Kinetic Studies of Donor‐Acceptor Cyclopropanes: Is Reactivity Predictably Related to Structural and Electronic Properties?

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 up to 50 compared to the standard (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. The obtained rate constants from the (3+2)‐cycloaddition correlated well with data from additionally studied (3+n)‐cycloadditions with nitrone (n = 3) and isobenzofuran (n = 4). A comparison of the kinetic data with bond lengths in the cyclopropane (obtained by X‐ray and computation), and 1H and 13C NMR shifts revealed no correlation. However, computed relaxed force constants of D‐A cyclopropanes proved to be a good indicator for the reactivity of the three‐membered‐ring.

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