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Knorr, Rudolf; Knittl, Monika; Behringer, Claudia; Ruhdorfer, Jakob; Böhrer, Petra (2017): Kinetics of alpha-(2,6-Dimethylphenl)vinyllithium: How To Control Errors Caused by Inefficient Mixing with Pairs of Rapidly Competing Ketones. In: Journal of Organic Chemistry, Vol. 82, No. 6: pp. 2843-2854
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Abstract

Kinetic studies are a suitable tool to disclose the role of tiny reagent fractions. The title compound 2 reacted in a kinetic reaction order of 0.5 (square root of its concentration) with an excess of the electrophiles ClSiMe3, 1-bromobutane (n-BuBr), or 1-iodobutane (n-BuI) at 32 degrees C in Et2O or in hydrocarbon solvents. This revealed that the tiny (NMR-invisible) amount of a deaggregated equilibrium component (presumably monomeric 2) was the reactive species, whereas the disolvated dimer 2 was only indirectly involved as a supply depot. Selectivity data (relative rate constants ?obs) were collected from competition experiments with the faster reactions of 2 in THF and the addition reactions of 2 to carbonyl compounds. This provided the rate sequences of Et2C=O > dicyclopropyl ketone > t-Bu-C(=O)-Ph > diisopropyl ketone >> t-Bu2C=O > ClSiMe3 > n-BuI > n-BuBr approximate to (bromomethyl)cyclopropane (but t-Bu2C=O < ClSiMe3 in THF only) and also of cyclopropanecarbaldehyde > acetone >= t-BuCH=O. It is suggested that a deceivingly depressed selectivity (1 < kappa(obs) < k(A)/k(B)), caused by inefficient microscopic mixing of a reagent X with two competing substrates A and B, may become evident toward zero deviation from the correlation line of the usual inverse (1/T) linear temperature dependence of ln ?obs.