Browsing by Author "Pipim, George Baffour"

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    Peri-, Chemo-, Regio-, Stereo- and Enantio-Selectivities of 1,3-dipolarcycloaddition reaction of C,N-Disubstituted nitrones withdisubstituted 4-methylene-1,3-oxazol-5(4H)- one: A quantummechanical study
    (Elsevier Inc., 2020-01-21) Pipim, George Baffour; Opoku, Ernest; Tia, Richard; Adei, Evans
    The peri-, chemo-, regio-, stereo- and enantio-selectivities of 1,3-dipolar cycloaddition reaction of C,N-disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)-one have been studied usingdensity functional theory (DFT) at the M06e2X/6-311G (d,p) level of theory. The 1,3-dipole preferentiallyadds chemo-selectively across the olefinic bond in a (3þ2) fashion forming the corresponding spi-rocycloadduct. The titled reaction occurs with poor enantio- and stereo-selectivities, but a high degree ofregio-selectivity is observed for the addition of the 1,3-dipole across the dipolarophile. Electron-withdrawing groups on the dipolarophile significantly reduce the activation barriers while electron-donating groups on the dipolarophile increase the activation barriers. Analysis of the HOMO andLUMO energies of the two reacting species indicates that the 1,3-dipole reacts as a nucleophile while thedipolarophile reacts as the electrophile. Investigation of the electrophilic Parr function (PþKÞat the variousreaction centers in the dipolarophile indicates that the 1,3-dipole selectively adds across the atomicspecies with the largest electrophilic Mulliken and NBO atomic spin densities which is in accordancewith the energetic trends observed.
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    Site‑, enantio‑ and stereo‑selectivities of the 1,3‑dipolar cycloaddition reactions of oxanorbornadiene with C,N‑disubstituted nitrones and dimethyl nitrilimines: a DFT mechanistic study
    (Springer-Verlag GmbH Germany, 2020-01-01) Opoku, Ernest; Arhin, Grace; Pipim, George Baffour; Adams, Anita Houston; Tia, Richard; et. al
    1,3-Dipolar cycloaddition of nitrones to oxanorbornadienes is an important method for the enantioselective synthesis of highly substituted 5-membered heterocycles such as furans and isoxazolidines, which have high utility in the chemical and pharmaceutical industries. The mechanism of the reaction and the effects of substituents on the (3 + 2) cycloaddition reactions (32CA) of C,N-dialkyl nitrones with a series of substituted oxanorbornadienes have been studied with focus on the site-selectivity (attack on the more substituted double bond of the oxanorbornadiene derivatives versus attack on the less substituted double bond), enantioselectivity and stereo-selectivity using density functional theory calculations at the M06/6-311++G(d,p) of theory. The results showed that the addition step to form the bicyclic isoxazolidines cycloadducts has generally low barriers compared to the cycloreversion step which converts the cycloadducts into furans and monocyclic isoxazolidines. Generally, electron-withdrawing substituents favour the nitrone attack on the highly substituted double bond, while electron-donating substituents favour the attack on less substituted double bond. The R enantiomers are generally favoured over the S enantiomers, and exo stereo-isomers are generally favoured over the endo stereo-isomers, irrespective of substituents.