A covalently bound dimeric derivative of pyrochlorophyllide a. A Possible model for reaction center chlorophyll
A covalently bound dimeric derivative of pyrochlorophyllide a. A Possible model for reaction center chlorophyll
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焦叶绿素 a 的共价结合二聚衍生物。
DOI：
10.1021/ja00433a066
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发表时间：
1976
影响因子：
15
通讯作者：
G. Closs
中科院分区：
文献类型：
--
作者：
S. Boxer;G. Closs
在方案I中有几个值得强调的机械特征。方程式6中烷基汞自由基的唯一归宿是断裂，这与计算出的此类物质的低键能（约6 kcal mol-1）12以及它们在其他过程（特别是脱汞）中的明显行为一致。2已知等式7中的烷基自由基从四氯化碳转移氯原子是容易的。13.然而，特别令人感兴趣的是，利用这种设施选择性地去除位于汞的邻位的氢，从而提供与其他工艺竞争的高收率的消除产物。为了获得这种激活的定量评估，我们进行了氢转移的竞争研究。测定了I在CCU溶液中与异辛烷和环己烷的反应活性。进行表II中所述的反应至小于10%的转化率，以保持基本恒定的底物比例。根据无引发剂时新戊基氯和2-氯-2，4，4-三甲基戊烷的相对产率，我们发现I中氢的反应活性比异辛烷中的高580倍。类似地，I中的α-氢比环己烷中的给定氢的反应性高600倍。以过氧化二丁酰为引发剂，直接测定环己烷和异辛烷的相对氢反应活性。平均值（每氢）为0-94，与从与汞的竞争中获得的间接值0.97很好地一致。14.在所研究的所有情况下，氯仿总是以相对于烷基氯化物等摩尔量形成，并且自由基双分子反应产生的副产物可以忽略不计。据我们所知，这些数据代表了在烷基金属中α，δ-氢被充分活化至均裂提取的第一个证据。[15]因此，它们与在氢化物转移形成的β-金属碳离子中观察到的超共轭效应有着有趣的关系，[17]并且类似的效应在一定程度上适用于自由基是合理的。我们希望将这些研究扩展到其他有机金属物种的还原消除，特别是过渡金属衍生物。18· 19
There are several mechanistic features in Scheme I which merit emphasis. Fragmentation as the exclusive fate of the alkyl-mercuric radical in eq 6 is consistent with the low bond energy (~ 6 kcal mol-1) calculated for such species12 and also with their apparent behavior in other processes, notably demer-curation. 2 Chlorine atom transfer from carbon tetrachloride by alkyl radicals in eq 7 is known to be facile. 13 Of particular interest, however, is the selective removal of a hydrogen located ß to mercury with such facility as to afford high yields of elimination products in competition with other processes. In order to obtain a quantitative assessment of this activation we carried out competition studies for hydrogen transfer. The reactivity of I towardhydrogen abstraction by tri-chloromethyl radical was determined in CCU solutions relative to isooctane and to cyclohexane. Reactions described in Table II were carried out to less than 10% conversions in order to maintain an essentially constant ratio of substrates. Based on the relative yields of neopentyl chloride and 2-chloro-2, 4, 4-trimethylpentane in theabsence of initiator, we find the hydrogen in I to be 580 times more reactive than that in isooctane. Similarly, the ß-hydrogen in I is 600 times more reactive than a given hydrogen in cyclohexane. As a cross-check, the relative hydrogen reactivities of cyclohexane and isooctane were de-termined directly using di-n-butyryl peroxide as initiator. The average (per hydrogen) value of &c6Hi2/&c8Hi8= 0-94 is in good agreement with the indirect value of 0.97 obtained from competition with the mercurial. 14 In all cases studied the chloroform was always formed in equimolar amounts relative to alkyl chlorides, and side products arisingfrom the bimolecular reactions of radicals were negligible. To our knowledge these data represent the first evidence for substantial activation of a, 8-hydrogen in a metal alkyl to homolytic abstraction. 15 As such they bear an interesting re-lationship to hyperconjugative effectsobserved in/3-metallo-carbonium ions formed by hydride transfers, 17 and it is rea-sonable that similar effects are applicable to radicals in measure. We hope to extend these studies to the reductive elimi-nation of other organometallic species, particularly those de-rived from transition metals. 18· 19