Carbon-13 nuclear magnetic resonance spectroscopy as a method to determine relative acidity of boron Lewis acids in pyridine complexes

Abstract

Complexes of pyridine and 2-ethylpyridine with a series of boron Lewis acids have been studied using ¹H, ¹¹B, and ¹³C n.m.r. The difference in chemical shifts (Δδ_{c – 4}) between the boron complex and the free pyridine were correlated with calorimetric data. The ¹³C data show that Δδ_{c – 4} of pyridines provides a measure of the strength of the N → B bond, thus allowing a scale of acidity for boron compounds to be derived.