Nitrile-forming eliminations from (E)-2,4,6-(NO 2 ) 3 C 6 H 2 CH=NOC 6 H 4 -2-X-4-NO 2 (1) promoted by R 3 NH/ R 3 NH + in 70 mol % MeCN(aq) have been studied kinetically. When X = NO 2 , the reactions exhibited secondorder kinetics as well as Brönsted β = 0.63 and |β lg | = 0.34-0.46, and an E2 mechanism is evident. As the leaving group was made poorer (X = H, Cl, and CF 3 ), Brönsted β value increased from 0.63 to 0.85-0.89 without much change in the |β lg | value E2, indicating that structure of the transition state changed to an E1cb-like with extensive C β -H bond cleavage, significant negative charge development at the β-carbon, and limited C α -OAr bond cleavage.Key Words : Elimination, E2 and Elcb-like, β-Aryl group effect
IntroductionBase-promoted elimination reactions of (E)-benzaldehyde O-aryloximes have been extensively investigated under various conditions. [1][2][3][4][5][6] We have employed mono-substituted phenyl and 2,4-dinitropheny groups as the β-aryl group and 4-nitrophenoxy, 2,4-dinitrophenoxy, and picrate as the leaving group, and HO − in 60% DMSO(aq), RO − in ROH, R 2 N in MeCN, and R 3 N/R 3 NH + in 70 mol % MeCN(aq) as the base-solvent system, respectively. In all cases, the reactions proceeded by the E2 mechanism despite the fact that the reactants have syn stereochemistry, poor leaving, and sp 2 hybridized β-carbon atom, all of which favor E1cb-or E1cb-like transition state. Very recently, we discovered an (E1cb) irr mechanism for the reaction of (E)-2,4,6-(NO 2 ) 3 C 6 H 2 CH=NOC(O)C 6 H 5 (2a) with R 2 NH in MeCN.
7The result has been attributed to the strong electron withdrawing ability of β-aryl group and the poor leaving group ability the benzoate.The result is somewhat surprising, since MeCN is a poor anion solvating solvent that stabilizes E2 transition state with maximum charge dispersal. We were interested in learning whether the occurrence of in the (E1cb) irr mechanism might be due to the strongly anion-stabilizing β-aryl group in 2a. To provide a better insight the nitrileforming transition state, we have now studied the reactions of (E)-2,4,6-(NO 2 ) 3 C 6 H 2 CH=NOC 6 H 4 -4-X (1a-d) with R 3 N/R 3 NH + in 70 mol % MeCN(aq) (eq. 1). Since the pK a values of benzoic acid and 4-nitrophenol are identical in MeCN,8 a similar mechanism should be anticipated, if the above interpretation could be applied. Herein, we report the similarities and differences in the nitrile-forming eliminations from 1a-d and 2a.
Results(E)-2,4,6-Trinitrobenzaldehyde O-aryloximes 1a-d were prepared by reacting appropriate O-arylhydroxylamines with (E)-2,4,6-trinitrobenzaldoxime in ethanol as described previously.
9-11The reactions of 1a with Et 3 N/Et 3 NH + in 70 mol % MeCN(aq) produced 2,4,6-trinitrobenzonitrile in 94%. No trace of (E)-2,4,6-trinitrobenzaldehyde oxime could be detected by TLC. For all reactions, the yields of aryloxides determined by the comparing the absorbance of the infinity samples from the kinetic studies with those of the authentic aryloxides were in the range of 95-98%.The r...