The Acylation of Cycloheptatriene

Abstract

The reaction of cycloheptatriene with acyl halides in the presence of a Lewis-acid catalyst has been studied. In particular reaction of benzoyl fluoride and boron trifluoride in methylene chloride, benzoyl fluoride and borontrifluoride in liquid SO2 and a series of acyl chlorides and aluminium chloride in methylene chloride, with cycloheptatriene gave tropylium salts in up to 30% yield, without the simultaneous formation of an aldehyde. Thus a process other than direct hydride-ion transfer to the electrophilic acylium species was responsible for the formation of the tropylium salts. A mechanism has been proposed for these reactions. Additional products have been isolated from the above reactions and by slight variations in the reaction conditions either 1-acylcyloheptatrienes or substituted deoxybenzoins have been prepared. This method is more convenient than any other preparations of acylcycloheptatrienes, and the reaction also provides the first example of the introduction of an electrophilic species into the cycloheptatriene [please see Thesis]-electron system. Normally in such reactions only extensive polymerisation and low yields of tropylium salts are recorded. The preparation of substituted deoxybenzoins is also of synthetic importance, isomerically pure products being conveniently obtained. The normal synthesis, i.e. by Friedel-Crafts acylation of the substituted benzene with phenyl acetyl chloride, leads to mixed products, depending upon the orientating effects of the substituent on the benzene ring. The identification of a chloro-ketone intermediate allowed a coherent mechanism for the formation of both series of compounds to be postulated. The evidence supporting this mechanism is discussed in detail. The ready synthesis of 1-benzoylcycloheptatriene by these reactions made possible an investigation of the properties of this compound. Thermolysis of 1-benzoylcycloheptatriene gave deoxybenzoin. Photolysis, provided a convenient synthesis of 2—benzoylcycloheptatriene by a highly specific 1,7 hydrogen shift and hydride-ion transfer to trityl fluoroborate gave the novel benzoyltropylium fluoroborate. The characterisation of this compound by spectroscopic and chemical techniques, is described.