The reaction of cycloheptatriene with acyl halides in the presence of Lewis acids. A convenient synthesis of 1-acylcycloheptatrienes

Abstract

The reaction of cycloheptatriene with acyl halides in the presence of a Lewis acid does not involve a hydride-atom> transfer. The reaction has been used to prepare two series of compounds, ( a ) benzyl ketones, and (b) l-acylcycloheptatrienes. An intermediate common to both reactions has been identified, allowing a coherent mechanism to be postulated.Hydride-atom Transfer. --Cycloheptatriene undergoes a hydride-atom transfer reaction to give the tropylium ion. This was used to synthesise tropylium fluoroborate in almost quantitative yield from cycloheptatriene and trityl fluoroborate.l described the reaction of cycloheptatriene with t-butyl halides and a Lewis acid to give the corresponding tropylium salts in ca. 20% yield. They claimed these salts were formed by halidehydride exchange but did not detect t-butane.Harmon et aL3 discussed the reaction of cycloheptatriene with boron halides to form the tropylium ion. Hydride-atom transfer reactions and electrophilic attack on the triene system were considered as possible mechanisms.The acyl fluoride-Lewis acid fluoride complexes described by Olah4 are extremely reactive. It was hoped that they would be hydride-acceptors in lowtemperature reactions, as in Scheme 1.