104549-51-1Relevant articles and documents
Nucleophilic Addition to Olefines. 19. Abnormally High Intrinsic Barrier in the Reaction of Piperidine and Morpholine with Benzylideneacetylacetone
Bernasconi, Claude F.,Kanavarioti, Anastassia
, p. 7744 - 7751 (2007/10/02)
The title reaction leads to the formation of the zwitterionic Michael adduct T+/- (PhCH(R2NH+)C(COCH3)2-) which is in rapid acid-base equilibrium with its anionic form T-(PhCH(R2N)C(COCH3)2-).Rate (k1, k-1) and equilibrium constants (K1) for nucleophilic addition and the pKa of the T+/- adducts were determined in 50percent Me2SO - 50percent water at 20 deg C.From an interpolation of the rate constants to K1 = 1 an intrinsic rate constant, log k0 = 0.3, was determined.This value deviates negatively by approximately 2.5 log units from a correlation of log k0 for amine addition to five olefins of the type PhCH=CXY, with log k0 for the deprotonation of the corresponding carbon acids CH2XY.Two major factors are believed to contribute to this depressed intrinsic rate constant or enhanced intrinsic barrier: (1) steric inhibition of resonance in T+/- 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 showes that steric hindrance to optimal ?-overlap in the adduct T+/- is already present in the substrate); (2) intramolecular hydrogen bonding in T+/-, which is inferred from abnormally high pKa values and whose development lags behind C-N bond formation.These effects are shown to be manifestation of the Principle of Nonperfect Synchronization.
