Quantum yields for the photoinduced release of seven different commonly used leaving groups (LGs) from the o-nitroveratryl protecting group were measured. It was found that these quantum yields depend strongly on the nature of the LGs. We show that the quantum efficiency with which the LGs are released correlates with the stabilization that these LGs provide to o-nitrobenzyl-type radicals because radical stabilizing groups weaken the C–H bond that is cleaved in the photoinduced hydrogen atom transfer step, and hence lower the barrier for this process. At the same time these substituents lower the endothermicity of the thermal hydrogen atom transfer and thus increase the barrier for the reverse process, thereby enhancing the part of the initially formed aci-nitro intermediates which undergo cyclization (which ultimately leads to LG release). Radical stabilization energies computed by DFT methods are thus a useful predictor of the relative efficiency with which LGs are photoreleased from o-nitrobenzyl protecting groups.
测量了邻硝基
藜芦碱保护基团光诱导释放七个不同常用离去基团(LG)的量子产率。结果发现,这些量子产率在很大程度上取决于 LG 的性质。我们的研究表明,LGs 释放的量子效率与这些 LGs 为邻硝基苄基自由基提供的稳定作用有关,因为自由基稳定基团会减弱在光诱导氢原子转移步骤中被裂解的 C-H 键,从而降低这一过程的障碍。同时,这些取代基降低了氢原子热转移的内热,从而增加了反向过程的障碍,因此增加了最初形成的酰硝基中间体发生环化(最终导致 LG 释放)的部分。因此,通过 DFT 方法计算出的自由基稳定能可以有效预测 LG 从邻硝基苄基保护基团中光释放的相对效率。