Journal of the American Chemical Society p. 6950 - 6954 (1991)
Update date:2022-08-16
Topics:
Bunting, John W.
Kanter, James P.
Second-order rate constants have been measured (aqueous solution, I = 1.0, 25 deg C) for the hydroxide ion catalyzed elimination reactions of 12 N-(2-(4-nitrophenyl)ethyl)pyridinium cations (3) bearing a variety of substituents in the pyridine ring.Broensted 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 E1cB mechanism.These plots are characterized by βlg = -0.17 for the rate-determining deprotonation for pKBH < 6.5, and βlg = -0.39 for the rate-determining expulsion of the pyridine nucleofuge from the carbanionic intermediate for pKBH > 6.5.Elimination reactions in basic D2O 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 N-<2-(4-nitrophenyl)ethyl>quinuclidinium cations that have also been reported to have βlg = -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 1-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 E1cB reactions of 3 are similar to the analogous reactions of N-(2-cyanoethyl)pyridinium cations (1) in displaying a change in the rate-determining step with nucleofuge basicity; however, theβlg values for 1 and 3 are quite different for both k1 and k2/k-1.
View MoreContact:+86-571-86491666
Address:SHI XIANG ROAD
Contact:+86-512-56795332
Address:No227 Shuanglong Rd, Fenghuang, Zhangjiagang
Contact:+86-15527136576
Address:Wuhan city
Wuhan Shangrisyn chemicals Technology Co.,Ltd
Contact:+86-027-84466317 __ +86-15387123698
Address:wuhan - china
Contact:+86-25-83719363
Address:106-7 Chunnan Rd, Chunxi Town, Gaochun, Nanjing, China
Doi:10.1016/j.tetlet.2008.04.071
(2008)Doi:10.1016/j.apcata.2013.03.038
(2013)Doi:10.1016/j.jallcom.2016.10.209
(2017)Doi:10.1016/j.polymer.2014.04.036
(2014)Doi:10.1055/s-1974-23277
(1974)Doi:10.1002/anie.201805505
(2018)