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 MeONHMe with beta-methoxy-alpha-nitrosbilbene (1-OMe) in 50% Me(2)SO-50% water is reported. With MeONHMe, the reaction leads to the expected enamine 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 enamine. 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 which allows their spectroscopic and kinetic characterizations. These reactions represent the first examples of a nucleophilic vinylic substitution by amine nucleophiles in which the intermediate is directly observable. Structure-reactivity comparisons between the MeONHMe reaction and the reaction of 1-OMe with piperidine and morpholine reported previously are consistent with a relatively weak dependence of the nucleophilic 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 observable in the reaction with the relatively weakly basic MeONH(2) and MeONHMe but not with more strongly basic amines. MeONH(2) and MeONHMe show the enhanced reactivity expected for alpha-effect nucleophiles, but it is mainly reflected in an enhanced equilibrium constant for nucleophilic addition while the effect on k(1) is relatively small. Steric effects are shown to play a major role in the MeONHMe reaction. One type of steric effect is caused by crowding in the intermediate which reduces 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 reduces the push by the nitrogen lone pair of the intermediate and hence decreases k(2) for leaving group expulsion; this latter effect is stronger than the effect of crowding in the intermediate.