Methyltrioxorhenium as a Lewis Acid in the Prins Cyclization of Benzaldehyde and Isoprenol

Abstract

Methyltrioxorhenium (MTO) as a Lewis acid was used in the acid catalyzed reaction of benzaldehyde and isoprenol leading to the formation of 4-methyl-2-phenyl-tetrahydro-2H-pyran-4-ol. The influence of reaction conditions on the reaction course was investigated. Under optimal reaction conditions (70 °C, a molar ratio of reactants 1:1, 1 wt% of MTO, no solvent, 100 mol% of water) the selectivity to desired product was 76%. The addition of water to the reaction mixture evoked the decrease of the reaction rate and slight increase of selectivity (without water addition the selectivity was approximately 70%). The typical composition of isomers of the desired product was cis:trans = 30:70. The mechanism of Prins cyclization using methyltrioxorhenium as the catalyst was offered. This mechanism explains the low concentration of intermediate hemiacetal in the reaction mixture using MTO as the catalyst. MTO was also heterogenized on silica, alumosilicate and alumina support. Characterization confirmed the successful attachment and homogeneous distribution of MTO on the supports, but kinetic measurement showed high leaching of MTO from pure silica and aluminosilicate support. MTO/alumina was reused with a low decrease of conversion.