Cyclisation of (phenylthio)acetaldehyde diethylacetal in the presence of dealuminated beta zeolites: an unexpected Meerwein–Ponndorf–Verley reduction

Abstract

Zeolite beta was dealuminated by thermal and acid treatment methods. Cyclisation of (phenylthio)acetaldehyde diethylacetal was carried out over these dealuminated zeolites with a view of obtaining benzo[b]thiophene. The yield of benzo[b]thiophene is higher in the presence of acid-dealuminated beta zeolites than in the presence of their thermally dealuminated counterparts. The product formation was thus greatly affected by the treatment methods. Thermally dealuminated beta zeolite unexpectedly favoured Meerwein–Ponndorf–Verley (MPV) reduction of the intermediate, namely, thiophenyl acetaldehyde rather than cyclisation. The different aluminium species present in the modified zeolites were analysed using DRIFT and 27 Al MAS–NMR spectra. Such unexpected MPV activity is correlated to the partly framework aluminium species.