ChemInform Abstract: C‐13‐KERNRESONANZSPEKTREN VON 1‐X‐HEXAMETHYL‐ UND 1‐X‐4‐HYDROXY‐PENTAMETHYLBENZOLONIUM‐IONEN

Structure and Dynamics of Reactive Intermediates in Reaction Mechanisms. σ- and π-Complexes in Electrophilic Aromatic Substitutions

“…(b) The signals for both the aromatic carbons and the methyl carbons in the 13 C NMR spectra exhibit the same 1: 2: 2: 1 intensity patterns as observed in the 1 H NMR spectra …”

“…In contrast, the NMR spectrum of chlorohexamethylbenzenium ion does not change with temperature (in the range of −70 to −20 °C), which points to rather high activation barriers for Cl + migration . All the above-mentioned conclusions based on 1 H NMR results have been confirmed by low-temperature 13 C NMR studies carried out by Koptyug et al in 1973, , which establish the carbocationic character of chloro-, nitro-, bromo-, methyl-, and sulfonic acid-substituted hexamethylbenzenium σ-complexes as well as protonated hexamethylbenzene.…”

“…Historically, NMR spectroscopy is the first method successfully employed to observe and identify reactive intermediates in electrophilic aromatic substitutions. ,− Among all spectroscopic techniques, NMR spectroscopy provides the most detailed information on the structure of the investigated substrates within a short data acquisition time. More precise structural data can only be obtained by X-ray crystallography which however requires the preparation of single crystals.…”

“…To compare our synthetic methods under mild conditions (see the previous section) with the published procedures (in superacids or magic acid), ,, we first examine the 1 H NMR spectra in deuterated dichloromethane at low (−80 °C) temperature. Table summarizes the 1 H NMR data for σ- and π-complexes of various electrophiles with hexamethylbenzene.…”

Isolation, X-ray Structures, and Electronic Spectra of Reactive Intermediates in Friedel−Crafts Acylations

“…The same insensitivity of λ max was observed with changes in solvent polarity (CH 3 COCl, PrNO 2 ) and counteranion (SbCl 6 - ). It is noteworthy that no changes in the absorption spectra of acylium cations were observed upon the addition of the sterically hindered hexaethylbenzene (HEB). , Similarly, only very weak (indeterminate) bands were detected when either pentamethylbenzene or durene was added under the same (low-temperature) conditions . Such a spectral behavior accompanying the exposure of different acyl cations to hexamethylbenzene is strongly reminiscent of the formation of σ-adducts as benzenium cations with other cationic electrophiles such as E + = H + , CH 3 + , NO 2 + , and Br + summarized recently, i.e., Although this characteristic absorption band experiences a noticeable red shift with strong electrophiles such as E + = NO 2 + and Br + , it will be rather invariant among the HMB adducts of the family of carbon-centered cations such as E + = CH 3 + , ClCH 2 + , CH 3 C + O, and PhC + O, which are unlikely to exert appreciably different perturbations of the cyclohexadienyl chromophore (especially at its nodal position) owing to their essential carbocationic character …”