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

• Published in: Journal of organic chemistry 2005-05, Vol.70 (10), p.4013-4021
• Main Authors: Davlieva, M. G, Lindeman, S. V, Neretin, I. S, Kochi, J. K
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo0501588

Reactive intermediates in the Friedel−Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV−vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies.