Zur Kenntnis der Hexaalkylbenzole. 1. Mitteilung. Anomale FRIEDEL‐CRAFTS‐Reaktionen von Hexaäthyl‐ und Hexamethylbenzol

Abstract

An anomalous type of FRIEDEL‐CRAFTS reaction was observed with hexaethyland hexamethylbenzene. With a number of acid chlorides and γ‐dicarboxylic anhydrides the reaction proceeded by elimination of an alkyl group with production of the corresponding pentaalkylphenyl ketones and γ‐pentaalkylbenzoyl acids respectively.

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…[33] This presumably proceeds through electrophilic attack by [(C 5 H 5 )Fe(CO) 2 ] + to form an intermediate that readily eliminates ethylene, leading to the s-complex 23 that, in turn, loses two carbonyl ligands( Scheme 3); this parallels the facile acylation of HEB relative to that of HMB. [34] Deuterium labelling experiments showedthat the hydrogen in the pentaethylbenzene ring is derived from ap rotonic sourcei nt he medium, and not from an ethyl group. [33] In the case of the closely relatedh exa(phenethyl)benzene, the free ligand has the classic D 3d structure (a), but if complexed, as in [(h 6 -C 6 (CH 2 CH 2 Ph) 6 Fe(h 5 - Figure 9).…”

“…Notably, the attempted preparation of the [(HEB)Fe(C 5 H 5 )] + cation directly from HEB and [(C 5 H 5 )Fe(CO) 2 Br] in the presence of AlCl 3 resulted instead in the loss of an ethyl substituent and formation of [(C 6 Et 5 H)Fe(C 5 H 5 )] + ( 22 ); the X‐ray crystal structure of the PF 6 − salt revealed that all five ethyl substituents were oriented distal . This presumably proceeds through electrophilic attack by [(C 5 H 5 )Fe(CO) 2 ] + to form an intermediate that readily eliminates ethylene, leading to the σ‐complex 23 that, in turn, loses two carbonyl ligands (Scheme ); this parallels the facile acylation of HEB relative to that of HMB . Deuterium labelling experiments showed that the hydrogen in the pentaethylbenzene ring is derived from a protonic source in the medium, and not from an ethyl group…”

“…46 °C) readily furnished pentaethylacetophenone ( 79 ) in 80 % yield. In contrast, acylation of HMB is a much more difficult process; typically, if benzoyl chloride and HMB are heated in tetrachloroethane at 75–85 °C for 7 h the product was obtained in only 37 % yield along with other tarry material . In the case of HEB, the driving force is the ready elimination of ethylene, as depicted in Scheme .…”

The Aromatic Carbon–Carbon ipso‐Substitution Reaction

“…Nevertheless, the acylation of hexaethylbenzene by terephthaloyl chloride occurred quite efficiently as illustrated in Scheme 3. [10] For a successful ipso-substitution reaction in less substituted aromatics, however, a tertiary alkyl group such as a tertbutyl group has to be replaced. What was interesting was the case of tert-butylated azulenes.…”