Asian Research Thesis Index

Abstract

The mechanism of the Zn(II) catalyzed oxidation of benzylic alcohol to benzaldehyde, ester and amide by three different oxidants (H2O2, TBHP, and CH3OOH) is investigated through density functional theory methods and compared with the similar oxidation mechanisms of other late transition metals. Inner sphere, intermediate sphere and outer sphere mechanisms have been analyzed. The effect of pyridine-2- carboxylic acid (ligand) and halides (Br2 and I2) is studied for benzaldehyde and ester formation reactions. Two new reactions are predicted such as oxidation of thiol to thioester and oxidation of benzylamine to benzaldimine and guanidine. The same set of calculations is repeated for newly predicted reactions as were performed for ester and amide formations. The inner sphere mechanism involving β-hydride elimination is found kinetically more demanding in all oxidation reactions. Ligand showed profound effect on rate of the reaction. In the presence of a ligand, intermediate sphere mechanism is found more plausible because of steric effect. In the absence of a ligand, the outer sphere mechanism is found more favorable. Mechanism of Zn(OTf)2 catalyzed hydroamination-hydrogenation of alkynes with amines is investigated through density functional theory methods. Both inner sphere and outer sphere mechanisms for nucleophilic attack of nitrogen on electrophilic alkyne centre to deliver imine have been investigated for the hydroamination reaction. Four different possibilities of hydrogen activation for the hydrogenation of imine to deliver amine have also been studied. These competitive reactions differ regarding the fate of proton and hydride generated from heterolytic cleavage of H2. The inner sphere mechanism is kinetically more demanding and is not believed to contribute significantly to the progress of the reaction under the experimental conditions. Outer sphere route for nucleophilic attack of non-coordinated amine on coordinated alkyne is found the most plausible. The overall energy barrier for outer sphere mechanism in amine adduct can also be surpassed under the reaction conditions, therefore this mechanism cannot be excluded safely. For hydrogenation reaction, heterolytic hydrogen cleavage involving proton shift on triflate ligand and hydride to metal is found most plausible over the competitive H2 cleavage reactions. The mechanism of TiCl4 mediated formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3- dielectrophiles is also studied with B3LYP method 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. Four different mechanisms involving direct-direct, direct-conjugate, conjugate-direct and conjugate-conjugate 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 are 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.

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