DSpace
 

KNUSTSpace >
Theses / Dissertations >
College of Science >

Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/11014

Title: Exploring the Peri-, Chemo-, and Regio-Selectivity of addition of Metal Oxides to Ketenes: a DFT computational study
Authors: Ahmed, Issahaku
Issue Date: 14-Oct-2017
Abstract: Ketenes are excellent precursors for catalytic asymmetric reactions, creating chiral centers mainly through addition across their C=C bonds and C=O bonds. Density functional theory calculations at the MO6/LACVP* and B3LYP/LACVP* levels of theory have been employed in a systematic investigation of the peri-, chemo- and regio- selectivity of the addition of transition metal oxo complexes of the type MO3L (M = Re, Tc, Mn; L = Cl, O-, OCH3, CH3) to substituted ketenes O=C=C(CH3)(X) [X = CH3, H, CN, Ph] with the aim of elucidating the effects of substituents on the mechanism of the reactions. The [2 + 2] addition pathway, across the C=C or C=O (depending on the ligand), is the most preferred in the reactions of dimethyl ketene with all the metal complexes studied. The [2 + 2] pathway is also the most preferred in the reactions of ReO3Cl with all the substituted ketenes studied except when X = Cl. Thus of all the reactions studied, it is only the reaction of ReO3Cl with O=C=C(CH3)(Cl) that prefers the [3 + 2] addition pathway. Reactions of dimethyl ketene with ReO3L favours addition across C=O bonds of the ketene when L = O- and CH3 but favours addition across C=C bonds when L = OCH3 and Cl. In the reactions of ReO3Cl with substituted ketenes, addition across C=O bonds is favoured only when X = H while addition across C=C bonds is favoured when X = CH3, Cl, Ph, CN. The order in the activation energies of the reactions of dimethyl ketenes with the metal complexes ReO3L with respect to changing ligand L is O- < CH3O- < Cl- < CH3 while the order in reaction energies is CH3 < CH3O- < O- < Cl-. For the reactions of substituted ketenes with ReO3Cl, the order in activation barriers is CH3 < Ph < CN < Cl < H while the reactions energies follow the order Cl < CH3 < H < Ph < CN. In the reactions of dimethyl ketenes with ReO3L, the trend in the selectivity of the reactions with respect to ligand L is Cl- < CH3O- < CH3 < O- while the trend in selectivity is CH3 < CN < Cl < Ph in the reactions of ReO3Cl with substituted ketenes. In the reactions of TcO3L (L = Cl, O-, OCH3, CH3) to substituted ketenes O=C=C(CH3)(X) [X = H, CH3, Cl, CN, Ph] the [2 + 2] addition across the C=C bond of the ketenes is the preferred pathway while the [3 + 2] addition across the C=C bond of the ketenes is the preferred pathway for L = Cl, OCH3. The order in the activation energies of the preferred [3 + 2] and [2 +2] pathways for addition of dimethyl ketenes to the metal complexes TcO3L with respect to changing ligand L is O- < Cl < CH3 < CH3O- while the order in reaction energies is CH3 < CH3O- < O- < Cl. For the reactions of substituted ketenes with TcO3Cl, the order in activation barriers for the preferred addition pathways is CH3 < Ph < H < Cl< CN while the reactions energies follow the order Cl < CH3 < H < Ph < CN. In the reactions of dimethyl xv ketenes with TcO3L, the trend in the selectivity of the reactions is Cl < CH3O- < CH3 < O- while the trend in selectivity is CH3 < H < CN < Cl < Ph in the reactions of TcO3Cl with substituted ketenes. In the reactions of MnO3L (L = Cl, O-, OCH3, CH3) to substituted ketenes O=C=C(CH3)(X) [X = H, CH3, Cl, CN, Ph] the [3 + 2] addition across the C=C is the preferred pathway for all the reactions studied irrespective of the ligand or substituents on the ketene except for L = O which undergo stepwise addition pathway. In the reaction of MnO3Cl with the substituted ketenes (O=C=C(CH3)(X); X = H, CH3, Cl, CN, Ph), the [2 + 2] addition across the C=O of the ketene is preferred for L = OCH3 over C=C of the ketene. No [2 + 2] addition pathways were located except for L = OCH3. The order in the activation energies of the preferred [3 + 2] and [2 +2] pathways for addition of dimethyl ketenes to the metal complexes MnO3L with respect to changing ligand L is O- < Cl < CH3 < CH3O- while the order in reaction energies is CH3 < CH3O- < Cl < O- . For the reactions of substituted ketenes with MnO3Cl, the order in the activation energies for the preferred addition pathways is O- < Cl < CH3 < CH3O- while the order in reaction energies is CH3 < CH3O- < Cl < O-. For the reactions of substituted ketenes with MnO3Cl, the order in activation barriers for the preferred addition pathways is Cl < H < CN < CH3 < Ph while the reactions energies follow the order H < CH3 < CN < Ph < Cl. In the reactions of dimethyl ketenes with MnO3L, the trend in the selectivity of the reactions is Cl- < CH3O- < CH3 < O- while the trend in selectivity is H < Cl < CH3 < CN < Ph in the reactions of MnO3Cl with substituted ketenes (O=C=C(CH3)(X); X = H, CH3, Cl, CN, Ph). Generally, reactions involving a change in oxidation state of metal from the reactant to the product have high activation barriers while reactions that do not involve a change in oxidation state have low activation barriers. The changes in oxidation state were observed for substituents or ligands with inductive effect. A triplet zwitterionic intermediate is formed in the reactions of the MO3L with the substituted ketenes for all the metals. The reactions of dimethyl ketene with MO3L (L = Cl, O-, OCH3, CH3) will most likely lead to the formation of an ester precursor for each metal. For both [3 + 2] and [2 + 2] addition, low activation barriers are obtained when the substituent on the ketene is electrondonating while high activation barriers are obtained when the substituent is electron-withdrawing. The results show that the reactions of ketenes with MnO3L complexes have lower activation barriers for the preferred [3 + 2] and [2 + 2] addition pathways than those of the ReO3L and TcO3L complexes reported in the literature. Thus the MnO3L complexes may be better catalysts for the activation of the C=C bonds of substituted ketenes than the reported ReO3L and TcO3L complexes and is in the order Mn < Tc < Re.
Description: A thesis submitted to the Department of Chemistry, College of Science, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirement for the award of the degree of Master of Philosophy in Physical Chemistry, 2015.
URI: http://hdl.handle.net/123456789/11014
Appears in Collections:College of Science

Files in This Item:

File Description SizeFormat
Issahaku Ahmed.pdf191.62 kBAdobe PDFView/Open

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

 

Valid XHTML 1.0! DSpace Software Copyright © 2002-2010  Duraspace - Feedback