Photoinduced hydrogen abstraction from phenols by aromatic ketones. A new mechanism for hydrogen abstraction by carbonyl n,pi(*) and pi,pi(*) triplets
Photoinduced hydrogen abstraction from phenols by aromatic ketones. A new mechanism for hydrogen abstraction by carbonyl n,pi(*) and pi,pi(*) triplets
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DOI：
10.1021/ja961973v
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发表时间：
1996-12-11
影响因子：
15
通讯作者：
StPierre, MJ
中科院分区：
文献类型：
--
作者：
Leigh, WJ;Lathioor, EC;StPierre, MJ
纳秒激光闪光光解研究已经进行了分子间和分子内酚氢提取的动力学烷氧基苯乙酮，5-烷氧基茚满酮，4-烷氧基二苯甲酮三联体在乙腈和苯溶液中。关于通过羰基n，pi* 和pi，pi* 三联体提取的几何要求的信息来自于一系列酮的结果，所述酮含有通过对氧乙基键连接的对酚部分。对于所有这些化合物，氘动力学同位素效应的三重态寿命在乙腈中，解决方案，表明三重态衰变是由远程酚氢，从而产生相应的phenolic-hemipinacol双自由基的分子内提取率。双自由基也被检测到，并且在每种情况下比三重自由基的寿命长一个数量级。对于三种化合物，分子内过程的速率遵循与在母体甲氧基取代的酮通过对甲酚的双分子淬灭速率中观察到的相同的趋势。观察到烷氧基茚满酮衍生物偏离这一趋势：其中酚氢不可能平面内接近羰基n-轨道。对甲酚对一系列取代二苯甲酮的双分子猝灭速率常数的变化趋势表明，对于n，ns三重态抽提，电子给体取代和电子受体取代的酮的猝灭机理是不同的。对于π，π * 三重态和供体取代的（n，π * 三重态）二苯甲酮，抽象提出发生的机制，涉及的氢键激基复合物的中间，产生相应的自由基通过顺序的电子和质子转移。因此，通过这种机制淬灭的速率常数主要取决于酮三重态的碱性和苯酚的氧化电位。
Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,pi* and pi,pi* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile,solution,indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl-hemipinacol biradical. The biradicals:have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecular quenching of the parent methoxy-substituted ketones by p-cresol. Deviation from this trend is observed for the alkoxyindanone derivative,: where an in-plane approach of the phenolic hydrogen to the carbonyl n-orbital is not possible. The trends in the rate constants for bimolecular quenching of a series of substituted benzophenones by p-cresol indicate that for n,ns triplet abstractions, the quenching mechanism is different for electron donor-substituted and electron acceptor-substituted ketones. For pi,pi* triplets and donor-substituted (n,pi* triplet) benzophenones, abstraction is proposed to occur by a mechanism involving the intermediacy of a hydrogen-bonded exciplex, which yields the corresponding radicals by sequential electron- and proton-transfer. The rate constant for quenching by this mechanism thus depends mainly on the basicity of the ketone triplet state and, the oxidation potential of the phenol.