(674e) 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