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 H and 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.
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