THE ALDOL REACTION OF LITHIOPINACOLONATE WITH SUBSTITUTED BENZALDEHYDES - SOLVENT AND STRUCTURE EFFECTS ON AGGREGATION, ELECTROCHEMISTRY, AND HEAT OF REACTION

The reaction of lithium pinacolonate with seven para-substituted benzaldehydes has been examined in tetrahydrofuran and acetonitrile. Reactions were clean, complete, and instantaneous at 25 °C, and heats of reaction were determined by titration calorimetry. Fairly good (R = 0.96) correlations between heats of reaction and the usual Hammett σp substituent parameters were obtained although the ionization of substituted benzoic acids is scarcely an appropriate model for the aldol reaction in nonpolar media. The question of electron transfer as a feasible mechanism was tested by using Eberson's criterion for estimating the free energy of activation from the free energy of electron transfer using the oxidation potential of lithium pinacolonate and the reduction potentials of the aldehydes by second harmonic alternating current voltammetry (SHACV) and cyclic voltammetry (CV). Calculated barriers are sufficiently endergonic to eliminate the SET pathway on the basis of this criterion. The effect of lithium perchlorate as a supporting electrolyte suggests a significant interaction between the lithium ion and the aldehyde group, which in fact, was detected through the chemical shifts of 13C NMR spectra of the benzaldehydes upon the addition of lithium perchlorate. Unusual, and unexplained, effects on the oxidation potential were observed from varying the working electrode from platinum to glassy carbon. Aggregation numbers for the initial reactants and for the aldolate product were determined in both solvents by means of vapor pressure osmometry, cryoscopy, and 6Li NMR. © 1990, American Chemical Society. All rights reserved.