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

• Published in: Bulletin of the Chemical Society of Japan 2007, Vol.80 (1), p.195-203
• Main Authors: Maeda, Hideaki, Irie, Maki, Than, Soe, Kikukawa, Kiyoshi, Mishima, Masaaki
• Format: Article
• Language: English
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.80.195

The free energy changes (ΔG) for the reaction B2Li+ = Li+ + 2B were obtained in the gas phase for m- and p-substituted acetophenones based on the measurement of ligand-exchange equilibria of the type LiA2+ + 2B = LiB2+ + 2A using a FT-ICR spectrometer. A plot of ΔG (relative lithium cation basicity, ΔLCB[B2Li+]) values against the ΔGB (proton basicity) showed a limited linear correlation for meta substituents and para 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+ complex compared to that in the protonated ion. Based on the correlation analysis using the Yukawa–Tsuno equation, , the substituent effect on ΔLCB[B2Li+] of acetophenone was characterized by a ρ value of −12.5 and an r+ value of 0.49. A ρ value of −8.3 was determined for ΔLCB[BLi+] based on the reaction LiB+ = Li+ + B from a ρ value for ΔLCB[B2Li+] by using a factor of 1.5 that was obtained from a linear relationship between calculated ΔLCB[B2Li+] and ΔLCB[BLi+]. In comparison with the corresponding results for other Lewis cation basicities, it was shown that the ρ value decreases in the order of H+ (−11.6) > Me3Si+ (−11.0) > Me3Ge+ (−9.7) > Li+ (−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+ basicity is consistent with largely ionic (ion–dipole interaction) nature of the bonding interaction between Li+ and the carbonyl oxygen atom. The r+ 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+ to acetophenone ligand, the moderate r+ value of 0.49 observed for the Li+ complex is interpreted to be due to the redistribution of the induced positive charge in the acetophenone moiety by binding with Li+.