Oxidation-isomerization of an olefin to allylic alcohol using titania-silica and a base co-catalyst

The potential of a two-step, one-pot synthesis of alpha,beta-unsaturated alcohols from olefins is illustrated on the example of 4-hydroxy-isophorone using titania-silica aerogels and basic co-catalysts. Kinetic analysis of the complex reaction network revealed that titania-silica is efficient in the epoxidation of beta-isophorone (3,5,5-trimethyl-3-cyclohexen-1-one) with TBHP at 353 K, whereas the acid/base-catalyzed in situ rearrangement to 4-hydroxy-isophorone (4-hydroxy-3,5,5-trimethyl-cyclohex-2-enone) is slow. Addition of solid bases such as CaO, Na2CO3, and KF/CaF2 remarkably accelerated the rearrangement and up to 77.5% selectivity at 83% conversion was achieved in 3 h. Stronger bases (K2CO3, BaO, Mg-Al-O-Bu-t, guanidine bases) inhibited the epoxidation reaction due to deactivation of the isolated Ti sites, and favored isomerization of beta-isophorone to alpha-isophorone. Hydrophobization of titaniasilica by covalently bound surface phenyl groups greatly suppressed oligomerization and isomerization of beta-isophorone but did not improve the selectivity to 4-hydroxy-isophorone. (C) 2000 Academic Press.