Kinetics and mechanisms of electrophilic addition. II. Thermochemical-kinetic approach to transition state structure
Kinetics and mechanisms of electrophilic addition. II. Thermochemical-kinetic approach to transition state structure
复制标题
亲电加成的动力学和机制。
DOI：
10.1021/jo00954a012
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发表时间：
1973
影响因子：
3.6
通讯作者：
R. Mcdonald
中科院分区：
文献类型：
--
作者：
K. Yates;R. Mcdonald
用燃烧热法直接测定了六对几何异构的1，2-二取代乙烯的异构化反应速率（Δ H_4），并与文献值进行了比较。这些AH·，值通过测量在该溶剂中的溶解热来校正到乙酸条件。在大多数情况下，用停流法测定了10种上述烯烃和几种感兴趣的烯烃在几个温度下在乙酸中溴化的双分子速率常数。获得了每种异构体的AH+和AS* 值。还分析了这些反应的二溴化物-乙酰氧基溴产物分布和产物立体化学。在某些情况下，观察到立体定向反加成，并在其他变量的立体选择性。然而，在所有情况下，起始顺式和反式异构体之间的初始焓差在溴化过渡态增加。它的结论是，虽然桥过渡态参与在所有情况下，这些可能会导致。桥连的或开放的阳离子中间体，这取决于连接基团的性质。研究还表明，在这些反应中，随着非对映异构体产物的AAH和AAF值接近，反应物的（顺-反）通常不会沿着反应坐标单调地降低。这是根据Roberts和Kimball 1的众所周知的环状溴离子中间体来解释的，但现在清楚的是，这些反应可以显示可变的立体选择性，范围从主要的顺式到排他性的反加成。[2]中间体的性质已被证明强烈地依赖于起始烯烃结构和溶剂，现在更合理的是提出一系列可能的离子中间体，其中I和III是极端。
The enthalpies of isomerization (AH4 of six pairs of geometrically isomeric 1, 2-disubstituted ethylenes were either determined directly from heat of combustion measurements or obtained from literature values. These AH·, values were corrected to acetic acid conditionsby measuring heats of solutionin that solvent. Bimolecular rate constants (fa) for bromination of ten of the above olefihs, and several more of interest, were determined at several temperatures in acetic acid using stopped flow methods in most cases. Values of AH+ and AS* were obtained for each isomer. These reactions were also analyzed for dibromide-acetoxybromide product distribution and product stereochemistry. In some cases stereospecific anti addition was observed, and in others variable stereoselectivity was obtained. However, in all cases the initial enthalpy difference between starting cis and trans isomers was increased at thebromination transition state. It is concluded that, although bridged transition states are involved in all cases, these may lead to. either bridged or open cationic intermediates, depending on the nature of the attached groups. It is also shown that in these reactions large values of AAH and AAF (cis-trans) for the reactants do not generally decrease monotonically along the reaction coordinate as small AAH and AAF values for the diastereomeric products are approached.For a long time, electrophilic bromine additions to olefins were considered to be generally stereospecific and anti, which was explained in terms of the wellknown cyclic bromonium ion intermediate of Roberts and Kimball, 1 but it is now clear that these reactions can show variable stereoselectivity, ranging all the way from predominant syn to exclusive anti addition. 2 The nature of the intermediate has been shown to depend strongly on starting olefinstructure and on the solvent, and it is now more reasonable to propose a spectrum of possible ionic intermediates, of which I and Ill are extremes.