Concerted and stepwise mechanisms in the Diels–Alder and Michael reactions of furans with methyl 3-nitroacrylate — Experimental and theoretical studies

Abstract

The Diels–Alder reactions of the furans (1a–1c) and methyl 3-nitroacrylate (2) produce the cycloadducts via a concerted mechanism. Then, the products subsequently undergo retro-Diels–Alder reactions to give the Michael adducts via a stepwise mechanism. We have used both experimental and theoretical methods to investigate the mechanism for, and the regio- and stereo-selectivity in, the Diels–Alder and Michael reactions. To account for the selectivity observed in these additions, we examined the frontier molecular orbitals involved, and to locate the transition states (TS), we employed a B3LYP/6-31G* level density functional calculation. We were able to explain the regio- and stereo-selectivity of the four possible Diels–Alder adducts observed in the experiments by a comparison of their calculated activation energies, which ranged from 11 to 18 kcal/mol (1 cal = 4.184 J). The s-cis forms of the dienophile (2) were found to be more stable than the s-trans forms, both in the ground state and in the transition state. In addition, the endo transition structures for the ester group exhibited a lower energy barrier by 0.3 kcal/mol than their exo counterparts, a value that is in disagreement with the experimental results. The presence of the nitro group in 2 may play an important role in determining the selectivity. Our attempts to find a stepwise mechanism leading to the Diels–Alder adduct via a zwitterion were unsuccessful. We did find that two stepwise processes lead to the formation of Michael adducts generated via an electrophilic attack by 2 at the α position of 1b and 1c, and an intramolecular proton-transfer mediated by the formation of a four-membered structure. The potential energies for these reactions showed values in the following ranges: 17 kcal/mol for 5a and 5b, and 11 to 12 kcal/mol for 7a and 7b for the first step, and, for the proton-transfer step, 48–51 kcal/mol for 5a and 5b, and 41–45 kcal/mol for 7a and 7b. Solvent effects in chloroform on the Michael addition did not appear in the electrophilic attack step (less than 4 kcal/mol), but the transition state of the later process was stabilized by 6–13 kcal/mol.Key words: Diels–Alder reactions, Michael reactions, density functional method, furan, retro-Diels–Alder reactions, solvent effect.