Nucleophilic additions to olefins. 19. Abnormally high intrinsic barrier in the reaction of piperidine and morpholine with benzylideneacetylacetone

The title reaction leads to the formation of the zwitterionic Michael adduct T/sup +/-/ (PhCH(R/sub 2/NH/sup +/)C(COCH/sub 3/)/sub 2//sup -/) which is in rapid acid-base equilibrium with its anionic form T/sup -/ (PhCH(R/sub 2/N)C(COCH/sub 3/)/sub 2//sup -/). Rate (K/sub 1/, k/sub -1/) and equilibrium constants (K/sub 1/) for nucleophilic addition and the pK/sub a/ of the T/sup +/-/-adducts were determined in 50% Me/sub 2/SO-50% water at 20/sup 0/C. From an interpolation of the rate constants to K/sub 1/ = 1 an intrinsic rate constant, log k/sub 0/ = 0.3, was determined. This value deviates negatively by approximately 2.5 log units from a correlation of log k/sub 0/ for amine addition to five olefins of the type PhCH=CXY, with log k/sub 0/ for the deprotonation of the corresponding carbon acids CH/sub 2/XY. Two major factors are believed to contribute to this depressed intrinsic rate constant or enhanced intrinsic barrier: (1) steric inhibition of resonance in T/sup +/-/ with the steric effect developing ahead of C-N bond formation (this conclusion is supported by an X-ray crystallographic study of p-methoxybenzylideneacetylacetone which shows that steric hindrance to optimal ..pi..-overlap in the adduct T/sup+/-/ is already present in the substrate); (2) intramolecular hydrogen bonding inmore » T/sup +/-/, which is inferred from abnormally high pK/sub a/ values and whose development lags behind C-N bond formation. These effects are shown to be manifestations of the Principle of Nonperfect Synchronization.« less