Model compound studies of the β-O-4 linkage in lignin: Absolute rate expressions for β-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl radical
Model compound studies of the β-O-4 linkage in lignin: Absolute rate expressions for β-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl radical
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DOI：
10.1021/jo025581k
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发表时间：
2002-11-15
影响因子：
3.6
通讯作者：
Franz, JA
中科院分区：
文献类型：
--
作者：
Kandanarachchi, PH;Autrey, T;Franz, JA
测定了1-苯基-2-苯氧乙醇-1-基（PhC）苯氧自由基β-断裂的Arrhenius速率表达式。（OH）CH2OPh（V）.酮基自由基V被苯硫酚竞争捕获生成PhCH（OH）CH_2OPh，与β-断裂竞争生成苯氧基自由基和苯乙酮。本文提出了一种用邻苯乙醇（PhCa）提取氢原子的基本速率表达式。（OH）CH_3，来自苯硫酚，log（k（abs）/M ~（-1）s ~（-1））=（8.88 +/-0.24）-（6.07 +/-0.34）/θ，θ = 2.303RT。初级保健自我终止率。使用Smoluchowski方程，采用母体醇PhCH（OH）CH 3的实验扩散系数作为自由基的模型，计算（OH）CH 3。用夺氢基反应测定了1-苯基-2-苯氧基乙醇-1-基（V）β-断裂苯氧基的活化能垒：log（k（beta）/s（-1））=（12.85 +/-0.22）-（15.06 +/-0.38）/θ，k（beta）（298 K）ca. (64.0 s（-1）在苯中），log（k（beta）/s（-1））=（12.50 +/- 0.18）-（14.46 +/- 0.30）/θ，k（beta）（298 K）= 78.7 s（-1）在含有0.8 M 2-丙醇的苯中。B3 LYP/cc-PVTZ电子结构计算预测，分子内的α-OH和酮自由基（V）的-OPh离去基团之间的氢键稳定基态和过渡态结构。计算的活化势垒为14.9千卡/摩尔，与实验活化势垒吻合得很好。
Arrhenius rate expressions were determined for beta-scission of phenoxyl radical from 1-phenyl-2phenoxyethanol-1-yl, PhC.(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenol to yield PhCH(OH)CH2OPh in competition with beta-scission to yield phenoxyl radical and acetophenone. A basis rate expression for hydrogen atom abstraction by see-phenethyl alcohol, PhCa.(OH)CH3, from thiophenol, log(k(abs)/M-1 s(-1)) = (8.88 +/- 0.24) - (6.07 +/- 0.34)/theta, theta = 2.303RT, was determined by competing hydrogen atom abstraction with radical self-termination. Self-termination rates for PhC.(OH)CH3 were calculated using the Smoluchowski equation employing experimental diffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogen abstraction basis reaction was employed to determine the activation barrier for the beta-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(k(beta)/s(-1)) = (12.85 +/- 0.22) - (15.06 +/- 0.38)/theta, k(beta)(298 K) ca. (64.0 s(-1) in benzene), and log(k(beta)/s(-1)) = (12.50 +/- 0.18) - (14.46 +/- 0.30)/theta, k(beta)(298 K) = 78.7 s(-1) in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculations predict that intramolecular hydrogen bonding between the alpha-OH and the -OPh leaving group of ketyl radical (V) stabilizes both ground- and transition-state structures. The computed activation barrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.