Stepwise Diels-Alder: More than Just an Oddity? A Computational Mechanistic Study
2012 (English)In: Journal of Organic Chemistry, ISSN 0022-3263, E-ISSN 1520-6904, Vol. 77, no 15, 6563-6573 p.Article in journal (Refereed) Published
We have employed hybrid DFT and SCS-MP2 calculations at the SMD-PCM–6-311++G(2d,2p)//6-31+G(d) level to investigate the relationship between three possible channels for forming a Diels–Alder adduct from a highly nucleophilic diene and moderately to highly electrophilic dienophiles. We discuss geometries optimized using the B3LYP and M06-2X functionals with the 6-31+(d) basis set. The transition states and intermediates are characterized on the basis of geometric and electronic properties, and we also address the possibility of predicting detectability of a zwitterionic intermediate based on its relative stability. Our results show that a conventional Diels–Alder transition state conformation yields intermediates in all four investigated cases, but that these are too short-lived to be detected experimentally for the less activated reactants. The stepwise trans pathway, beginning with a conjugate addition-like transition state, becomes increasingly competitive with more activated reactants and is indeed favored for the most electrophilic dienophiles. Addition of a trans diene leads to a dead-end as the trans intermediates have insurmountable rotation barriers that prohibit formation of the second bond, unless another, heterocyclic intermediate is formed. We also show that introduction of a hydrogen bond donating catalyst favors a stepwise pathway even for less activated dienophiles.
Place, publisher, year, edition, pages
2012. Vol. 77, no 15, 6563-6573 p.
Density-Functional Theory, Main-Group Thermochemistry, Noncovalent Interactions, Quantitative Characterization, Cycloaddition Reactions, Organic-Molecules, Metal-Free, M06 Suite, Energies, Butadiene
IdentifiersURN: urn:nbn:se:kth:diva-101708DOI: 10.1021/jo301176tISI: 000307037700021ScopusID: 2-s2.0-84864609647OAI: oai:DiVA.org:kth-101708DiVA: diva2:548767
FunderSwedish Research Council
QC 201209032012-09-032012-08-312012-10-10Bibliographically approved