THIOL CATALYSIS OF GEOMETRIC ISOMERIZATION OF BENZALDEHYDE SEMI-CARBAZONE BY NUCLEOPHILIC-ADDITION
THIOL CATALYSIS OF GEOMETRIC ISOMERIZATION OF BENZALDEHYDE SEMI-CARBAZONE BY NUCLEOPHILIC-ADDITION
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DOI：
10.1021/jo01316a023
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发表时间：
1979-01-01
影响因子：
3.6
通讯作者：
SAYER, JM
中科院分区：
文献类型：
--
作者：
CONLON, PR;SAYER, JM
脂肪和芳香族硫醇催化了(Z)-苯甲醛缩氨基酮的异构化反应。提出的反应机制包括快速形成四面体加成中间体，该中间体经历快速键旋转、氮反转和/或质子交换，然后损失硫醇以生成同分异构体氨基脲。该反应为测定碳氮双键上巯基的亲核加成速率提供了一种方便的方法，在四面体加成中间体极不利于平衡形成的情况下，该反应被用于研究该加成反应的详细机理，该加成反应在25℃下在水中的加成遵循速率规律/zth±i= &rs-[RS"~]+ ArshIRSH]；对于弱碱硫醇&rsh很大，/？不能检测到rs。对于pKB为7.9-10.3的硫醇，速率常数£rs-与硫醇的pKB （/3nuc^ 0.13）关系不大或没有关系。pKB bbb5.5的硫醇的速率常数6rsh与斜率为log Izrsh与pKB -1.1的斜率呈布朗斯蒂德型关系，与dnuc约为0的巯基阴离子对质子化氨基脲的速率决定加成机制一致。对于碱性较低的硫醇（pKB 2.7 ~ 5.5）， log &rsh与pKa的斜率约为-0.65，对应于阴离子的finuc为0.35。对于强碱性硫醇，阴离子攻击质子化氨基脲的/3nuc和计算出的速率常数与RS与质子化氨基脲的速率决定扩散控制反应一致，与氨基脲被巯基质子化形成的络合物的速率决定重组的“一次相遇”机制一致，或在“观察者”巯基阴离子存在下H30+将氨基脲质子化速率决定。弱碱性硫醇的/3nuc~ 0.35值可能反映了br0nsted型图中速率决定步骤或曲率的变化，这是由较弱亲核试剂的碳硫键形成所对应的过渡态结构的变化引起的。在某些情况下，催化C= N双键的几何异构化可以为该过程提供较低的能量途径，而非催化机制2涉及键的旋转或氮的反转。亚胺和相关化合物的几何异构化的可能的cat-分析机制包括(a)氮上的质子化，这降低了CN键的双键性质，促进了键的旋转，(b)碱催化的烯胺形成（对于含有一个氢的化合物），以及(c)亲核加成，随后在氮上反转或质子交换，然后消除亲核试剂。本文描述了苯甲醛缩氨基酮（eq 1）在巯基催化下的ZE异构化反应
The isomerization of (Z)-to (E)-benzaldehyde semicarbazone is catalyzed by aliphatic and aromatic thiols. The proposed reaction mechanism involves rate-determining formationof a tetrahedral addition intermediate which undergoes fast bond rotation, nitrogen inversion, and/or proton exchange, followed by loss of thiol to generate the isomeric semicarbazone. The reaction, which provides a convenient method for measuring rates of nucleophilic ad-dition of thiols to a carbon-nitrogen double bond in a system where equilibrium formation of the tetrahedral addition intermediate is highly unfavorable, has been used toinvestigate the detailed mechanism of this addition reac-tion in waterat 25 C. Addition of stronglybasic thiols to thesemicarbazone follows the rate law/zth¡ 0i= &rs-[RS"~]+ ArshIRSH]; for weaklybasic thiols &rsh is large and/? rs-cannot be detected. Rate constants,£ rs-, show little or no dependence on the pKB of the thiol (/3nuc^ 0.13) forthiols of pKB 7.9-10.3. Rate constants, 6rsh, for thiols of pKB> 5.5 follow a Br0nsted-type relationship with a slope of log Izrsh vs. pKB of-1.1, consistent with a mechanism involving rate-determining addition of the thiol anion to the protonated semicarbazone with dnuc approximately 0. For less basic thiols (pKB 2.7-5.5) the slope of log &rsh vs. pKa is ca.-0.65, correspondingto finuc for the anion of 0.35. For strongly basic thiols the/3nuc of 0 and calculated rate constants for anion attack on the protonated semicarbazone are consistent with a rate-determining diffusion-controlled reaction of RS “with the protonated semicarbazone, a “one-encounter” mechanism with rate-determining reorganization of the complex formed upon protonation of the semicarbazone by the thiol, or rate-determining semicarbazone protonation by H30+ in thepres-ence of “spectator" thiol anion. Thevalue of/3nuc~ 0.35 for weakly basic thiols may reflect a change in rate-determining step or curvature in the Br0nsted-type plot that results from a change in transition-state structure corre-sponding to significant carbon-sulfur bond formation for the poorer nucleophiles.Catalysis of geometric isomerization of the C= N double bond can, under some circumstances, provide a lower energy pathway for this process than uncatalyzed mechanisms2 involving bond rotation or inversion at nitrogen. Possiblecat-alytic mechanisms for geometric isomerization of imines and related compounds include (a) protonation at nitrogen, 3 which decreases the double-bond character of the CN bond and facilitates bond rotation,(b) base-catalyzed enamine formation4 (for compounds possessing an-hydrogen), and (c) nucleophilic addition6 followed by inversion or proton ex-change atnitrogen and subsequent elimination of the nucle-ophile. In this paper we describe a facile thiol-catalyzed ZE isomerization of benzaldehyde semicarbazone (eq 1) that is