Eklund, L; Axelsson, AK; Nordahl, A; Carlson, R
The use of zeolites as catalysts in Diels-Alder reactions of isoprene, 2, has been studied. The regioselectivity in zeolite-catalyzed reactions 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 (1c), cyclohex-2-enone (1d), phenylethene (1e), ethyl propynoate (1f) and propenenitrile (1g). The following catalysts were investigated: zeolites: ZSM-5, mordenite, zeolite Y-152, zeolite Y-45, and zeolite beta; Lewis acids: aluminum tribromide, aluminum trichloride, antimony pentachloride, antimony triiodide, boron trifluoride-diethyl ether, manganese difluoride and zinc dichloride. The zeolite-catalyzed reactions afforded the Diels-Alder product with the dienophiles 1a-1d, but not with the other dienophiles, 1e-1g. The dienophiles which failed to undergo cycloaddition in the presence of zeolites also did so or afforded poor results with the Lewis acid catalysts. With the exception of the reaction 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 Lewis acid catalysis.The experimental conditions were studied in detail 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 experimental details are given for the preparative use of the method. Examples are provided by the reactions with the dienophiles 1a, 1c and 1d catalyzed by zeolite beta.Factors determining the regioselectivity of zeolite-catalyzed Diels-Alder reaction are briefly discussed.