Stabile organische elektrisch-neutrale Lewis-Säuren

Abstract

The electrically neutral organic Lewis acids described here are reversibly titrable by various bases. This property depends on their structure, characterized by a multiple C–C-bond polarized in one direction by two strong electron acceptor groups.
Knoevenagel condensation and related methods may be used for their synthesis.
The reaction with bases leads to the formation of an isolable adduct (anbadon). The structure of the anbadons [LB^⊖ in equ. (1)] is verified by means of UV and NMR spectroscopy as well as by preparative methods.
Using electrochemical methods and bases B^⊖ corresponding to the solvents HB the virtual acidity constants K'_L = K_L ∙ I_HB are measured (K_L is the true acidity constant, I_LB the ion product of the solvent). The pK'_L values of a number of compounds measured by different methods in different solvents are summarized in table 1. The comparison to the pK values of Broenstedt acids (table 2) and their solvent dependence (fig. 3) is given as well. The competition between Lewis acid and Broenstedt acid behaviour arising in certain structures is discussed.
The pK'_L values correlate well with σ⁺ Hammett constants as well as with theoretical magnitudes (ΔE_π) derived by HMO.
Such Lewis acids undergo the following typical reactions discussed briefly: Michael addition, retro Knoevenagel reaction, reaction with diazoalkanes, Diels-Alder additions. Some catalytic effects of Lewis acids have been proved. Finally the photochemical behaviour is reviewed briefly.