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End date: 2019 × Start date: 2010 × Type: Text × Subject: Catalysis × Subject: Transition metals × WGL Institute: LIKAT ×

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Now showing 1 - 2 of 2
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    Theoretical mechanistic investigation of zinc(ii) catalyzed oxidation of alcohols to aldehydes and esters
    (London : RSC Publishing, 2016) Nisa, Riffat Un; Mahmood, Tariq; Ludwig, Ralf; Ayub, Khurshid
    The mechanism of the Zn(II) catalyzed oxidation of benzylic alcohol to benzaldehyde and ester by H2O2 oxidant was investigated through density functional theory methods and compared with the similar oxidation mechanisms of other late transition metals. Both inner sphere and intermediate sphere mechanisms have been analyzed in the presence and absence of pyridine-2-carboxylic acid (ligand). An intermediate sphere mechanism involving the transfer of hydrogen from alcohol to H2O2 was found to be preferred over the competitive inner sphere mechanism involving β-hydride elimination. Kinetic barriers associated with the intermediate sphere mechanism are consistent with the experimental observations, suggesting that the intermediate sphere mechanism is a plausible mechanism under these reaction conditions. The oxidation of alcohols to aldehydes (first step) is kinetically more demanding than the oxidation of hemiacetals to esters (second step). Changing the oxidant to tert-butyl hydrogen peroxide (TBHP) increases the activation barrier for the oxidation of alcohol to aldehyde by 0.4 kcal mol−1, but decreases the activation barrier by 3.24 kcal mol−1 for oxidation of hemiacetal to ester. Replacement of zinc bromide with zinc iodide causes the second step to be more demanding than the first step. Pyridine-2-carboxylic acid ligand remarkably decreases the activation barriers for the intermediate sphere pathway, whereas a less pronounced inverse effect is estimated for the inner sphere mechanism.
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    Mechanistic insight of TiCl4catalyzed formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles
    (London : RSC Publishing, 2015) Nisa, Riffat Un; Maria, Maria; Wasim, Fatima; Mahmood, Tariq; Ludwig, Ralf; Ayub, Khurshid
    The mechanism of TiCl4 mediated formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles is studied at the B3LYP level of density functional theory (DFT) to rationalize the experimental regioselectivity. Methyl and trifluoromethyl substituted 1,3 dielectrophiles are studied theoretically since they show different regioselectivities. Two different mechanisms involving 1,2 and 1,4 addition of 1,3-bis(silyl enol ethers) on 1,3-dielectrophiles are studied for each dienophile. The intramolecular transition metal catalyzed and non-catalyzed dynamic shift of the silyl moiety is also studied. The structure of the 1,3 dienophile and the associated Mulliken charges are the driving forces for different regioselectivities in methyl and trifluoromethyl dienophiles.