Um Ik-Hwan, Kim Eun-Hee, Lee Ji-Youn
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
J Org Chem. 2009 Feb 6;74(3):1212-7. doi: 10.1021/jo802446y.
A kinetic study is reported for nucleophilic substitution reactions of O-Y-substituted phenyl thionobenzoates (1a-h) and O-4-nitrophenyl X-substituted thionobenzoates (2a-f) with N(3)(-) and CN(-) in 80 mol % H(2)O-20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Brønsted-type plot for the reactions of 1a-h with N(3)(-) exhibits a downward curvature, i.e., the slope (beta(lg)) changes from -1.10 to -0.33 as the leaving group basicity decreases, indicating that the reactions proceed through a stepwise mechanism with a change in rate-determining step (RDS). In contrast, the Brønsted-type plot for the corresponding reactions with CN(-) is linear with a beta(lg) value of -0.33. This value is similar to that found previously for the reactions of 1a-h with OH(-) (-0.35). Besides, sigma(o) constants result in much better Hammett correlation than sigma(-) constants. Thus, the reactions with CN(-) and OH(-) have been concluded to proceed through a stepwise mechanism in which departure of the leaving group occurs after RDS. Reactions of 2a-f with N(3)(-) and CN(-) result in nonlinear Hammett plots. However, the Yukawa-Tsuno plots for the same reactions exhibit excellent linearity with r = 0.5 +/- 0.1, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by ground state stabilization through resonance interactions between the electron-donating substituent and the thio carbonyl functionality. Calculation of the k(1) values (nucleophile attack as RDS) for the reactions of 1a-h with N(3)(-) indicates that azide ion is more reactive than OH(-) toward the thione esters, although the former is over 11 pK(a) units less basic than the latter. The high polarizability of N(3)(-) has been suggested to be responsible for its great affinity for the polarizable thione esters 1a-h and 2a-f.
报道了在25.0±0.1℃下，O - Y - 取代的苯硫代苯甲酸酯（1a - h）和O - 4 - 硝基苯基X - 取代的硫代苯甲酸酯（2a - f）与N₃⁻和CN⁻在80 mol% H₂O - 20 mol% DMSO中的亲核取代反应的动力学研究。1a - h与N₃⁻反应的布朗斯特型曲线呈现向下的曲率，即随着离去基团碱性降低，斜率（β(lg)）从 - 1.10变为 - 0.33，这表明反应通过逐步机理进行，速率决定步骤（RDS）发生了变化。相比之下，与CN⁻的相应反应的布朗斯特型曲线是线性的，β(lg)值为 - 0.33。该值与先前发现的1a - h与OH⁻反应的值（ - 0.35）相似。此外，σ(o)常数比σ( - )常数能给出更好的哈米特相关性。因此，已得出与CN⁻和OH⁻的反应通过逐步机理进行，其中离去基团在RDS之后离去。2a - f与N₃⁻和CN⁻的反应导致非线性哈米特图。然而，相同反应的汤川 - Tsuno图呈现出极好的线性，r = 0.5±0.1，这表明非线性哈米特图不是由于RDS的变化，而是由给电子取代基与硫羰基官能团之间的共振相互作用导致的基态稳定化引起的。对1a - h与N₃⁻反应的k(1)值（亲核试剂进攻作为RDS）的计算表明，叠氮离子对硫酮酯的反应性比OH⁻高，尽管前者的碱性比后者低超过11个pK(a)单位。有人认为N₃⁻的高极化率是其对可极化硫酮酯1a - h和2a - f具有高亲和力的原因。
