Gas-Phase Lithium Cation Basicities of Acetophenones : A Linear Relationship between Lithium Cation and Proton Basicities

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    • Than Soe [他] THAN Soe
    • Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University
    • MISHIMA Masaaki
    • Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University


The free energy changes (Δ<I>G</I>) for the reaction B<SUB>2</SUB>Li<SUP>+</SUP> = Li<SUP>+</SUP> + 2B were obtained in the gas phase for <I>m</I>- and <I>p</I>-substituted acetophenones based on the measurement of ligand-exchange equilibria of the type LiA<SUB>2</SUB><SUP>+</SUP> + 2B = LiB<SUB>2</SUB><SUP>+</SUP> + 2A using a FT-ICR spectrometer. A plot of Δ<I>G</I> (relative lithium cation basicity, ΔLCB[B<SUB>2</SUB>Li<SUP>+</SUP>]) values against the ΔGB (proton basicity) showed a limited linear correlation for <I>meta</I> substituents and <I>para</I> electron-withdrawing groups with a slope of 1.06. Negative deviations observed for strong π-donor susbstituents suggested that the resonance effect of these substituents is reduced in the Li<SUP>+</SUP> complex compared to that in the protonated ion. Based on the correlation analysis using the Yukawa–Tsuno equation, <SUB></SUB>, the substituent effect on ΔLCB[B<SUB>2</SUB>Li<SUP>+</SUP>] of acetophenone was characterized by a ρ value of −12.5 and an <I>r</I><SUP>+</SUP> value of 0.49. A ρ value of −8.3 was determined for ΔLCB[BLi<SUP>+</SUP>] based on the reaction LiB<SUP>+</SUP> = Li<SUP>+</SUP> + B from a ρ value for ΔLCB[B<SUB>2</SUB>Li<SUP>+</SUP>] by using a factor of 1.5 that was obtained from a linear relationship between calculated ΔLCB[B<SUB>2</SUB>Li<SUP>+</SUP>] and ΔLCB[BLi<SUP>+</SUP>]. In comparison with the corresponding results for other Lewis cation basicities, it was shown that the ρ value decreases in the order of H<SUP>+</SUP> (−11.6) > Me<SUB>3</SUB>Si<SUP>+</SUP> (−11.0) > Me<SUB>3</SUB>Ge<SUP>+</SUP> (−9.7) > Li<SUP>+</SUP> (−8.3). The decrease in the ρ value indicates that the positive charges at the Lewis cation moiety of the adduct ion increases in this order. The small ρ value for the Li<SUP>+</SUP> basicity is consistent with largely ionic (ion–dipole interaction) nature of the bonding interaction between Li<SUP>+</SUP> and the carbonyl oxygen atom. The <I>r</I><SUP>+</SUP> value that indicates the degree of charge-delocalization into the aromatic π-system also decreases in the same order. In spite of a small charge transfer from Li<SUP>+</SUP> to acetophenone ligand, the moderate <I>r</I><SUP>+</SUP> value of 0.49 observed for the Li<SUP>+</SUP> complex is interpreted to be due to the redistribution of the induced positive charge in the acetophenone moiety by binding with Li<SUP>+</SUP>.


  • Bull. Chem. Soc. Jpn.  

    Bull. Chem. Soc. Jpn. 80(1), 195-203, 2007-01-15 

    The Chemical Society of Japan

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