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

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.

Source: KNUST