取代基对非共轭二烯自动氧化区域选择性的影响。
Substituent effects on regioselectivity in the autoxidation of nonconjugated dienes.
机构信息
Department of Chemistry and Center in Molecular Toxicology, Vanderbilt University, Nashville, Tennessee 37235, USA.
出版信息
J Am Chem Soc. 2009 Apr 22;131(15):5635-41. doi: 10.1021/ja900040d.
Free radical oxidation of several 1,4-dienes was carried out in the presence of variable concentrations of alpha-tocopherol to investigate the effect of diene structure on product distribution. Oxidations carried out at low tocopherol concentration gave only C-1 and C-5 conjugated diene hydroperoxides, while higher concentrations of the antioxidant resulted in formation of substantial amounts of the nonconjugated C-3 diene hydroperoxide. Increasing size of the substituents at C-1 and C-5 of the diene favors kinetic products arising from oxygen addition at the nonconjugated position, C-3, of the pentadienyl radical intermediate. Substituents at C-1 or C-5 of the pentadienyl radical also have a significant effect on the regioselectivity of the conjugated diene hydroperoxides formed, larger substituents directing oxygen addition to the pentadienyl radical at the site of least steric hindrance. This trend is also observed in oxidations of omega-3 and omega-6 linolenate fatty acid esters. Groups at C-1 and C-5 of the diene can influence product distribution based upon (a) steric demand in the oxygen-radical reaction and (b) the influence of substituents on the rearrangement of the C-3 peroxyl radical to give conjugated diene products.
在不同浓度的 α-生育酚存在下，对几种 1,4-二烯进行了自由基氧化，以研究二烯结构对产物分布的影响。在低生育酚浓度下进行的氧化仅产生 C-1 和 C-5 共轭二烯氢过氧化物，而抗氧化剂的浓度较高时则会形成大量非共轭 C-3 二烯氢过氧化物。二烯 C-1 和 C-5 上取代基的增大有利于从戊二烯基自由基中间体的非共轭位置（C-3）加成氧而产生动力学产物。戊二烯基自由基 C-1 或 C-5 上的取代基也对形成的共轭二烯氢过氧化物的区域选择性有显著影响，较大的取代基将氧加成到戊二烯基自由基的位阻最小的位置。这种趋势在 ω-3 和 ω-6 亚麻酸酯的氧化中也观察到。二烯的 C-1 和 C-5 上的基团可以根据（a）氧自由基反应中的空间需求和（b）取代基对 C-3 过氧自由基重排以生成共轭二烯产物的影响来影响产物分布。
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