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Title: 1,3-Dipolar [3 + 2] cycloaddition reactions of N,C,C-trisubstituted nitrones with ring-acceptor methylenecyclopropanes: a computational study
Authors: Mensah, Mary
Amepetey, Evans Elikem
Tia, Richard
Adei, Evans
Keywords: Methylenecyclopropanes
Nitrones
Regioselectivity
Density functional theory
Spirocyclopropanes
Issue Date: 1-Dec-2016
Publisher: Springer International Publishing
Citation: Mensah et al. SpringerPlus (2016) 5:2072 DOI 10.1186/s40064-016-3758-0
Abstract: Background: 1,3-Dipolar [3+ 2]-cycloaddition of nitrones to the carbon–carbon double bonds of methylenecy-clopropanes yields a mixture of regioisomeric 4- and 5-isoxazolidines. The mechanisms of the reactions of N,C,C-trisubstituted nitrones with ring acceptor substituted dimethyl methylenecyclopropanes-1,2-dicarboxylate and aryl methylidene cyclopropanes-1,1-dicarboxylate have been investigated with the Becke 3-Parameter Lee–Yang–Par exchange–correlation functional, a Hartree–Fock DFT hybrid functional, to delineate the factors responsible for the regioselectivity of these class of reactions.Findings: The energetics of the reaction of the phenyl-substituted nitrone with unsubstituted methylenecyclopro-pane indicate that the formation of the 5-spirocyclopropane is favored over the 4-spirocyclopropane kinetically and thermodynamically. However, the energetics of the reaction of the same phenyl nitrone with vicinal ester (–CO2CH3)-substituted methylenecyclopropane show an inversion in the regioselectivity favoring the formation of the 4-regioiso-mer over the 5-regioisomer. For the reactions of N,C,C-trisubstituted nitrone with vicinal ester (–CO2CH3)-substituted methylenecyclopropane (–R1=H, –R2=Ph and –R1=CH3 and –R2=CO2CH3) and geminal ester (–CO2CH3)-substituted methylenecyclopropane (R3=H, R4=H; R3=OCH3, R4=CH3; and R3=H, R4=Cl), the energetics indicate that the 5-spiro-cyclopropane is favored over the 4-spirocyclopropane. The calculations also indicate that electron-donating groups increase regioselectivity of the 5-regioisomers over the 4-regioisomers.Conclusion: The regioselectivity of these reactions is determined by both electronic and steric factors. The pathways with the lower activation barrier leads to the more stable regioisomer in all cases, implying that the pathways that are kinetically favored are also thermodynamically favored. However, it is also clear from the energetics that these reac-tions are not reversible and are therefore under kinetic control. Therefore the selectivity of the reactions is governed solely by the difference in activation barriers leading to the two isomers and not in any way by the thermodynamic stability of the isomers formed.
Description: An article published by Springer International Publishing and also available at DOI 10.1186/s40064-016-3758-0
URI: http://hdl.handle.net/123456789/12685
Appears in Collections:College of Science

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