Lewis acid addticts of ,-unsaturated carbonyl and nitrile compounds. A calorimetric study

Enthalpies of complexation of the α,β-unsaturated Lewis bases crotonaldehyde, 1; methyl crotonate, 2; mesityl oxide, 3; and crotononitrile, 4; with each of the Lewis acids SnCl 4 , TiCl 4 , SbCl 5 , and BCl 3 were measured in solution in CH 2 Cl 2 . The results indicate that while a qualitative agreement exists in the ranking of Lewis acids for each of the Lewis bases, there is only a modest quantitative correlation. Similarly, when compared with an nmr-derived Lewis acid scale, the enthalpies were found to agree with the qualitative ranking of Lewis acids, but exhibited only a poor correlation.