近日，英国布里斯托大学教授Varinder K. Aggarwal及其研究小组开发出不需金属催化剂、光诱导即可发生的C(sp³)-H键硼化反应。相关论文于2020年10月28日发表在《自然》杂志上。

研究人员报道了一个与以往反应机理不同的、利用氢原子转移催化的无金属硼化反应，其中C(sp³)-H键均裂形成的烷基自由基直接与双硼试剂发生硼化反应。这个反应是通过由紫光诱导的、在N-烷氧基邻苯二甲酰亚胺基氧化剂和含氯氢原子转移催化剂之间发生的电子转移反应来进行的。反常的是，在这一反应中，更强的伯碳C-H键比更弱的仲碳、叔碳甚至苯环上的C-H键更易被硼化。

机理研究表明，这种高甲基选择性是因为在反应中形成了一个氯化自由基硼“酸盐”复合物，该复合物选择性切断没有空间位阻的C-H键。通过采取这样一种光引发氢原子转移策略，这一无金属的C(sp³)-H键硼化反应使不活泼的烷烃可以在温和条件下转变为更有价值的有机硼化合物，且该反应的选择性有别于现有的金属催化策略。

据介绍，硼酸及其衍生物是化学科学中最有用的试剂之一，其应用横跨医药、农用化学品和功能材料等领域。通过催化使C-H键发生硼化反应是一种将许多不同含硼基团引入有机分子的有力途径，因为这样一来可以在不对反应物预活化的情况下，直接对化学原料的C-H键功能化。这些反应传统上依赖于能切断C-H键的贵金属催化剂，因此对芳香性的C(sp²)-H键——而不是脂肪性的C(sp³)-H键——选择性更高。

附：英文原文

Title: Metal-free photoinduced C( sp 3 )–H borylation of alkanes

Author: Chao Shu, Adam Noble, Varinder K. Aggarwal

Issue&Volume: 2020-10-28

Abstract: Boronic acids and their derivatives are some of the most useful reagents in the chemical sciences, with applications spanning pharmaceuticals, agrochemicals and functional materials. Catalytic C–H borylation is a powerful method for introducing these and other boron groups into organic molecules because it can be used to directly functionalize C–H bonds of feedstock chemicals without the need for substrate pre-activation. These reactions have traditionally relied on precious-metal catalysts for C–H bond cleavage and, as a result, display high selectivity for borylation of aromatic C(sp2)–H bonds over aliphatic C(sp3)–H bonds. Here we report a mechanistically distinct, metal-free borylation using hydrogen atom transfer catalysis, in which homolytic cleavage of C(sp3)–H bonds produces alkyl radicals that are borylated by direct reaction with a diboron reagent. The reaction proceeds by violet-light photoinduced electron transfer between an N-alkoxyphthalimide-based oxidant and a chloride hydrogen atom transfer catalyst. Unusually, stronger methyl C–H bonds are borylated preferentially over weaker secondary, tertiary and even benzylic C–H bonds. Mechanistic studies indicate that the high methyl selectivity is a result of the formation of a chlorine radical–boron ‘ate’ complex that selectively cleaves sterically unhindered C–H bonds. By using a photoinduced hydrogen atom transfer strategy, this metal-free C(sp3)–H borylation enables unreactive alkanes to be transformed into valuable organoboron reagents under mild conditions and with selectivities that contrast with those of established metal-catalysed protocols.

DOI: 10.1038/s41586-020-2831-6

Source: https://www.nature.com/articles/s41586-020-2831-6