Extent of charge transfer in the photoreduction of phenyl ketones by alkylbenzenes.
Extent of charge transfer in the photoreduction of phenyl ketones by alkylbenzenes.
复制标题
烷基苯光还原苯基酮时的电荷转移程度。
DOI：
10.1021/ja00284a041
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发表时间：
1986
影响因子：
15
通讯作者：
R. Wake
中科院分区：
文献类型：
--
作者：
P. Wagner;R. Truman;A. Puchalski;R. Wake
用闪光动力学、稳态猝灭和量子产率测量相结合的方法，测定了不同环代二苯甲酮(BPS)、苯乙酮(APs)和..CAPα、..CAPα、..CAPα三氟苯乙酮(TFAs)与甲苯和对二甲苯的三态反应速率常数。还测定了由相同的酮与对伞花烃反应生成的伯基和叔基的相对数量。对于所有三种类型的酮，速率常数与三线态酮的还原潜势有很好的相关性。甲苯-d/sub8/和对二甲苯-d/sub10/的动力学同位素效应的大小随着酮变得更容易还原而减小。所有的酮三元组与烷基苯的反应主要是通过电荷转移机制进行的，随着酮变得更难还原，速度决定步骤从络合转变为氢转移。最不活跃的AP三联体也可能通过简单的氢原子提取而发生显著的反应。那些具有n，..pi..*最低三联体的酮(所有BPS和一些AP)与p-伞花烃反应，得到伯三基/叔丁基比率，该比率与叔丁氧基显示的0.40值相差不超过2倍；那些具有..pi..，..pi..*最低三联体(TFA和一些AP)的酮提供有利于初级自由基的比率，并且随着三联体酮还原潜力的增加而变化幅度不超过一个数量级。伞花烃乘积比率的变化反映了n，..pi.*和..pi..，..pi.*三联体对伞花烃攻击的不同方向，以及激基复合体中不同程度的部分电子转移，它们不是紧密的自由基离子对。
Rate constants for triplet-state reaction of various ring-substituted benzophenones (BPs), acetophenones (APs), and ..cap alpha..,..cap alpha..,..cap alpha..-trifluoroacetophenones (TFAs) with toluene and p-xylene have been determined by a combination of flash kinetics, steady-state quenching, and quantum yield measurements. The relative amounts of primary and tertiary radicals formed by reaction of the same ketones with p-cymene have also been measured. For all three types of ketones, rate constants correlate well with triplet ketone reduction potentials. The magnitude of the kinetic isotope effects observed with toluene-d/sub 8/ and p-xylene-d/sub 10/ diminishes as the ketones become easier to reduce. All of the ketone triplets react with alkylbenzenes primarily by a charge-transfer mechanism, with the rate-determining step changing from complexation to hydrogen transfer as the ketones become harder to reduce. The least reactive AP triplets probably react significantly via simple hydrogen atom abstraction as well. Those ketones with n,..pi..* lowest triplets (all BPs and some APs) react with p-cymene to give primary/tertiary radical ratios that vary no more than a factor of 2 from the 0.40 value displayed by tert-butoxy radicals; those with ..pi..,..pi..* lowest triplets (TFAs and some APs) give ratios that favor primary radicals and that vary by an order of magnitude withmore » the triplet ketone reduction potential. The variation in cymene product ratios reflects different orientations for attack on cymene by n,..pi..* and ..pi..,..pi..* triplets and differing degrees of partial electron transfer within the exciplexes, which are not tight radical ion pairs.« less