KINETICS OF NITROXIDE RADICAL TRAPPING .2. STRUCTURAL EFFECTS
KINETICS OF NITROXIDE RADICAL TRAPPING .2. STRUCTURAL EFFECTS
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DOI：
10.1021/ja00039a006
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发表时间：
1992-06-17
影响因子：
15
通讯作者：
INGOLD, KU
中科院分区：
文献类型：
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作者：
BOWRY, VW;INGOLD, KU
激光闪光光解和动力学竞争产物研究表明，在室温下异辛烷中，碳中心自由基与持久性氮氧化物偶联的速率常数k(T)取决于偶联的位阻程度和碳自由基的共振稳定程度。由于氮氧化物结构和碳自由基结构的变化，可以观察到空间诱导的k(T)量级的减小。因此，对于任何特定的碳自由基k(T)对于Bredt规则保护的氮氧化物，9-氮杂环[3.3.1]壬烷- n-氧（ABNO）和nortropane- n-氧，k(T)最大，而对于“通常”的二叔烷基氮氧化物，k(T)沿着系列减小，1,1,3,3-四甲基异吲哚-2-氧大于或等于2,2,5,5-四甲基哌啶-1-氧(Tempo) >二叔丁基氮氧化物，即k(T)从五元环到六元环再到非环结构。环丙基和三苯基甲基分别以最快和最慢的速率被捕获，对应的k(T)值为3.0 × 10(9)和1.2 × 10(8) M-1 s-1， ABNO为2.1 × 10(9)和2.1 × 10(9)
Laser flash photolysis and kinetic competition product studies have demonstrated that in isooctane at ambient temperatures thc rate constant for coupling of carbon-centered radicals with persistent nitroxides, k(T), depends upon the degree of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of k(T) are observed for changes in both the structure of the nitroxide and the structure of the carbon radical. Thus, for any particular carbon radical k(T) is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the ''usual'' di-tert-alkyl nitroxides k(T) decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl greater-than-or-equal-to 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., k(T) decreases on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl and triphenylmethyl are trapped at thc fastest and slowest rates, respectively, the corresponding k(T) values being 3.0 x 10(9) and 1.2 x 10(8) M-1 s-1 for ABNO and 2.1 X 10(9) and