Gas-phase lithium cation basicities of acetophenones: A linear relationship between lithium cation and proton basicities

作者：Maeda Hideaki; Irie Maki; Than Soe; Kikukawa Kiyoshi; Mishima Masaaki^*

来源：Bulletin of the Chemical Society of Japan, 2007, 80(1): 195-203.

DOI：10.1246/bcsj.80.195

The free energy changes (Delta G) 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 LiA(2)(+) + 2B = LiB2+ + 2A using a FT-ICR spectrometer. A plot of Delta G (relative lithium cation basicity, Delta LCB[B2Li+]) values against the Delta GB (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 pi-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, Delta LCB = rho(sigma(o) + r(+) Delta(sigma) over bar (+)(R)), the substituent effect on Delta LCB[B2Li+] of acetophenone was characterized by a rho value of -12.5 and an r(+) value of 0.49. A rho value of -8.3 was determined for Delta LCB[BLi+] based on the reaction LiB+ = Li+ + B from a rho value for Delta LCB[B2Li+] by using a factor of 1.5 that was obtained from a linear relationship between calculated Delta LCB[B2Li+] and Delta LCB[BLi+]. In comparison with the corresponding results for other Lewis cation basicities, it was shown that the rho value decreases in the order of H+ (-11.6) > Me3Si+ (-11.0) > Me3Ge+ (-9.7) > Li+ (-8.3). The decrease in the rho value indicates that the positive charges at the Lewis cation moiety of the adduct ion increases in this order. The small rho 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 pi-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+.

• 出版日期2007-1-15