Reaction between Phenols and Isoprene under Zeolite Catalysis. Highly Selective Synthesis of Chromans and o-Isopentenylphenols

Abstract

Chromans 3 and o-isopentenylphenols 4 are synthesi- zed in satisfactory to good yields and selectivities by the reaction of phenols and isoprene in the presence of the commercially available acid faujasite zeolite HSZ-360. The preparation of fine chemicals following environmen- tally friendly strategies represents a challenging goal in the field of synthetic organic chemistry. 1 In the last ten years this approach has had a great development, mainly due to the use of solid acids such as clays and zeolites, (up to that time nearly exclusively employed in petrochemical processes). 2 It is now well stated that the use of these het- erogeneous catalysts represents the best answer both to the new environmental legislation and to the old chemis- try requirements: the processes they promote produce the minimum of pollution and show good 3 yields frequently accompanied by excellent selectivities. As part of a programme designed to develop new selective and preparatively useful methods based on the use of solid acids as catalysts for the preparation of fine chemicals, previous investigations of our laboratory have described the synthesis of 2H-1-benzopyrans from phenols and a- alkynols using a commercial HY-zeolite in typically a het- erogeneously catalyzed process. 4 The a-alkynols, first dehydrate into the corresponding enynes which subsequently react with phenols in a regio- selective way to form 2H-1-benzopyrans; in all cases the sole products arise from interaction of the phenol oxygen with the olefinic double bond and the ortho carbon of the phenol ring with the acetylenic framework of the enyne in- termediate. On the basis of these results we investigated the reaction of isoprene with phenols to produce chromans (3,4-dihydro-2H-1-benzopyrans) over solid acid catalysts. The synthesis of chromans or o-isopentenylphenols (2-(3- methylbut-2-enyl)phenols), 5 important classes of natural and biologically active compounds, has been extensively studied by many researchers, but it's chemo- and regiose- lective control is only achieved by using sophisticated cat- alytic systems such as conveniently prepared acid-base combinations 6 or transition metal complexes, 7 whereas the simple acid catalysis is described to result in less se-