Acid zeolites as catalysts in organic reactions. Acetylation of cyclohexene and 1-methylcyclohexene

Treatment of cyclohexene (1a) and 1-methylcyclohexene (1b) with acetyl chloride or acetic anhydride has been carried out in the presence of a series of acid zeolites including Y, beta Mor and ZSM-5. Complex mixtures containing conjugated and unconjugated acetylcyclohexenes together with other products arising from the formal addition of hydrochloric acid, acetic acid, acetyl chloride and acetic anhydride to the C=C double bond were generally obtained. Mor and ZSM-5 were found to be less active but more selective catalysts than Y and beta. These results have been compared with those achieved using silica-alumina (25%), phosphotungstic acid (HPW) supported on silica, and AlCl3. Silica-alumina does not catalyze the acetylation of cyclohexene with acetyl chloride, while phosphotungstic acid was found to be less active than tridirectional, large pore zeolites. In the case of AlCl3, acetylchlorocyclohexanes were the main products. Kinetics of the acetylation for 1b established that the beta,gamma-unsaturated 6-acetyl-1-methylcyclohexene was the primary product, which undergoes subsequent rearrangement to the thermodynamically more stable 1-acetyl-2-methylcyclohexene. In contrast, 1a affords exclusively the alpha,beta-saturated 1-acetylcyclohexene. Based on semiempirical hamiltonians AM1 quantum chemical calculations of the optimum chair conformation of the corresponding carbocationic intermediates, this different behavior of cyclohexene and 1-methylcyclohexene has been interpreted in terms of stereoelectronic control owing to the lack of an axial alpha hydrogen in the intermediate derived from 1-methylcyclohexene.