Oxygen versus carbon acidity in the side-chain fragmentation of 2-, 3-, and 4-arylalkanol radical cations in aqueous solution: The influence of the distance between the OH group and the aromatic ring
Oxygen versus carbon acidity in the side-chain fragmentation of 2-, 3-, and 4-arylalkanol radical cations in aqueous solution: The influence of the distance between the OH group and the aromatic ring
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DOI：
10.1021/ja990352
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发表时间：
1999-07-21
影响因子：
15
通讯作者：
Steenken, S
中科院分区：
文献类型：
--
作者：
Baciocchi, E;Bietti, M;Steenken, S
用脉冲辐解法研究了2-、3-和4-(4-甲氧基苯基)醇自由基阳离子在水中的衰变，反应产物由稳态伽马辐解实验或由真正的单电子氧化剂12-钨钴(III)酸钾促进的反应确定。研究发现，所有2-芳基烷醇自由基阳离子都以扩散控制的速度与-OH反应，生成C-α-C-β键断裂产物。这表明这是由醇基上的去质子化引起的反应。在酸性介质(pH=4)中，这些自由基阳离子的衰变速度较慢，导致C-α-H去质子化(对于2-(4-甲氧基苯基)乙醇(1(.+))和1-(4-甲氧基苯基)-2-丙醇(4(.+)或C-α-C-β键断裂(对于1-苯基-2-(4-甲氧基苯基)乙醇(5(.+))和2-methyl-1-phenyl-3-(4-methoxyphenyl)-2-propanol(6(.+)。3-(4-甲氧基苯基)丙醇自由基阳离子(2(.+))在酸性介质(pH=4)中以接近1(.+)的速度发生C-α-H去质子化反应。相反，在碱性介质中(pH=10)，2(.+)生成3-(4-甲氧基苯基)丙醛的速率比1(.+)低近5倍，这再次表明O-H去质子化促进了反应。用4-(4-甲氧基苯基)-1-丁醇自由基阳离子(3(.+))，在-OH存在和不存在的情况下都能观察到C-α-H去质子化的产物。对这些结果进行了机理二分法的讨论，即碳对氧酸化，它似乎对2-和3-芳基烷醇起作用，而对4-芳基烷醇自由基阳离子只观察到碳酸。
The decay of 2-, 3-, and 4-(4-methoxyphenyl) alkanol radical cations in water has been kinetically investigated by pulse radiolysis, the reaction products being determined either by steady-slate gamma-radiolysis experiments or by reactions promoted by potassium 12-tungstocobalt(III)ate, a bona fide one-electron oxidant. It was found that all 2-arylalkanol radical cations react with -OH at a diffusion-controlled rate leading to C-alpha-C-beta bond cleavage products. This suggests a reaction induced by deprotonation at the alcoholic OH group. In acidic medium (pH = 4), the rates of decay of these radical cations are much lower leading to C-alpha-H deprotonation (for 2-(4-methoxyphenyl)ethanol (1(.+)) and 1-(4-methoxyphenyl)-2-propanol (4(.+))) or C-alpha-C-beta bond cleavage produces (for 1-phenyl-2-(4-methoxyphenyl)ethanol (5(.+)) and 2-methyl-1-phenyl-3-(4-methoxyphenyl)-2-propanol (6(.+))). The 3-(4- methoxyphenyl)propanol radical cation (2(.+)) reacts in acidic medium (pH = 4) at a rate close to that of 1(.+), undergoing C-alpha-H deprotonation. In contrast, in basic medium (pH = 10) 2(.+) produces 3-(4-methoxyphenyl)propanal, with a rate similar to 5-fold lower than that of 1(.+), again indicating a reaction promoted by O-H deprotonation. With 4-(4-methoxyphenyl)-1-butanol radical cation (3(.+)), products of C-alpha-H deprotonation were observed both in the presence and in the absence of -OH. These results are discussed in terms of a mechanistic dichotomy, that is, carbon versus oxygen acidify, which appears to be operating for 2- and 3-arylalkanols whereas with 4-arylalkanol radical cations only carbon acidity is observed.