Exploring the Peri-, Chemo-, and Regio-Selectivity of addition of Metal Oxides to Ketenes: a DFT computational study
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Date
August, 2015
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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