A DFT mechanistic study of the generation of azomethine ylides from thering-opening reactions of stabilized aziridines and follow-up 1,3-dipolarcycloaddition reactions with acetaldehyde
This work investigated computationally with density functional theory calculations at the M06/6-311G∗level,the ring opening reaction of various methyl-, phenyl- and carbonyl- substituted aziridines to obtain azomethineylides and the subsequent 1,3-dipolar cycloaddition reaction with acetaldehyde leading to 3-methyl and 4-methyl regioisomers andendo- andexo- stereoisomers. The activation barrier for the electrocyclic ring openingof the parent aziridine is very high (51.3 kcal/mol) but is lowered by at least 15.5 kcal/mol upon methyl andester group substitutions. In the reaction of 1,3-diphenyl-2,2-methoxycarbonylaziridineA2with acetaldehyde,the ring opening step is rate-determining with an activation barrier of 28.9 kcal/mol. The activation barrier forthe formation of the 4-methyl isomer from this reaction is at least 7.4 kcal/mol lower than that for the formationof the 3-methyl isomer, which is in accord with the experimentally-observed regioselectivity. Also, the formationof theexoisomer is more favoured than theendoisomer as the barrier of the former is 2.7 kcal/mol compared to6.1 kcal/mol for the latter. There is an inverse correlation between the activation barriers for the electrocycliccleavage of the aziridines and the electrophilicities of the resulting azomethine ylides. The results are rationa-lized in terms of perturbation molecular orbital theory.
An article published by Elsevier B.V. and also available at https://doi.org/10.1016/j.comptc.2018.10.002
Acetaldehyde, Aziridine, Azomethine ylide, Oxazolidines: Mechanistic study
S. Nantogma et al. A DFT mechanistic study of the generation of azomethine ylides from thering-opening reactions of stabilized aziridines and follow-up 1,3-dipolarcycloaddition reactions with acetaldehyde. https://doi.org/10.1016/j.comptc.2018.10.002