MECHANISMS OF GENERAL ACID AND BASE CATALYSIS OF REACTIONS OF WATER AND ALCOHOLS WITH FORMALDEHYDE
MECHANISMS OF GENERAL ACID AND BASE CATALYSIS OF REACTIONS OF WATER AND ALCOHOLS WITH FORMALDEHYDE
复制标题
DOI：
10.1021/ja00485a032
复制
发表时间：
1978-01-01
影响因子：
15
通讯作者：
JENCKS, WP
中科院分区：
文献类型：
--
作者：
FUNDERBURK, LH;ALDWIN, L;JENCKS, WP
所观察到的甲醛水合物和半缩醛的裂解和形成的一般碱催化通过n类机制进行，涉及质子转移到或从离开的氧原子。这一结论是基于（1）pxy= d 0/~ dpAfig= d 0 ig/-dpAfeH+= 0.09和pf= d 0 ig/-dpAfig =-0.20的结构-反应性关系，这是对该机理的预期，并且与先前在动力学明确的n类催化中观察到的行为相似，（2）对于乙醇根离子对甲醛的酸催化攻击，需要大于扩散控制极限的速率常数，根据可选的E类机理，和（3）观察到的速率增加和速率降低与离去醇中的给电子取代基，这取决于催化剂和离去基团的pAf。结构-反应性关系和分步机理所需的速率常数提供了质子转移和C-0裂解是一致的证据。所观察到的这些反应的一般酸催化通过质子转移到甲醛氧原子或从甲醛氧原子转移的e类机理进行。这一结论是基于（1）结构-反应性关系，包括pxy= da/-dpÁTg= 0.022的值，该值与一般酸催化的硫醇阴离子与乙醛加成的pxy= 0.026的值相似，（2）绝对速率常数和a与n类反应模型的缩醛水解的速率常数的比较，和（3）对于质子化甲醛的碱催化水合，根据可选择的N类机理，速率常数需要大于扩散控制极限。的值，结构-反应性关系，和速率常数，将需要一个逐步的机制表明，该机制涉及一个更大的组成部分的质子转移的过渡态相比，更强的亲核试剂的反应，并继续朝着一个完全协调的反应机制的趋势作为攻击亲核试剂的碱性降低。的酸和碱催化反应的过渡态的性质被描述的反应坐标图，所定义的观察到的结构-反应性关系。
The observed general base catalysis of the cleavage and formation of formaldehyde hydrate and hemiacetals pro-ceeds through a class n mechanism involving proton transfer to or from the leavingoxygen atom. This conclusion is based on (1) structure-reactivity relationships with pxy= d0/~ dpAfig= d0ig/—dpAfeH+= 0.09 and pf= d0ig/—dpAfjg=—0.20 that are expected for this mechanismand are similar to the behavior observed previously for kinetically unambiguous class n catalysis,(2) a requirement for rate constants larger than the diffusion-controlled limit for the acid-catalyzed attack of ethoxide ion on formaldehyde, according to the alternative class e mechanism, and (3) the observation of both rate increases and rate decreases with electron-donating substituents in the leaving alcohol, depending on the pAf of the catalyst and leaving group. The structure-reactivity relationshipsand the rate constants that would be required for a stepwise mechanism provide evidence that proton transfer and C-0 cleavage are concerted. The observed general acid catalysis of these reactions proceeds through a class e mechanism with proton transfer to or from the formaldehyde oxygenatom. This conclusion is based on (1) structure-reactivity relationships including a value of pxy= da/—dpÁTg= 0.022 that is similar to the value of pxy= 0.026 for the gen-eral-acid-catalyzed addition of thiol anions to acetaldehyde,(2) comparison of absolute rate constantsand a with those for ac-etal hydrolysis, a model for a class n reaction, and (3) a requirement for rate constants larger than the diffusion-controlled limit for the base-catalyzed hydration of protonated formaldehyde according to the alternative class n mechanism. The values of a, the structure-reactivity relationships, and the rate constants that would be required for a stepwise mechanism show that the mechanism involves a larger component of proton transfer in the transition state compared with the reactions of stronger nu-cleophiles and continues a trend toward a fully concerted reaction mechanism as the basicity of the attacking nucleophile is decreased. The properties of the transition states of the acid-and base-catalyzed reactions are described in terms of reaction coordinate diagrams that are defined by the observed structure-reactivity relationships.