Substituent effects on the photocleavage of benzyl-sulfur bonds. Observation of the ''meta effect''
Substituent effects on the photocleavage of benzyl-sulfur bonds. Observation of the ''meta effect''
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DOI：
10.1021/jo9606923
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发表时间：
1996-10-04
影响因子：
3.6
通讯作者：
Jensen, AW
中科院分区：
文献类型：
--
作者：
Fleming, SA;Jensen, AW
用苯基苯硫醚研究了苯硫键的光裂解机理。四个实验表明该反应经过一个自由基中间体。首先，观察到的光产物都可以用自由基机制来解释。其次，自由基中间体被五己基系链束缚。第三，对4-NO2衍生物的类似物进行紫外分析表明，没有外络合物或电子转移途径。第四，a值与取代苯基苯基硫化物损失的准产率之间没有很强的相关性。氧对量子产率的影响最好在连续的冷冻泵-解冻循环彻底排出样品后观察。结果表明，氧会减弱取代基效应。在脱气样品的光解过程中，元取代衍生物比对取代衍生物表现出更显著的差异。元衍生物裂解效率最高的顺序为：3-CN > 3-NO2 > 3-CF3 > 3-CH3 > 3-OCH3。这些发现被3-OCH3衍生物形成苄基自由基的自由基ipso的电子密度增加和3-CN衍生物形成苄基自由基的自由基ipso的电子密度降低所证实。
Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO2 derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between a values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO2 > 3-CF3 > 3-CH3 > 3-OCH3. These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH3 derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.