![]() We were intrigued, however, by simple ring-opening reactions of nucleophiles with ECPs because of their relation to Michael additions and nucleophilic substitutions at sp 3-hybridized carbon centers ( Fig. More recently, the Werz group used in situ 19F NMR kinetics to survey substituent effects on the reactivity of SnCl 4-complexed 1,1-di(alkoxycarbonyl)-substituted cyclopropanes in (3 + 2)-cycloadditions with p-fluorobenzaldehyde, which had been reported to follow a complex mechanism. focused on a few reactions of dimedone- and Meldrum’s acid derived ECPs with pyridines. Previous kinetic studies by McKinney et al. Reactivity of electrophilic cyclopropanes.ĭespite their key role in organic synthesis, a reliable and broadly applicable characterization of the polar reactivity of ECPs had not been available until recently. Additionally, (3 + 2), (3 + 3), and (4 + 3) cycloadditions, , as well as rearrangements and ring-opening reactions of functionalized cyclopropanes offer access to many, structurally diverse building blocks. The manifold of options for chemical transformations of such donor-acceptor substituted cyclopropanes has been reviewed frequently, ,, ,, ,, ,, , and includes various modes for the preparation of carbo- and heterocycles. To further enhance the synthetic versatility of electrophilic cyclopropanes (ECPs) electron-donating groups can be installed at C2 of the cyclopropane ring ( Fig. Cyclopropanes with electron-accepting groups can thus be potent σ-electrophiles that undergo polar reactions with nucleophiles to give methylene-extended Michael adducts. The reactivities of cyclopropanes are governed by the ring strain, which boosts the thermodynamic driving force of ring-opening reactions by more than 100 kJ mol −1, as well as by the electronic nature of the substituents. Īlso substituted cyclopropanes constitute a versatile class of compounds. Owing to their importance, the kinetics of Michael additions have extensively been investigated to characterize the electrophilic reactivities of electron-deficient π-systems, ,,. Variation of the electronic properties of the aryl groups at C2 gave rise to parabolic Hammett relationships.Īdditions of nucleophiles to the electron-deficient π-systems of Michael acceptors are among the best understood organic reactions and frequently used in organic synthesis for C–C- or C–X bond formations. Interestingly, cyclopropanes with aryl substituents at the C2 position reacted faster than their unsubstituted analogues. The experimentally determined second-order rate constants k 2 for cyclopropane ring-opening reactions were then compared to those of related Michael additions. Monitoring the kinetics of non-catalytic ring-opening reactions with a series of thiophenolates and other strong nucleophiles, such as azide ions, in DMSO provided the inherent S N2 reactivity of electrophilic cyclopropanes. The polarization of the C1–C2 bond in 1-acceptor-2-donor-substituted cyclopropanes not only favorably enhances reactivity toward nucleophiles but also directs the nucleophilic attack toward the already substituted C2 position. ![]() Consequently, functionalized cyclopropanes are frequently used building blocks in organic synthesis. Analogous reactions at cyclopropanes with additional C2 substituents allow one to access difunctionalized products. Cyclopropanes that carry an electron-accepting group react as electrophiles in polar, ring-opening reactions.
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