A kinetic study on the pivaloyl chloride promoted H-phosphonate condensation step in the presence of differently substituted pyridines has been carried out. The kinetics of the system follow second order kinetics with a 1 ∶ 1 ratio of nucleoside 3′-H-phosphonate and nucleoside component (with free 5-OH). Irrespective of the pyridine derivative used, the reaction rate is dependent on the concentration of this as well as of the hydroxy component and H-phosphonate. A reactive intermediate is indirectly identified from the kinetic evidence. This intermediate is suggested to be a pyridinium adduct formed by attack of the pyridine derivative on the initially formed mixed phosphonic carboxylic anhydride. Considerably lower rates of condensation in the presence of sterically hindered pyridines further support the existence of nucleophilic catalysis in the reactions with non-hindered pyridines. In addition, the rate of reaction in the presence of pyridines with pKa values above ∼4.8 is enhanced by an increase in pivaloyl chloride concentration. The main reason for this enhancement is most likely the fact that pivaloyl chloride removes pivalate ion, which retards the reaction by influencing the equilibrium between the mixed anhydride and the pyridinium intermediate. Although the observed rate constants are composed of several constants, their temperature dependence gives some indication of the nature of the transition-state of the rate-limiting step. Entropies of activation are estimated to be slightly positive, suggesting a transition state arising from attack of the hydroxy component on the pyridinium intermediate but involving a fair degree of bond breakage to the leaving group, i.e. the pyridine.
在不同取代的
吡啶存在下,对新
戊酰氯促进 H-
膦酸缩合步骤进行了动力学研究。该体系的动力学遵循二阶动力学,核苷 3′-H-
膦酸盐和核苷成分(游离 5-OH)的比例为 1 ∶ 1。无论使用哪种
吡啶衍生物,反应速率都取决于
吡啶衍生物以及羟基成分和 H-
膦酸盐的浓度。从动力学证据中间接确定了一种反应中间体。据推测,这种中间体是
吡啶衍生物对最初形成的混合
膦酸羧酸酐的攻击所形成的
吡啶鎓加合物。在存在立体受阻
吡啶的情况下,缩合率要低得多,这进一步证明了在与非受阻
吡啶的反应中存在亲核催化作用。此外,pKa 值高于 4.8 的
吡啶存在时,反应速率会随着特
戊酰氯浓度的增加而提高。这种增强的主要原因很可能是新
戊酰氯去除了
特戊酸根离子,而
特戊酸根离子会影响混合酸酐和
吡啶中间体之间的平衡,从而延缓反应的进行。尽管观察到的速率常数由多个常数组成,但它们对温度的依赖性在一定程度上表明了限速步骤过渡状态的性质。据估计,活化熵略呈正值,这表明过渡状态是由羟基成分对
吡啶中间体的攻击引起的,但涉及到离去基团(即
吡啶)相当程度的断键。