Hydrolysis of cyclic phosphoramides. Evidence for syn lone pair catalysis
作者:Andrés Núñez、Dyanna Berroterán、Oswaldo Núñez
DOI:10.1039/b300916e
日期:——
Hydrolysis between 1.5 < pH < 4 of five and six membered cyclic phosphoramides has been followed by UV and 31PNMR spectroscopy. The observed rates fit the equation: kobs
=
kH2O
[H+]/([H+]
+
Ka)
+
k′H2O, where kH2O and k′H2O are the pseudo first-order rate constants of water attack on the protonated phosphoramide and its unprotonated form, respectively, and Ka is the phosphoramide acidity equilibrium constant. Although, faster hydrolysis rates on the five membered ring are expected due to the energy released in going from a strained cyclic to a “strained free” trigonal-bipyramidal-pentacoordinated intermediate, with one of the cyclic nitrogens occupying the apical position, these compounds react slightly faster (kH2O values) but slower regarding the k′H2O values than the six membered analogs. The balance in reactivity is attributed to the additional stability obtained in the six membered cyclic compounds by a syn orientation of the two lone pairs of the cyclic nitrogen to the water attack. This stabilization does not exist in the five membered phospholidines since the water attack is perpendicular to the electron pairs of the cyclic nitrogen. In agreement with the incoming water orientation, the product ratios from the hydrolysis show that in the five membered rings the main product is the one produced by endocyclic cleavage; meanwhile, in the six membered cyclic phospholines the kinetic product is the one produced by exocyclic cleavage. The syn orientation of two electron pairs on nitrogen stabilizes the transition state of water approach to the phosphoramides by ca. 3 kcal mol−1 when compared to the orthogonal attack.
在1.5 < pH < 4的条件下,五元环和六元环磷酰胺的水解过程通过紫外光谱和31P核磁共振波谱进行了研究。观察到的反应速率符合以下方程:kobs = kH2O [H+]/([H+] + Ka) + k′H2O,其中kH2O和k′H2O分别是水分子攻击质子化磷酰胺和非质子化磷酰胺的假一级速率常数,Ka是磷酰胺酸度平衡常数。虽然预计五元环的水解速率会更快,因为从受限环状结构转变为“无受限”三角双锥五配位中间态时释放能量,其中一个环状氮原子占据顶端位置,但这些化合物的反应速率(kH2O值)略快,而关于k′H2O值则较慢于六元环的类似物。反应活性的平衡归因于六元环状化合物在水攻击时通过syn取向使环状氮的两个孤对电子与水攻击方向一致所获得的额外稳定性。这种稳定化在五元环磷酯中不存在,因为水攻击方向垂直于环状氮的电子对。与进来的水取向一致,水解产物的比例表明,在五元环中主要产物是由环内断裂产生的;而在六元环磷脂中,动力学产物是由环外断裂产生的。氮原子上两个电子对的syn取向通过与正交攻击相比,大约稳定了磷酰胺水接近过渡态的3 kcal mol−1。