Antiaromatic Dianions: Dianions of Dixanthylidene by Reduction and Attempted Excited-State Deprotonation

Abstract

Reduction of dixanthylidene with potassium or lithium resulted in formation of the antiaromatic dianion in high yield. Attempts to form the dianion by excited-state deprotonation of dixanthene with n-butyllithium/TMEDA resulted in formation of the tetraanion from deprotonation ortho to the oxygen. Orientation of the sp3 hydrogens presumably allows preferential deprotonation of the xanthene rings.

Antiaromaticity Proved by the Anisotropic Effect in 1H NMR Spectra

“…Concerning aforementioned antiaromatic molecules ( 2 – 7 , 9 , and 10 ) this property is, employing our approach, in 11 2– and 12 – rather large and in line with the parameter ΣNICS(1) zz /ring area as the measure of (anti)­aromaticity . However, this is a computational result, found out already by Black et al and needs further experimental proof.…”

“…3 The structure of 11 2− is in complete agreement with ref 28: the two oxaanthracene moieties are twisted by 90°to each other, and the two via C-9 connected moieties are structurally in complete agreement with those in 12 − . As noted, 28 both compounds are antiaromatic and, employing our basis approach, 3 are similar in antiaromaticity [11 2− ICSS (−0.1 ppm) = 6.9 Å), 12 − ICSS (−0.1 ppm) = 7.5 Å). Concerning aforementioned antiaromatic molecules (2−7, 9, and 10) this property is, employing our approach, 3 in 11 2− and 12 − rather large and in line with the parameter ΣNICS(1) zz /ring area as the measure of (anti)aromaticity.…”

“…However, we never tried to compare the antiaromaticity of different structures employing our approach, and because Black et al found different answers concerning the antiaromaticity of 9-oxaanthracene anion 12 – and of the corresponding 10-dimeric dianion 11 2– (Scheme ) when employing various methods [NICS(1) zz , magnetic susceptibility exaltation Λ and average 1 H chemical shifts], we attempted our method, and this more so, as employing proton chemical shifts to quantify (anti)­aromaticity proves to be a dubious parameter . This is the topic of this paper.…”

Quantification and Visualization of the Anisotropy Effect in NMR Spectroscopy by Through-Space NMR Shieldings

“…With these encouraging results in hand we studied, employing our approach [1], the relative antiaromaticity of the oxaanthracene anion 11 À and dioxaanthracene dianion 10 2À (Scheme 3.5) which proved to be unclear [41] employing the ring-current effect of these antiaromatic moieties. The antiaromaticity of one half of the dianion 10 2À compared with the one of the anion 11 À was found to be consistent with the magnetic susceptibility data of 11 À compared with a single ring moiety of 10 2À (cf.…”

Reduction of Benzylidene Dibenzo[a,d]cycloheptenes: Over-Reduction of Antiaromatic Dianions to Aromatic Tetraanions

“…12,13 More recently, we have prepared antiaromatic dianions containing dibenzotropylium anions 14 as well as those containing heterocyclic antiaromatic systems such as the dianion of dixanthyidene. 15 Herein we report the attempt to prepare and characterize dianions of substituted benzylidene dibenzocycloheptenes (1) through reduction with lithium or potassium. We anticipated that the electron-donating (destabilizing) group on the benzyl substituent would enhance the antiaromaticity of the dibenzotropylium anion just as electron-withdrawing groups on the benzyl substituent of the analogous benzylidene fluorene dications had enhanced the antiaromaticity of the fluorenyl system.…”