Carbo‐Benzene's Aromaticity, Before and Beyond: A Tribute to Nozoe

Abstract

Beyond rigorous inspection of the literature, selected comments and drawings from the Nozoe Autograph Books show how concepts and targets of the chemistry of today already existed 40 years ago. This is illustrated for the chemistry of carbo-mers, which was actually unknowingly inspired by chemists of the golden age. After a personal travel through time and pages by the last author of this essay, who meets autographs by people who have been important in his career, in particular on the occasions of Nozoe's visits to France or past ISNA events, unpublished results in non-benzenoid chemistry are dedicated to Nozoe's memory. Experimental advances are first disclosed on the comparative chemistry of the three ortho, meta and para isomers of tetraphenyl-carbo-benzene C18 H2 Ph4 , the known ortho isomer serving as a reference. The two CH vertices of these chromophores make their [6]pericyclyne precursors quite sensitive and their yields and purifications challenging to optimize. The meta isomer was, however, isolated in 18% yield, and described by NMR and UV-vis absorption spectroscopy and crystallography. The para isomer was characterized both in pure form and in a 1:1 mixture with a single isomer of its HCl adduct, including by X-ray diffraction analysis of a co-crystal showing that the interior of the C18 ring of the HCl adduct is filled by the Cl atom. Theoretical advances are then offered regarding the "aromatic character" of a triple bond, by showing that the topological acyclic reference of acetylene is the Möbius-twisted cyclobutadiene. Acetylenoids are finally proposed as a novel class of non-benzenoid aromatics.

Steric/π‐Electronic Insulation of the carbo‐Benzene Ring: Dramatic Effects of tert‐Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

“…The [6]pericyclynediol 27 (Scheme ) was first envisaged as a precursor of the [6]pericyclynedione 28 , which is the tetra‐ tert ‐butyl analogue of 29 widely used for the synthesis of tetraphenyl‐ carbo ‐benzenes of type 3 (Figure ) . However, treatment of 21 with the bromomagnesium salt of 13 did not give any trace of 27 , leading to unreacted triyne and to the diol 22 , likely resulting from a magnesium‐mediated reduction of 21 .…”

“…Subsequent double O‐methylation and C‐desilylation allowed access to 13 in 58 % yield over four steps only. This shorter procedure was also applied in the preparation of the phenylated triyne 14 , which has been widely used for the synthesis of carbo ‐benzenes of type 3 (Figure ) . The triynes 18 – 20 and 13 were obtained as quasi‐statistical mixtures of stereoisomers, and used as such in subsequent steps.…”

How carbo-benzenes fit molecules in their inner core as do biologic ion carriers?

“…The out-of-plane tensor component (NICS(1) zz ) is supposed to be an even more accurate index as it avoids the in-plane components. The aromatic character of carbobenzenes has been discussed in detail previously [52,53], in particular using the topological analysis of electron localization function (ELF) [54,55], or Schleyer's NICS indices [56,57]. Now, the analysis of the effect of guest insertion on the aromaticity of the carbo-benzene host ring is undertaken by the comparison of NICS(1) indices values mainly.…”

“Carbo-aromaticity” and novel carbo-aromatic compounds

“…the precursor 2e of the quadrupolar carbo-benzene 1e was obtained in a higher 31% yield through a [8+10] route (Scheme 1). 43 In the absence of HCl, the formation of 3 can be prevented, but the reaction is then slower, leading to partial polymerization. The carbo-benzenes 1c and 1d were then isolated as pure samples after reductive aromatization by treatment with SnCl 2 /HCl, 42,45 while 1e was obtained in mixture with a side-product 3 resulting from the addition of HCl to a triple bond of one of the edges of the macrocycle.…”