A change in the rate-determining step in the E1cB reactions of N-[2-(4-nitrophenyl)ethyl]pyridinium cations

Abstract

Second-order rate constants have been measured (aqueous solution, I = 1.0, 25 °C) for the hydroxide ion catalyzed elimination reactions of 12 7V-(2-(4-nitrophenyl)ethyl)pyridinium cations (3) bearing a variety of substituents in the pyridine ring. Bronsted plots as a function of the basicity of the pyridine leaving group are concave-down, which is consistent with a change in rate-determining step within an ElcB mechanism. These plots are characterized by ftg = -0.17 for the rate-determining deprotonation for < 6.5, and ftg = -0.39 for the rate-determining expulsion of the pyridine nucleofuge from the carbanionic intermediate for pÁfBH > 6.5. Elimination reactions in basic D20 occur without any significant incorporation of deuterium into the 4-nitrostyrene product, and require the presence of a hydrogen-bonded carbanionic intermediate in which nucleofuge expulsion occurs faster than exchange of hydrogen-bonding water molecules. Rate-determining deprotonation in these elimination reactions occurs 50-fold more slowly than for the corresponding reactions of the 7V-[2-(4-nitrophenyl)ethyl]quinuclidinium cations that have also been reported to have ft, = -0.17, but which do not show an analogous change in the rate-determining step upon variation of the nucleofuge basicity. The analogous elimination of the l-methyl-3-[2-(4-nitrophenyl)ethyl]imidazolium cation occurs a further 30-fold more slowly than that predicted for 3 having a pyridine leaving group of the same basicity as 1-methylimidazole. The ElcB reactions of 3 are similar to the analogous reactions of A'-(2-cyanoethyl)pyridinium cations(1) in displaying a change in the rate-determining step with nucleofuge basicity; however, the ftg values for 1 and 3 are quite different for both and k2/k"t.We have recently reported1 a detailed kinetic study of the hydroxide ion catalyzed elimination of pyridines from 7V-(2cyanoethyl)pyridinium cations (1) in aqueous solution for pyridine leaving groups having pAfBH in the range 1.5-9.7. Br0nsted plots as a function of pAfBH showed a clear change in the rate-determining step within an ElcB mechanism. Thus, ftg = -0.30 for rate-determining deprotonation (i.e. (ElcB)irrev or AxhDHl + DN)2 for pAfBH < 5.8 is transformed into ftg = -0.93 for rate-determining leaving group expulsion from the carbanionic intermediate (i.e. (ElcB)rev or AxhDH + DN<) for pAfBH > 5.8. X ;n-ch2ch2cn * ] < + CH2=CHCN + H20 (1)A closely related reaction, the hydroxide ion catalyzed elimination of quinuclidines from Ar-(2-(4-nitrophenyl)ethyl)quinuclidinium cations (2), showed3 4"5 no such change in the rate-determining step with leaving-group basicity, although other evidence firmly established the ElcB mechanism for these reactions. Rate-determining deprotonation, with ftg = -0.17, was found for quinuclidine leaving groups having pKm covering the range 3-11. X :n-ch2ch2c6h4no2 *oh["OH] + CH2=CHC6H4N02 + H20 (2) appears to implicate the quite different activating groups (cyano

Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base

“…This result requires that elimination occurs within a solvated (hydrogen-bonded) intermediate in which bromide ion departure occurs more rapidly than exchange of the HOD molecule with bulk D 2 0 (i.e., k, >> k, in Scheme 2). A similar observation has been reported for rate-determining nucleofuge departure in the elimination reactions of 10: X = 4-N(CH,), in aqueous base (9) and also for the analogous eliminations from 2-cyanoethyl sulfides (16). Such a process indicates that the elimination from l m corresponds to an ( E l~b )~, ,~~~~ (i.e., AXhDH*DN) mechanism (17, 18).…”

“…Quantitative comparisons can also be made of the range pKBH = 2-10. These data now allow detailed comreactivities in these systems and in related elimination reparisons of the reactivities of 1 , 3 , and 4 in base-catalyzed actions in which substituted pyridines are the nucleofugic eliminations in aqueous solution, and also demonstrate that leaving groups (8,9).…”

“…[8] and [9] in aqueous solution are linearly correlated with pKBH for the corresponding substituted pyridinium cations: text of the rate-determining bromide ion departure from the carbanionic intermediate that is deduced above from the concave-down Bronsted plot. This result requires that elimination occurs within a solvated (hydrogen-bonded) intermediate in which bromide ion departure occurs more rapidly than exchange of the HOD molecule with bulk D 2 0 (i.e., k, >> k, in Scheme 2).…”

“…[8]) and a = 0.33 for the deprotonation of N-di(ethoxycarbony1)methyl pyridinium cations (7) (eq. [9]). These a values are consistent with the expectation that the delocalization of the negative charge in the conjugate base of 7 into the carbonyl substituents will result in the thermodynamic acidity of 7 being less sensitive to X-substituents than are the acidities of 1 (conjugate base = 5) and 6, for which the negative charge in each conjugate base is essentially localized on the atom adjacent to the ring nitrogen atom.…”

Investigation of Catalysis by Acids, Bases, Other Small Molecules and Enzymes

“…An interesting example of this method related to acid-base catalysis is the hydroxide-catalysed elimination reaction of 4-nitrophenylethylpyridinium ions (Fig. 11.7A) which is interpreted to involve a carbanion intermediate (see Chapter 9 for the related base-induced elimination from N-(4-nitrophenylethyl)quinuclidinium ion) [16]. A free energy plot which is concave upwards is diagnostic of a mechanism with parallel pathways.…”