Strength of Lewis acid–Lewis base interactions. A 13C NMR study of the ether interchange in boron trifluoride complexes

Abstract

The intramolecular chemical shift difference previously used for calibration of Brønsted acid strengths (the Δδ method) has been extended to the study of Lewis acid–base interactions. The exchange between an ether complexed with boron trifluoride and another ester is fast on the ¹³C NMR timescale (75.4 MHz, 22 °C) and allows the determination of the equilibrium constant by interpolation of chemical shifts. Using boron trifluoride diethyl etherate as the standard, equilibrium constants for the reaction (Et₂O·BF₃ + ROR′⇌Et₂O + RR′O·BF₃) have been measured both in dichloromethane and without solvent for R, R′: Bu, Bu; Me₂CH, Me₂CH; Me, CH₂CH₂Cl; CH₂CH₂Cl, CH₂CH₂Cl; Me, C₆H₅; Et, C₆H₅. The values agree in some cases with literature values obtained by other methods and do not agree in other cases. The electronic and steric effects of substituents upon the stability of complexes are evidenced in the results.