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Abstract The kinetics of (3+2) cycloaddition reactions of 18 different donor–acceptor cyclopropanes with the same aldehyde were studied by in situ NMR spectroscopy. Increasing the electron density of the donor residue accelerates the reaction by a factor of up to 50 compared to the standard system (donor group=phenyl), whereas electron‐withdrawing substituents slow down the reaction by a factor up to 660. This behavior is in agreement with the Hammett substituent parameter σ. The obtained rate constants from the (3+2) cycloadditions correlate well with data from additionally studied (3+ n ) cycloadditions with a nitrone ( n =3) and an isobenzofuran ( n =4). A comparison of the kinetic data with the bond lengths in the cyclopropane (obtained by X‐ray diffraction and computation), or the 1 H and 13 C NMR shifts, revealed no correlation. However, the computed relaxed force constants of donor–acceptor cyclopropanes proved to be a good indicator for the reactivity of the three‐membered ring.
Kreft et al. (Tue,) studied this question.