(544fe) Mechanistic Insights into the Prins Condensation of Formaldehyde with Butene Isomers over H-[Al]-ZSM5 Catalyst

As steam crackers in the U.S shift their attention from naphtha to ethane feedstock, there is a tight supply of some commodity chemicals. Among these is isoprene, an important monomer in synthetic rubbers. Prins condensation between isobutene and formaldehyde can be used as an on-purpose method to form isoprene by way of dehydration of the Prins product, 3-methyl-3-buten-1-ol.^{1, 2}

The thermal Prins reaction follows a concerted mechanism through a single, rather asynchronous transition state complex with a six-membered ring geometry. The reaction is quite slow, with activation energy of ca. 36 kcal/mol or higher, but it can be catalyzed by Lewis acids via coordination of the enophile and an electronic mechanism readily understood in terms of frontier molecular orbital theory—the Lewis acid lowers the LUMO of the enophile and closes the energy gap to the HOMO of the ene. Over Lewis acids, the single transition state retains its six-membered ring geometry, but becomes more zwitterionic in character and substantially more asynchronous. Overall, the mechanism of the reaction over Lewis acids is quite well understood, elucidated by quantum chemical calculations.^{3, 4}

The mechanism over Brønsted acids is not as well studied. We will present quantum chemical calculations showing that the Brønsted acid-catalyzed reaction follows only a two-step mechanism that formally entails protonation of the formyl group, electrophilic attack on the alkenyl group and deprotonation of the resulting carbocation. Detailed free energy profiles and electronic analysis will be presented.

The butene-formaldehyde co-adsorbed complex requires very modest activation; intrinsic free energy barriers of 3.1, 4.6 and 5.7 kcal/mol for isobutene, 1-butene and cis-2-butene, respectively, at 150 °C. This trend seems intuitive and to be tracking the relative stabilities of the resulting intermediate carbocations (1°<2°<3°). We will argue, however, that the barrier of the electrophilic attack is not determined by the stability of the resulting carbocation but rather by the nucleophilic character of the carbon atom being attacked. This explains why the barrier for 1-butene is somewhat lower than that for cis-2-butene even though both lead to 2° carbocations.

The second transition state involves deprotonation of the carbocation and is rate controlling, because proton transfer between carbon and oxygen atoms is generally slow. The corresponding intrinsic barriers are 7.9, 6.7 and 13.2 kcal/mol for isobutene, 1-butene and cis-2-butene, respectively. For all three isomers, the transition state is characterized by a delocalized proton with partial charges ranging between 0.44 and 0.49e. The fractional charges are consistent with the Mulliken charge transfer picture—and indeed our current understanding—of proton transfer, according to which proton transfer is activated by electron density shift from the lone pair of the acceptor atom to the donor-H antibonding orbital. Within this framework, we will rationalize the observed intrinsic barriers: the lower the energy of the , the stronger the electronic coupling to the lone pair of the acceptor and thus the more facile the proton transfer.

From the overall free energy profiles and according to the energy span model, the turnover frequency determining apparent activation energy of the Prins reaction is given by the energy span between the second transition state and the most stable, preceding intermediate. For isobutene, 1-butene and cis-2-butene the free energy spans are 8.5, 9.3 and 17.5 kcal/mol, respectively, at 150 °C, in complete agreement with experimental reactivity observations.

References

1. Sushkevich, V. L.; Ordomsky, V. V.; Ivanova, II, Isoprene synthesis from formaldehyde and isobutene over Keggin-type heteropolyacids supported on silica. Catalysis Science & Technology 2016, 6 (16), 6354-6364.
2. Pavlov, O. S.; Karsakov, S. A.; Pavlov, S. Y., A new technology for the production of isoprene from isobutene-containing C-4 fractions and formaldehyde: Prospects for industrial reconstruction. Theoretical Foundations of Chemical Engineering 2011, 45 (4), 487-491.
3. Fu, H.; Xie, S. W.; Fu, A. P.; Ye, T. X., Theoretical study of the carbonyl-ene reaction between formaldehyde and propylene on the MgY zeolite. Computational and Theoretical Chemistry 2012, 982, 51-57.
4. Yang, Q. W.; Tong, X. L.; Zhang, W. Q., Influence of Lewis acids and substituents on carbonyl-ene reactions: A density functional theory study. Journal of Molecular Structure-Theochem 2010, 957 (1-3), 84-89.