H-type Zeolite-Catalyzed 1,4-Addition of Benzene Derivatives to Labile Acrolein

The 1,4-addition of benzene derivatives to acrolein is a straightforward way to synthesize 3-arylpropanals. A survey of acid catalysts for the 1,4-addition of methoxy-substituted benzenes to acrolein revealed that H-Beta and H-Y were the most suitable catalysts. We hypothesized three side-reactions: (1) the double 1,4-addition of acrolein to the starting benzene derivatives, (2) the Friedel Crafts-type alkylation to the desired product, and (3) the self-polymerization of acrolein. The type (3) side-reaction was inhibited by two different methods which kept the concentration of acrolein low in the reaction mixture or in the zeolite pores. First, acrolein monomers were in situ generated through the gradual monomerization of an acrolein cyclic trimer. Second, using a reaction solvent lowered the acrolein concentration in the zeolite pores due to the competitive adsorption. We discovered that the content of monomeric acrolein in a solvent was closely related to the polarity of the solvent. Actually, both methods improved the yields for the 1,4 additions of 1,3-dimethoxybenzene to acrolein. Other electron rich benzene derivatives, such as phenol and N,N-dimethylaniline, were also applicable to the 1,4-addition reactions.