Sodium Ion as the Most Essential and Effective Element for the Enantio-Differentiating…
691
Table 2 Effect of the addition
Substrate
Additive
Enantioselectivity
Hydrogenation rate
of alkali metal salts on the
enantioselectivity and the
hydrogenation rate
Methyl acetoacetate
Alkali acetate
Alkali halide
Alkali halide
Na+ >K+ >Rb+ ≈Li+ >Cs+
Na+ >K+ >Rb+ >Li+ >Cs+
Na+ >K+ ≈Rb+ >Cs+ >Li+
Na+ >K+ ≈Li+ >Rb+ >Cs+
Na+ >K+ >Rb+ >Li+ >Cs+
Na+ >K+ ≈Rb+ ≈Li+ >Cs+
Methyl levulinate
enti et al. [47, 48]. In the present study, the values by Slater
electron of the oxygen atom of the substrate approaching
the position of the valence s-electron of the alkali metal
ion is to be calculated. As for the values by Clementi et al.
when considering the penetration effects of the s-orbitals, it
would not be appropriate for the present purpose. The cal-
oxygen atom of the ester carbonyl group of the substrate
was estimated using Coulomb’s law. It was revealed that the
interaction involving the sodium ion was the strongest. This
would result in the production of the most stable sodium
tartrate–substrate complex on the nickel surface, hence,
the highest concentration of the substrate and the highest
hydrogenation rate on the enantio-differentiating site. This
would be the reason that the highest enantio-selectivity was
attained by the addition of sodium ions. The order of the
enantio-selectivity and hydrogenation rate agreed with the
order of the strength of the interaction between the alkali
metal ion and the oxygen atom of the ester carbonyl group,
except for the lithium ion.
Zeff
value
r2
was Na+ >Li+ >K+ >Rb+ >Cs+. The strongest interaction
between sodium tartrate and the substrate would result in
the most stable modifier–substrate complex, hence the
highest enantio-selectivity and hydrogenation rate.
Acknowledgements The authors thank Professor emeritus Tadao
Harada (Ryukoku University) for his valuable discussions and advice
throughout this study.
Meanwhile, the order of the effect of the addition of the
alkali metal salts on the enantio-selectivity and hydrogena-
tion rate of methyl acetoacetate and those of methyl lev-
Tables 1 and 2 agree except for the lithium ion. The lithium
the orders in Tables 1 and 2 concerning the lithium ion.
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