STRENGTH OF LEWIS-ACID LEWIS BASE INTERACTIONS - A C-13 NMR-STUDY OF THE ETHER INTERCHANGE IN BORON-TRIFLUORIDE COMPLEXES

The intramolecular chemical shift difference previously used for calib ration of Bronsted acid strengths (the Delta delta method) has been ex tended to the study of Lewis acid-base interactions. The exchange betw een an ether complexed with boron trifluoride and another ester is fas t on the C-13 NMR timescale(75.4 MHz, 22 degrees C) and allows the det ermination of the equilibrium constant by interpolation of chemical sh ifts. Using boron trifluoride diethyl etherate as the standard, equili brium constants for the reaction (Et2O . BF3 + ROR'reversible arrow Et 2O + RR'O . BF3) have been measured both in dichloromethane and withou t solvent for R, R': Bu, Bu; Me2CH, Me2CH; Me, CH2CH2Cl; CH2CH2Cl, CH2 CH2Cl; Me, C6H5; Et, C6H5. The values agree in some cases with literat ure values obtained by other methods and do not agree in other cases. The electronic and steric effects of substituents upon the stability o f complexes are evidenced in the results.