Acid-free, organocatalytic acetalization

The acid-free, organocatalytic acetalization of various aldehydes and ketones with N,N′-bis[3,5-bis(trifluoromethyl)phenyl]thiourea is presented. The neutral, double hydrogen bonding thiourea catalyst can be used at very low loadings of 0.01–1 mol% at room temperature to furnish the respective acetals in 65–99% yield at turnover frequencies around 600 h−1. Acid-labile TBDMS-protected as well as unsaturated aldehydes can be converted efficiently into their acetals utilizing this very mild and highly practical method.