The Influence of Boryl Substituents on the Formation and Reactivity of Adjacent and Vicinal Free Radical Centers
作者:John C. Walton、Andrew J. McCarroll、Qiao Chen、Bertrand Carboni、Roger Nziengui
DOI:10.1021/ja9944812
日期:2000.6.1
(boronic esters), by addition to vinyl boronate, and by hydrogen abstraction from alkyldioxaborolanes and observed by EPR spectroscopy. Unsymmetrically substituted α-boronate radicals displayed selective line broadening in their low-temperature spectra from which barriers to internal rotation about •CH2−B(OR‘)OR bonds were found to be 3 ± 1 kcal mol-1. Use of an empirical relationship between barrier height
通过从 1-溴烷基二氧杂硼烷(硼酸酯)中提取溴、添加到硼酸乙烯酯以及从烷基二氧杂硼烷中提取氢并通过 EPR 光谱观察,可以生成含有 α-硼酸酯取代基的自由基。不对称取代的 α-硼酸酯自由基在其低温光谱中显示出选择性谱线加宽,从中发现关于 •CH2-B(OR')OR 键的内旋障碍为 3 ± 1 kcal mol-1。使用势垒高度和键离解能之间的经验关系导致 BDE[(RO)2BCH2-H] = 98.6 kcal mol-1。叔丁氧基自由基从 2,4,4,5,5-pentamethyl-1,3,2-dioxaborolane 夺氢的速率常数是根据竞争性 EPR 和产品研究确定的,发现相对较小,与未活化的速率常数相当甲基。发现从双(4,4,5,5-四甲基-1,3,2-二氧杂硼烷-2-基)甲烷中提取氢极其困难。α-硼酸盐自由基的结构和能量...