Ecofriendly ketalization of isatin using heteropolycompounds as catalysts

Abstract

The dioxolane ketals of isatin and 5-chloroisatin were prepared using an efficient and cleaner procedure than the usual synthetic routes. Keggin heteropolyacids, namely, heteropolyphosphotungstic acid (HPW), HPW/SiO2 and Cs2.2HPW, have proven to be useful catalysts, allowing to the fast, selective and solvent-less preparation of ketals, with high yields and 100% selectivity. The homogeneous HPW catalyst may be successfully heterogenized by partial exchange of H+ for Cs+ but some evidenceswere found of significant homogeneous contributions of leachedHPWwhen the silica supported catalysts (HPW/SiO2) was used in the ketalization reaction. The Cs2.2HPW (Cs2.2H0.8PW12O40) catalysts may be repeatedly used but a progressive activity losswas assigned to residual organic materials that could not be removed after several dichloromethane washings.