. Can. J. Chem. 60,809 (1982). Enthalpies of complexation of the a,P-unsaturated Lewis bases crotonaldehyde, 1; methyl crotonate, 2; mesityl oxide, 3; and crotononitrile, 4; with each of the Lewis acids SnCl,, TiCl,, SbCl,, and BC1, were measured in solution in CW,Cl,. 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. RONALD F. CHILDS, D. LINDSAY MULHOLLAND et ALAN NIXON. Can. J. Chem. 60,809 (1982). On a mesure, en solution dans le CH,Cl,, les enthalpies de complexation des bases de Lewis a,P-non saturees, crotonaldehyde (I), crotonate de mtthyle (2), oxyde de mesityle (3) et crotononitrile (4) avec chacun des acides Lewis suivants: SnCl,, TiCl,, SbC1, et BC1,. Les resultats indiquent que des accords qualitatifs permettent d'ordonner chaque acide de Lewis pour chacune des bases de Lewis; toutefois la correlation quantitative n'est que modeste. De la mCme maniere, si on compare les enthalpies avec une tchelle d'acide de Lewis derivee de la rmn, on peut qualitativement ordonner les acides de Lewis mais la correlation est faible.[Traduit par le journal]Lewis acids are used to catalyse a variety of synthetically useful reactions of a,P-unsaturated carbonyl and nitrile compounds. In a previous paper we have reported a systematic examination of the nmr spectra of the complexes formed between some of these unsaturated bases and Lewis acids and have proposed an acid scale which, it was suggested, reflected the amount of charge induced at C3 of the base on complexation (1).There has been much discussion about and many attempts at correlating Lewis acid1Lewi.s base interactions (2). These include, among others, the hardlsoft concepts introduced by Pearson (3), the donorlacceptor numbers of Cutmann (4), Drago's E/C equation (3, and theoretical treatments of Mulliken and Pearson (6) and Klopman (7). The equilibrium constant or the heat of reaction is generally used to measure the strength of a Lewis acidlbase reaction.In our attempts to quantify the catalytic effects of Lewis acids on the reactions of unsaturated carbonyls and nitriles, it seemed of interest to compare the nmr-based scale mentioned above with the strength of the acidlbase interaction. As complex formation was essentially complete for nearly all the systems we had studied it seemed appropriate to base such a comparison on the heats of reaction. There have been some ,previous reports of the measurement of the BIi for reaction of unsaturated bases with Lewis acids (8) but insufficient for a meaningful comparison with our nmr-based scale.'Author to whom correspondence may be addressed.We report here a range of calorimetric measurements of these systems and examine the correlations with the nmr scale.
Results and discussion
Calorimetric meas~rrementsIdeally the en...
