Cyclisation of citronellal to isopulegol catalysed by hydrous zirconia andother solid acids

The cyclisation of citronellal to isopulegol is an important step in the sy nthesis of menthol. Several zirconia-based catalysts were evaluated for thi s reaction. Zirconium hydroxides and phosphated zirconia had very good acti vity and selectivity. Ammonia TPD and pyridine IR studies indicate the pres ence of strong Lewis together with weak Bronsted acid sites. Other strong a cids like sulfated zirconia, Amberlyst, and Nafion were very active but the selectivity toward isopulegol was poor. They catalysed side reactions such as dehydration, cracking, and etherification of isopulegol. Silica, with o nly weak Lewis acidity, showed very low activity. The presence of both Lewi s and Bronsted sites is therefore essential for the reaction. A reaction me chanism is proposed where the citronellal molecule binds to a zirconium Lew is acid site via the aldehyde oxygen and the jr-electrons of the double bon d so that the right configuration is attained for cyclisation. Subsequent p rotonation of the aldehyde via a neighbouring Bronsted acid site initiates the cyclisation to isopulegol. (C) 2001 Academic Press.