Dications of Fluorenylidenes. Electronic Effects on the Paratropicity/Antiaromaticity of 2,7-Disubstituted Fluorenyl Cations

Abstract

Oxidation of 2,7-disubstituted tetrabenzo[5.5]fulvalene derivatives 1a−d resulted in the formation of dications which are fluorenyl cations linked by a single bond. These fluorenyl cations exhibit significant paratropicity in the 1H NMR spectrum, which is attributed to an antiaromatic ring current. Interaction of the perpendicular ring systems is evident in the upfield shift of carbons a and a‘, presumably due to σ−p donation. The lack of variation in the upfield shift of carbons a and a‘, compared to previously reported systems, is attributed to the similarities of the geometries of 1a−d2+ . Substitution at a remote site affects the antiaromaticity of the unsubstituted fluorenyl cation, but the nature of the effect of the substituent is not understood. Direct substitution on the fluorenyl cation by substituents which increase electron density, either through inductive π polarization or through resonance, cause a paratropic shift in the probe proton.

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

“…W e have been involved in the preparation of antiaromatic dications via oxidation of unsaturated precursors [1][2][3][4][5][6][7][8] and ionization of diols for some time. 9,10 The antiaromaticity of these species can be evaluated by the same type of criteria used in the evaluation of aromatic compounds.…”

A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

“… The two additional π charges are delocalized from the central boryl anions/carbanions into the phenylene rings. The additional shielding per phenylene ring amounts to Σ(Δ δ )=−68.2 ppm in Na 4 [ 3 ] and −66.1 ppm [53, 54] in Li 2 [ B ]. Shielding of the two carbon atoms ortho to B or C‐9 in [ 3 ] 4− or [ B ] 2− , respectively, is distinctly different. This is likely due to the presence of nodal planes running close to the less shielded ortho ‐carbon atoms in the LUMOs of [ 3 ] 2− / B and HOMOs of [ 3 ] 4− /[ B ] 2− [16, 39] …”

“…The two additional π charges are delocalized from the central boryl anions/carbanions into the phenylene rings. The additional shielding per phenylene ring amounts to Σ(Δ δ )=−68.2 ppm in Na 4 [ 3 ] and −66.1 ppm [53, 54] in Li 2 [ B ].…”

Peterson Olefination: Unexpected Rearrangement in the Overcrowded Polycyclic Aromatic Ene Series

“…9 H ‐Fluoren‐9‐yltrimethylsilane (5): Compound 5 was prepared by a literature procedure with some modifications 13. Fluorene (0.5 g, 3.0 mmol) was dissolved in dry THF (20 mL) in a round‐bottomed flask containing a magnetic stirrer and fitted with a septum.…”

“…Peterson olefination reactions have been extensively applied by Mills et al in the synthesis of overcrowded bistricyclic aromatic enes (BAEs, 1 ), including heteromerous bifluorenylidenes ( 1 , X ≠ Y) and diphenylmethylidene fluorenes 13–15. The 3,6‐dimethyl derivative of the parent bifluorenylidene 2 , for example, was synthesized by addition of trimethylsilyl carbanion to 9 H ‐fluorene to give (9 H ‐fluoren‐9‐yl)trimethysilane, which upon treatment with n BuLi and 3,6‐methyl‐9 H ‐fluoren‐9‐one gave 3,6‐dimethylbifluorenylidene 15…”