, and reduces the aromatic ring current to about half of that expected in each ring. THE normal van der Waals distance of constrained and parallel aromatic rings is ca. 340 pm.l The consequences of constraining such x-systems at distances and geometric orientations other than the van der Waals optimum have

Aromaticity and Ring Currents

“…The large number of systematic classical and semiempirical quantum-mechanical investigations cited here have shown that ring-current theory can successfully account semiquantitatively for the relative trends in the experimental 1 H NMR chemical shifts of the condensed, benzenoid hydrocarbons provided that , before being confronted with these experimental shifts, shieldings at the various protons in these moleculescalculated via any of the several ring-current modelsare expressed as a ratio to the shielding calculated , by the same method , to prevail at a standard benzene proton , because of the ring current in benzene. The fact that this procedure appears efficiently to mask the effects of the large number of far-fetched and drastic approximations inherent in semiempirical ring-current calculations 2-4 has been emphasized on many occasions, by the present authors 77,139,172,187,210,218,220 and by numerous others. ,,− ,,, It may also be mentioned in passing that this practice of taking ratios to benzene was one that much exercised even the pioneers in this field, when they were dealing with properties of bulk magnetism (section III). Referring specifically to predominantly diamagnetic, alternant hydrocarbons, O'Sullivan and Hameka wrote that “London's calculation [our refs −118] is perhaps less accurate than Hall and Hardisson's [our ref ]” but that when only the ratios of London's results are used, it seems likely that “these ratios are quite accurate.” Furthermore, Davies has stated the opinion that “the comparative success of London's method [our refs −118] for aromatic hydrocarbons may be attributed to the dependence of the theoretical anisotropy on the square of the areas of the rings in a molecule” and that “any method that takes this into account is likely to give reasonable results for the ratio” of a given calculated anisotropy to that calculated, by the same method, for benzene (see also ref ).…”

“…The fact that this procedure appears efficiently to mask the effects of the large number of far-fetched and drastic approximations inherent in semiempirical ring-current calculations [2][3][4] has been emphasized on many occa- sions, by the present authors 77,139,172,187,210,218,220 and by numerous others. 169,174,[198][199][200]214,221,222 It may also be mentioned in passing that this practice of taking ratios to benzene was one that much exercised even the pioneers in this field, when they were dealing with properties of bulk magnetism (section III).…”

A Triphenylene-Based Triptycene with Large Free Volume Synthesized by Zirconium-Mediated Biphenylation

“…Triptycene can be extended by increasing the size of its arene blades. With a few exceptions, , this has been done by increasing the length of the blade in a direction perpendicular to the 3-fold axis of the triptycene (e.g., benzene blades to anthracene blades). Extended triptycenes have primarily served as starting materials for the synthesis of higher iptycenes .…”

Preparation of Tri‐ and Hexasubstituted Triptycene Synthons by Transition Metal Catalyzed Cross‐Coupling Reactions for Post‐Modifications

“…An increase in the extension of the triptycene periphery has previously been achieved by expanding the arene blades,7a,12c,21 conjugation through alkynes,8 and crown ethers2a to name a few. The strength of such a feature has been exploited in host guest chemistry,2a,22,23 molecular cage formation8 and complex iptycene construction 19.…”