Castro Enrique A., Saavedra Claudia, Santos José G., Umaña María I.
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
J Org Chem. 1999 Jul 23;64(15):5401-5407. doi: 10.1021/jo990084y.
The reactions of secondary alicyclic amines with the title substrates are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl), by following spectrophotometrically the release of 4-nitrophenoxide ion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. For the reactions of phenyl 4-nitrophenyl thionocarbonate (1), linear plots of k(obsd) vs [NH] (NH is the free amine) are obtained, except for the reaction with piperazinium ion, which shows nonlinear upward plots. The aminolysis of methyl 4-nitrophenyl thionocarbonate (2) exhibits nonlinear plots of k(obsd) vs [NH], except that with piperidine, which is linear. The Brönsted-type plot for 1 is linear with slope beta = 0.25, indicating that the formation (k(1) step) of a tetrahedral addition intermediate (T(+/-)) is rate determining. For the aminolysis of 2 (except piperidine), k(-)(1) approximately k(3)[NH] > k(2), where k(-)(1), k(3), and k(2) are the rate coefficients for amine expulsion, amine deprotonation, and leaving group expulsion from T(+/-), respectively. For the reaction of 2 with piperidine, k(-)(1) < k(3)[NH]; therefore, the k(1) step is rate limiting. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) The change of MeO by EtO in 2 does not affect the k(1), k(-)(1), or k(2) values. (ii) Substitution of MeO by PhO in 2 results in lower k(1) values due to steric hindrance. (iii) The change of 4-nitrophenoxy (NPO) by PhO in 2 lowers the k(1) values and enlarges those of k(-)(1). (iv) Secondary alicyclic amines are less reactive toward 2 than isobasic pyridines when the breakdown of T(+/-) is rate determining; this is mainly due to larger k(-)(1) values for the former amines. (v) The change of PhO by NPO in 1 changes the mechanism from stepwise to concerted. (vi) Substitution of NPO by PhO in 1 does not alter the k(1) values significantly. (vii) The change of NPO by Cl in 1 increases the k(1) values. (viii) Substitution of C=S by C=O in 1 shifts the rate-limiting step from k(1) to k(2) due to a larger k(-)(1)/k(2) ratio by this change.
在25.0℃、离子强度0.2（KCl）的水溶液中，通过分光光度法跟踪4 - 硝基苯氧离子的释放，对二级脂环胺与标题底物的反应进行了动力学研究。在胺过量的情况下，得到了准一级速率系数（k(obsd)）。对于苯基4 - 硝基苯基硫代碳酸酯（1）的反应，除了与哌嗪离子的反应呈现非线性上升曲线外，得到了k(obsd)对[NH]（NH为游离胺）的线性图。4 - 硝基苯基硫代碳酸甲酯（2）的氨解反应中，除了与哌啶的反应呈线性外，k(obsd)对[NH]的图呈现非线性。对于1的布朗斯台德型图是线性的，斜率β = 0.25，表明四面体加成中间体（T(+/-)）的形成（k(1)步骤）是速率决定步骤。对于2的氨解反应（除哌啶外），k(-)(1)≈k(3)[NH] > k(2)，其中k(-)(1)、k(3)和k(2)分别是胺排出、胺去质子化以及从T(+/-)中排出离去基团的速率系数。对于2与哌啶的反应，k(-)(1) < k(3)[NH]；因此，k(1)步骤是速率限制步骤。通过比较所研究的反应以及与类似氨解反应，可得出以下结论：（i）2中MeO被EtO取代不影响k(1)、k(-)(1)或k(2)值。（ii）2中MeO被PhO取代由于空间位阻导致k(1)值降低。（iii）2中4 - 硝基苯氧基（NPO）被PhO取代降低了k(1)值并增大了k(-)(1)值。（iv）当T(+/-)的分解是速率决定步骤时，二级脂环胺对2的反应活性低于等碱性吡啶；这主要是由于前一种胺的k(-)(1)值较大。（v）1中PhO被NPO取代使反应机理从分步变为协同。（vi）1中NPO被PhO取代不会显著改变k(1)值。（vii）1中NPO被Cl取代增加了k(1)值。（viii）1中C=S被C=O取代使速率限制步骤从k(1)转变为k(2)，因为这种变化导致k(-)(1)/k(2)比值增大。
