Browsing by Author "Ahmed, Issahaku"

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    Exploring the Peri-, Chemo-, and Regio-Selectivity of addition of Metal Oxides to Ketenes: a DFT computational study
    (August, 2015) Ahmed, Issahaku
    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.
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    Exploring the peri-, chemo-, and regio-selectivity of additionof manganese metal oxides MnO3L(L=Cl ,O , OCH3,CH3)tosubstituted ketenes: A computational study
    (Elsevier B.V. I., 2015-11-11) Ahmed, Issahaku; Tia, Richard; Adei, Evans
    Ketenes are interesting reactive intermediates that find a wide range of synthetic applications. Densityfunctional theory calculations at the MO6/LACVP⁄and B3LYP/LACVP⁄levels of theory have beenemployed to explore the peri-, chemo-, and regio-selectivity of the addition of manganese oxo complexesMnO3L (L = Cl, O , OCH3,CH3) to substituted ketenes O@C@C(CH3)(X) [X = H, CH3, Cl, CN, Ph] with the aimof elucidating the effects of substituents on the mechanism of the reactions. The results show that theconcerted [3+2] addition of the C@C bond of the ketene across the metal complex is the most preferredpathway in all the reactions studied (with respect to changing ligand L on the metal complex or sub-stituent X on the ketene) except in the reaction of MnO4 (i.e. for L = O ) with dimethyl ketene, which fol-lows only a stepwise addition pathway. [2+2] addition is found to be possible only in the reaction ofMnO3–OCH3with dimethyl ketene where the activation barrier for [2+2]C@Oaddition is 23.79 kcal/mol,which is far greater than the barrier for the [3+2] addition. The reactions of dimethyl ketene withMnO4 will most likely lead to the formation of an ester precursor and the reaction of MnO3Cl with thesubstituted ketenes would lead to the formation of an ester precursor, chlorohydrin precursor, acetalde-hyde and carbon monoxide (for X = H, Cl). Generally, reactions involving an increase in oxidation state ofmetal have higher activation barriers. For both [3+2] and [2+2] addition, low activation barriers areobtained when the substituent on the ketene is electron-donating while high activation barriers areobtained when the substituent is electron-withdrawing. The reactions of ketenes with MnO3L complexeshave lower activation barriers for the preferred [3+2] and [2+2] addition pathways as well as fewer sidereactions than those of the ReO3L complexes reported in the literature, a trend which was seen in our ear-lier work with reactions of group VII metals with olefins, implying that manganese oxo complexes effi-ciently and selectively catalyze specific reactions in oxidation of ketenes and olefins than do Re oxocomplexes and therefore Mn oxo complexes may be better catalysts for specific oxidation reactions ofketenes and olefins than Re complexes are.
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    Exploring the peri-, chemo-, and regioselectivity of addition oftechnetium metal oxides of the type TcO3L(L=Cl–,O–, OCH3,CH3) to substituted ketenes: a DFT computational study
    (NRC Research Press, 2016-02-02) Ahmed, Issahaku; Tia, Richard; Adei, Evans
    The addition of TcO3L(L=Cl,O–, OCH3,CH3) to substituted ketenes along various addition pathways was studied withdensity functional theory calculations to explore the peri-, chemo-, and regioselectivity of the reactions. In the reactions of TcO3L withdimethyl ketene, the results show that forL=O–and CH3, [1 + 1] addition to form a triplet zwitterionic intermediate is the preferredfirst step; for L = Cl, the [3 + 2]C=Caddition across the O–Tc–Cl bond is the preferred first step and forL=OCH3the [3 + 2]C=Cadditionacross the O–Tc–OCH3 bond is the preferred first step. In the reactions of TcO3Cl with substituted ketenes, [1 + 1] addition to form atriplet zwitterionic intermediate is the preferred first step for X = Ph, CN, and Cl; the [3 + 2]C=Caddition across the O–Tc–O bond of thecomplex is the preferred first step forX=H,while the [3 + 2]C=Caddition across the O–Tc–CH3 bond is the preferred first step. Reactionsinvolving a change in the oxidation state of metal have high activation barriers, while reactions that do not involve a change inoxidation state have low activation barriers. Reactions of ketenes with TcO3L complexes have lower activation barriers for thepreferred addition pathways than those of the ReO3L complexes reported in the literature. Thus, the TcO3L complexes may be bettercatalysts for the activation of the C=C bonds of substituted ketenes than the reported ReO3L complexes.
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    A quantum chemical study of the mechanisms of olefin addition to group 9 transition metal dioxo compounds
    (Springer International Publishing, 2016-12-01) Ahmed, Issahaku; Tia, Richard; Adei, Evans
    The mechanistic aspects of ethylene addition to MO2(CH2)(CH3) (M=Co, Rh, Ir) have been investigated with a Hartree–Fock/DFT hybrid functional at the MO6/LACVP* and B3LYP/LACVP* levels of theory to elucidate the reaction pathways on the singlet, doublet and triplet potential energy surfaces (PES). In the reaction of the IrO2CH2CH3 complex with ethylene, [3+2]C,O addition is the most plausible pathway on the singlet PES, the [3+2]O,O addition is the most favoured pathway on the doublet surface whiles the stepwise [1+1] addition involving the oxygen atom of the com-plex in the first step and the carbon atom of the complex in the second step is the most plausible pathway on the triplet PES. For the reaction of the RhO2(CH2)(CH3) complex, the [2+2]Rh,O addition pathway is the most favoured on the singlet surface, the [2+2]Rh,C is the most plausible pathway on the triplet PES and [3+2]C,O is the most plausible on the doublet surface. For the reactions of the CoO2(CH2)(CH3) complex, the [1+2]O addition is the most plausible on the singlet PES, [3+2]C=Co=O cycloaddition to form the five–membered intermediate is the most preferred path-way on the doublet PES, whiles on the triplet PES the preferred pathway is the [3+2] addition across the O=Co=O bond of the metal complex. The reactions of olefins with the Co dioxo complex have lower activation barriers for the preferred [3+2] and [2+2] addition pathways as well as fewer side reactions than those of the rhodium and iridium systems. This could imply that the cobalt dioxo complexes can efficiently and selectively catalyze specific reactions in oxidation of olefins than Rh and Ir oxo complexes will do and therefore Co oxo complexes may be better catalysts for specific oxidation reactions of olefins than Rh and Ir complexes are. The activation barriers for the formation of the four—or five-membered metallacycle intermediates through [2+2] or [3+2] cyclo-addition are lower on the triplet PES than on the singlet PES for the formation of similar analogues. There are fewer competitive reaction pathways on the triplet surface than on the singlet PES. Also, cycloadditions that seem impossible on the singlet PES seem possible on the doublet and or triplet PESs, this is the case typically for the Rh and Co complexes, illustrating the importance of multiple spin states in organometallic reactions.