Asymmetric hydroalkoxylation of non-activated alkenes: titanium-catalyzed cycloisomerization of allylphenols at high temperatures

Abstract

The asymmetric catalytic addition of alcohols (phenols) to non-activated alkenes has been realized through the cycloisomerization of 2-allylphenols to 2-methyl-2,3-dihydrobenzofurans (2-methylcoumarans). The reaction was catalyzed by a chiral titanium-carboxylate complex at uncommonly high temperatures for asymmetric catalytic reactions. The catalyst was generated by mixing titanium isopropoxide, the chiral ligand (aS)-1-(2-methoxy-1-naphthyl)-2-naphthoic acid or its derivatives, and a co-catalytic amount of water in a ratio of 1:1:1 (5 mol % each). This homogeneous thermal catalysis (HOT-CAT) gave various (S)-2-methylcoumarans with yields of up to 90 % and in up to 85 % ee at 240 °C, and in 87 % ee at 220 °C.

Keywords: alkenes; asymmetric catalysis; heterocycles; microwave chemistry; titanium.