Orientation Nitration of C—H Bond Using Azo Group as a Removable Directing Group: Regiospecific Synthesis ofo-Phenylenediamines

It is a difficult task to regiospecifically synthesize o-phenylenediamines from anilines via traditional elelectrophilic nitration process.This article described a method for the regiospecific synthesis of o-phenylenediamines from anilines.The procedure consisted of three steps, namely, the synthesis of azo compounds from anilines, the palladium-catalyzed azo group-directed ortho-orientation nitration of C-H bond, and the final reduction of the resulting o-nitro azo compounds to give o-phenylenediamines by the Zn/HCOOH system.In the process, the palladium-catalyzed azo group-assisted ortho-nitration of C-H bond is the key step for gaining the whole high regioselectivity.Keywords azo compounds; o-phenylenediamines; nitration; palladium catalysis; C-H bond activation 硝基芳烃在有机合成中具有广泛的用途 [1] .目前, 制备硝基芳烃最直接与实用的方法是传统的亲电硝化 方法(Electrophilic aromatic substitution, SEAr) [2] .但是亲 电硝化方法也存在一些自身难以克服的缺陷, 如: (1)区 域选择性较差, 特别是对一些单取代芳烃的硝化, 如乙 酰苯胺、苯酚等.(2)化学选择性较差.由于硝化过程较 难控制, 往往存在过度硝化问题.(3)底物与官能团兼容 性不够理想.由于亲电硝化反应往往在酸性和氧化性比 较强的体系中进行(如硝硫混酸体系), 从而影响底物与 官能团的兼容性, 还容易造成环境污染问题.(4)很难实 现不受取代基定位规则制约的定向硝化反应. 例如, 由 于定位规则限制, 传统亲电硝化方法不能实现由芳酮、 芳醛和芳酸区域专一性地合成邻硝基芳酮、 芳醛和芳酸.为克服传统亲电硝化方法存在的上述缺陷, 受近年 来过渡金属催化导向基团辅助的 C-H 键官能化反应的 启发 [3] ,