Investigation of the Acid Catalyzed Formaldehyde-Olefin Condensation Reaction

New carbon-carbon bonds can be formed via the “ene” reaction, a pericyclic process between an alkene having an allylic hydrogen atom (an “ene”) and an electron-deficient multiple bond (an enophile) to form two sigma-bonds with transposition of the pi-bond [1] (see Scheme 1).

The Prins reaction is an example of the “ene” reaction referring to condensation of formaldehyde with olefins [2,3]. Functionalization of simple olefins in this way results in versatile building-blocks. The commercial availability of lower olefins, especially those formed by on-purpose methods from their corresponding alkanes (C3 and C4), coupled with the versatility of formaldehyde as a one-carbon electrophile make this reaction potentially important. Prins condensation can form unsaturated alcohols, diols, alkyl dioxanes, pyran skeleton compounds and dienes. The reaction is an acid catalyzed addition of aldehydes to olefins and is traditionally catalyzed by homogeneous mineral acids such as sulphuric acid or homogeneous Lewis acids like SnCl₄, BF₃ and ZnCl₂ [4, 5].

We have investigated solid Lewis acid catalysis for the reaction. Acidic zeolite Beta catalysts have been synthesized and evaluated in the condensation of formaldehyde with propylene. Specifically, framework Sn-, Zr-beta, and Zn-Beta—the last prepared by ion exchange— were prepared and characterized. The catalytic activity was tested by autoclaving formaldehyde and propylene using an appropriate solvent at a temperature range of 120-180⁰C. We have found that Zn-Beta is the most active catalyst for this reaction. Different reaction pathways are followed (see Scheme 2) producing valuable products. The product spectra can be divided into two categories; compounds with four carbon atoms and compounds having five carbon atoms. C-4 products include the unsaturated alcohol, 3-buten-1-ol, which dehydrates to form 1,3-butadiene. Reaction of excess formaldehyde with propylene forms the alkyl dioxane, 1,3-Dioxane, 4-methyl- (C-5 product). This compound can potentially hydrolyzed to afford 1,3-butanediol and CH₂O (a non-favored pathway under our experimental conditions). Prins cyclization [6] (reaction between 3-buten-1-ol and formaldehyde) leading to the synthesis of tetrahydropyran-ol, which subsequently dehydrates to the corresponding dihydropyran is the dominant reaction route. Based on these data, it seems necessary to exert an accurate control of the experimental conditions and the catalyst used in order to selectively drive the reaction in a specific direction. On-going research is directed towards optimization of reaction conditions to minimize important undesired side reaction pathways as well as evaluation of different pore size zeolites to induce shape selectivity effects.

 

 

 

References

[1] M. Yamanaka and K. Mikami, Helvetica Chimica Acta, 85 (2002) 4264

[2] P. C. Bloys van Treslong Prins, Chem. Weekkbl. 1919, 1510

[3] P. C. Bloys van Treslong Prins, Chem. Weekkbl. 1919, 1072

[4] D. R. Adams and S. P. Bhatanagar, Synthesis 10 (1977) 661

[5] W. Fitzky, U.S. Patent 2,325,760, 1943

[6] C. Olier, M. Kaafarani, S. Gastaldi, M. P. Bertrand, Tetrahedron 66 (2010) 413