KINETICS OF THE REACTIONS OF BETA-METHOXY-ALPHA-NITRO STILBENE WITH METHOXYAMINE AND N-METHYLMETHOXYAMINE - DIRECT OBSERVATION OF THE INTERMEDIATE IN NUCLEOPHILIC VINYLIC SUBSTITUTION

A kinetic and spectroscopic study of the reactions of MeONH(2) and MeO NHMe with beta-methoxy-alpha-nitrosbilbene (1-OMe) in 50% Me(2)SO-50% water is reported. With MeONHMe, the reaction leads to the expected en amine substitution product, Ph(MeONMe)C=C(Ph)NO2 (1-MMA); with MeONH(2 ) the product at high pH is the anion, MeON=C(Ph)C(Ph)=NO2-, while at low pH it is the imine form MeON=C(Ph)CH(Ph)NO2 rather than the enamin e. At high pH and high amine concentrations, the SNV intermediates, Ph (OMe)(MeONR)CC(Ph)=NO2-, with R = H or Me, rise to detectable levels w hich allows their spectroscopic and kinetic characterizations. These r eactions represent the first examples of a nucleophilic vinylic substi tution by amine nucleophiles in which the intermediate is directly obs ervable. Structure-reactivity comparisons between the MeONHMe reaction and the reaction of 1-OMe with piperidine and morpholine reported pre viously are consistent with a relatively weak dependence of the nucleo philic addition step (k(1)) on amine basicity (beta(nuc) = 0.25) but a strong dependence of the leaving group expulsion step (k(2)) on amine pK(a) (beta(push) = 0.71); this explains why the intermediate is obse rvable in the reaction with the relatively weakly basic MeONH(2) and M eONHMe but not with more strongly basic amines. MeONH(2) and MeONHMe s how the enhanced reactivity expected for alpha-effect nucleophiles, bu t it is mainly reflected in an enhanced equilibrium constant for nucle ophilic addition while the effect on k(1) is relatively small. Steric effects are shown to play a major role in the MeONHMe reaction. One ty pe of steric effect is caused by crowding in the intermediate which re duces the rate and equilibrium constant for intermediate formation and enhances leaving group departure. The other is steric hindrance to pi -overlap in the product and the transition state leading to it which r educes the push by the nitrogen lone pair of the intermediate and henc e decreases k(2) for leaving group expulsion; this latter effect is st ronger than the effect of crowding in the intermediate.