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首页> 外文期刊>Chemistry: A European journal >Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity
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Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

机译：水溶液中1-芳基烷醇自由基阳离子侧链断裂的动力学和产物研究：氧与碳酸度

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摘要

A kinetic and product study of the side-chain fragmentation reactions of a series of 1-arylalkanol radical cations (4-MeOC_6H_4CH(OH)R~(centre dot+)) and some of their methyl ethers was carried out; the radical cations were generated by pulse radiolysis and #gamma# radiolysis in aqueous solution. The radical cations undergo side-chain fragmentation involving the C_(#alpha#)-H and/or C_(#alpha#)-C_(#beta#) bonds, and their reactivity was studied both in acidic (aH <= 4) and basic (aH 10-11) solution. At pH4, the radical cations decay with first-order kinetics, and the exclusive reaction is C_(#alpha#)-H deprotonation for 1~(centre dot+), 2~(centre dot+), and 3~(centre dot+) (R = H, Me, and Et, respectively) but C_(#alpha#)-C_(#beta#) bond cleavage for 5~(centre dot+), 6~(centre dot+), 7~(centre dot+) (R = tBu, CH(OH)Me, and CH(OMe) Me, respectively). Both types of cleavage are observed for 4~(centre dot+) (R = iPr). The radical cations of the methyl ethers 8~(centre dot+), 9~(centre dot+), and 10~(centre dot+) (R = H, Et, and iPr, respectively) undergo exclusive deprotonation, whereas C-C fragmentation predominates for 11~(centre dot+) (R = tBu). Large C_(#alpha#) deuterium kinetic isotope effects (4.5 and 5.0, respectively) were found for 1~(centre dot+) and its methyl ether 8~(centre dot+). Replacement of an #alpha#-OH group by OMe has a very small effect on the decay rate when the radical cation undergoes deprotonation, but a very large, negative effect in the case of C-C bond cleavage. It is suggested that hydrogen bonding of the #alpha#-OH group with the solvent stabilizes the transition state of the C-C bond fragmentation reaction but not that of the deprotonation process; however, other factors could also contribute to this phenomenon. The decay of the radical cations is strongly accelerated by HO~-, and all the #alpha#-OH substituted radical cations react with HO~- at a rate (approx= 10~(10)M~(-1)S~(-1)) very close to the limit of diffusion control and independent of the nature of the bond that is finally broken in the process (C-H or C-C). The methyl ether 8~(centre dot+), which exclusively undergoes C-H bond cleavage, reacts significantly slower (by a factor of ca. 50) than the corresponding alcohol 1~(centre dot+). These data indicate that 1-arylalkanol radical cations, which display the expected carbon acidity in water, become oxygen acids in the presence of a strong base such as HO~- and undergo deprotonation of the O-H group; diffusion-controlled formation of the encounter complex between HO~- and the radical cation is the rate-determining step of the reaction. It is suggested that, within the complex, the proton is transferred to the base to give a benzyloxyl radical, either via a radical zwitterion (which undergoes intramolecular electron transfer) or directly (electron transfer coupled with deprotonation). The latter possibility seems more in line with the general base catalysis (#beta# approx= 0.4) observed in the reaction of 5~(centre dot+), which certainly involves O-H deprotonation. The benzyloxyl radical can then undergo a #beta# C-C bond cleavage to form 4-methoxybenzaldehyde and R~(centre dot) or a formal 1, 2-H shift to form an #alpha#-hydroxybenzyl-type radical. The factors of importance in this carbon/oxygen acidity dichotomy are discussed.
机译：进行了一系列1-芳基烷醇自由基阳离子（4-MeOC_6H_4CH（OH）R〜（中心点+））及其一些甲基醚的侧链断裂反应的动力学和产物研究;自由基阳离子是通过在水溶液中通过脉冲辐解和＃γ＃辐解产生的。自由基阳离子经历侧链断裂，涉及C _（＃alpha＃）-H和/或C _（＃alpha＃）-C _（＃beta＃）键，并且在酸性条件下（aH <= 4）研究了它们的反应性和基本（aH 10-11）解决方案。在pH4下，自由基阳离子以一阶动力学衰减，排他的反应是C _（＃alpha＃）-H去质子化1〜（中心点+），2〜（中心点+）和3〜（中心点+）（ R分别为H，Me和Et），但C _（＃alpha＃）-C _（＃beta＃）键的裂解为5〜（中心点+），6〜（中心点+），7〜（中心点+）（R分别为tBu，CH（OH）Me和CH（OMe）Me）。两种类型的切割均观察到4〜（中心点+）（R = iPr）。甲基醚8〜（中心点+），9〜（中心点+）和10〜（中心点+）的自由基阳离子（分别为R = H，Et和iPr）经历专有的去质子化作用，而CC断裂则占11位〜（中心点+）（R = tBu）。发现1〜（中心点+）及其甲醚8〜（中心点+）具有较大的C _（＃alpha＃）氘动力学同位素效应（分别为4.5和5.0）。当自由基阳离子去质子化时，用OMe取代＃alpha＃-OH基团对衰变速率的影响很小，但在C-C键断裂的情况下，消极影响很大。提示＃alpha＃-OH基团与溶剂的氢键稳定了C-C键断裂反应的过渡态，但不能稳定去质子化过程的过渡态。但是，其他因素也可能导致这种现象。 HO〜-强烈促进了自由基阳离子的衰变，并且所有被＃alpha＃-OH取代的自由基阳离子与HO〜-反应的速率为（约= 10〜（10）M〜（-1）S〜（ -1））非常接近扩散控制的极限，并且与该过程中最终断裂的键的性质（CH或CC）无关。仅经历C-H键裂解的甲基醚8〜（中心点+）比相应的醇1〜（中心点+）的反应明显更慢（约50倍）。这些数据表明，在水中显示出预期的碳酸度的1-芳基链烷醇自由基阳离子在强碱如HO〜-的存在下变成氧酸，并经历O-H基团的去质子化。 HO〜-与自由基阳离子之间相遇配合物的扩散控制形成是反应的速率决定步骤。建议在络合物中，质子通过自由基两性离子（经历分子内电子转移）或直接（电子转移与去质子化）转移到碱基上，生成苄氧基。后一种可能性似乎更符合在5〜（中心点+）反应中观察到的一般碱催化（＃beta＃大约= 0.4），这当然涉及O-H去质子化。然后，苄氧基可经＃β＃C-C键裂解形成4-甲氧基苯甲醛和R〜（中心点）或形式上的1、2-H移位以形成＃α＃-羟基苄基型自由基。讨论了在这种碳/氧酸度二分法中的重要因素。

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