Protonation of conjugated carbonyl groups in sulfuric acid solution. I. Adaptation of the amide acidity function H_A for protonation of the carbonyl group in non-Hammett bases

Abstract

The protonation of a number of aromatic and a#-unsaturated aliphatic aldehydes, ketones, and carb~xylic~acids in sulfuric acid has been found to obey the HA acidity function, rather than Ho.

Stability and solvation of organic cations

“…Literature data for aromatic ketones substantially agree in indicating somewhat larger m*'s than for aliphatic ketones, consistent with greater delocalization of the positive charge, e.~. 0.46 for acetophenone and 0.67 for benzophenone [17,77,112]; and similar values are found for e,B-unsaturated ketones [17,77,79]. Attachment of electrondonating substituents to the aromatic rings further increases m*, and for 2,4,4'-trimethoxybenzophenone m* is l.ll [14,120].…”

“…Carboxylic acids have not been as extensively investigated as ketones [17,77,121]. Probably the best value is m* = 0.56 for benzoic acid [77], and by comparison with the acetone-acetophenone pair a value of 0.47 may be estimated for acetic acid.…”

“…This approach was developed by Bunnett and Olsen [8,9] following earlier suggestions by Grunwald [lO] and Kresge [Ill. It has since been applied widely [3,[12][13][14][15][16][17], especially by Modena and Scorrano and their co-workers [20,21], but unlike acidity functions, has not been seriously revised since its presentation more than twenty years ago. In part, the lack of revision reflects soundness of the original formulation, but there have also been developments [26,[27][28][29][31][32][33][34]] that need to be incorporated and criticisms [25] that deserve a response.…”

Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

“…First, we considered a chromophore activation mechanism in which protonation of the substrate lowers the relative energy of its first singlet excited state, thus making direct excitation by visible light achievable. This hypothesis is consistent with the red-shifted absorption of enones in sulfuric acid solution 39,40 . Second, we considered that the chiral catalyst and enone substrate might form an electron donor-acceptor complex [41][42][43][44][45] .…”

Kinetic studies on the rearrangement of Diels–Alder adducts of activated benzoquinones with (E)-1-trimethylsiloxybuta-1,3-diene

“…The spectrophotometric and HPLC analyses of the reaction mixtures, which were obtained by treatment of compounds 4 6 with hydrochloric acid in aqueous tetrahydrofuran and dioxane solutions, show the presence of the corresponding rearrangement products. These substances were identified in the mixtures by comparison with authentic samples of the dihydrobenzofuran 12 and arylcrotonaldehydes 10 and 11.…”