ZEOLITE AND LEWIS ACID CATALYSIS IN DIELS-ALDER REACTIONS OF ISOPRENE

The use of zeolites as catalysts in Diels-Alder reactions of isoprene, 2, has been studied. The regioselectivity in zeolite-catalyzed reacti ons was compared with the regioselectivity of the same reaction using Lewis acid catalysts. The reaction was studied with seven dienophiles: but-3-en-2-one (1a), 3-bromobut-3-en-2-one (1b), methyl propenoate (1 c), cyclohex-2-enone (1d), phenylethene (1e), ethyl propynoate (1f) an d propenenitrile (1g). The following catalysts were investigated: zeol ites: ZSM-5, mordenite, zeolite Y-152, zeolite Y-45, and zeolite beta; Lewis acids: aluminum tribromide, aluminum trichloride, antimony pent achloride, antimony triiodide, boron trifluoride-diethyl ether, mangan ese difluoride and zinc dichloride. The zeolite-catalyzed reactions af forded the Diels-Alder product with the dienophiles 1a-1d, but not wit h the other dienophiles, 1e-1g. The dienophiles which failed to underg o cycloaddition in the presence of zeolites also did so or afforded po or results with the Lewis acid catalysts. With the exception of the re action of 1b catalyzed by ZSM-5, which afforded a regioisomeric ratio of 13/87, the other zeolite-catalyzed reactions afforded regioisomeric ratios in the range 6/94-0/100 which is similar to that obtained by L ewis acid catalysis. The experimental conditions were studied in detai l for two zeolite catalysts (mordenite, zeolite beta) in the reaction of isoprene with 1d. The optimum experimental conditions with respect to yield and selectivity were established by multivariate techniques. In this reaction, zeolite catalysis was found to afford high yields of cis-6-methyl-6(7)-1-octalone. Under Lewis acid catalyzed conditions, this product is rapidly epimerized into the trans isomer. Full experim ental details are given for the preparative use of the method. Example s are provided by the reactions with the dienophiles 1a, 1c and 1d cat alyzed by zeolite beta. Factors determining the regioselectivity of ze olite-catalyzed Diels-Alder reaction are briefly discussed.