基于双重碳氢活化的γ,δ-不饱和酰胺与炔烃的脱氢环化反应

吡啶酮是一类重要的含氮杂环骨架,广泛存在于天然产物和药物分子中,是重要的化学转化中间体,其合成与修饰是现代医药学及化学领域的研究热点之一。杂环母核的官能团化修饰是该类化合物较为常见的衍生方式,但要求特定位点的反应基团预组装。相较而言,两个片段分子的直接偶联环化,是更为直接且具备较高实用性的合成类似杂环分子库的方式。近年来,过渡金属催化的丙烯酰胺与炔烃的氧化偶联制备吡啶酮类化合物取得了长足进展,关键活化步骤为过金属催化剂对酰胺β位sp2碳氢键的活化。然而,通过对更加易得的烷基酰胺进行sp3碳氢键活化制备杂环骨架依然具有较高的挑战性。其原因主要在于较低的α-酸性使得酰胺的脱氢反应变得异常困难。本课题组最近报道了温和条件下,铱催化的酰胺、酸及酮的空气脱氢反应。反应中产生的烯丙基-铱中间体被认为提高了酰胺的α-酸性,从而加速了脱氢过程。在此基础上,我们报道一种铑(Ⅲ)催化的γ,δ-不饱和酰胺与炔烃类化合物的脱氢环化新方法,制备一系列多取代的吡啶酮类化合物。催化循环历经酰胺导向铑(Ⅲ)对底物β位点的sp3碳氢活化,进而脱氢生成共轭的双烯酰胺中间体,随后酰胺基团再次导向铑(Ⅲ)对β位的sp2碳氢活化,进而与炔烃进行插入,环化获得吡啶酮。该反应对各种官能团具有较好的容忍性。γ-烯基结构不但促进第一步的酰胺脱氢,而且是杂环产物后修饰的重要位点。机理实验表明双烯酰胺的确为反应中间体之一。核磁实验显示酰胺脱氢迅速,而控制实验则表明炔烃的插入过程的选择性与其电性有密切的关系,有可能参与了该反应的速控步。

Dehydrogenative Annulation of γ, δ-Unsaturated Amides and Alkynes via Double C―H Activation

Pyridones represent an important family of heterocycles that exhibit a wide range of biological activities. They are often found in pharmaceutical agents and biomolecules. Several transition-metal-catalyzed transformations have been developed to access this family of heterocycles. Among them, C―H bond activation has recently emerged as a general strategy for the construction of substituted pyridones. In most cases, the core nitrogen-containing heterocycle is assembled via the dehydrogenative annulation of α, β-unsaturated amides and alkynes. Such processes involve a cascade sequence of N―H cleavage, sp² C―H activation, and annulation. Despite this progress, the more readily available α, β-saturated amides are rarely used. Ideally, tethering the direct dehydrogenation of an amide with the above-mentioned C―H annulation cascade would give a more practical synthesis of pyridones. Nevertheless, the dehydrogenation of amides under mild conditions is a synthetic challenge due to their intrinsic weak α-acidity. Recently, we have reported a general protocol for the aerobic dehydrogenation of γ, δ-unsaturated amides, acids, and ketones. A key Ir―allyl intermediate was believed responsible for enhancing the α-acidity of the amides studied, which enables the dehydrogenation step to occur under mild reaction conditions. Herein, we describe a new method for the synthesis of polysubstituted pyridones using γ, δ-unsaturated amides and alkynes. In the presence of RhCp*Cl₂]₂, the dehydrogenation step occurs via β-C―H bond activation. The resulting π-allyl―Rh intermediate undergoes an accelerated dehydrogenation reaction to afford the doubly unsaturated amide. This in-situ generated dienamide undergoes sp² C―H activation at the β-position and a subsequent alkyne insertion/cyclization reaction to yield the target heterocycle. Regeneration of the Rh catalyst is accomplished using an external oxidant and completes the streamlined double C―H activation and double dehydrogenation catalytic cycle. Various functional groups are well tolerated. The γ-alkenyl moiety not only facilitates the direct dehydrogenation of amides, but also serves as a handle for further derivatization of the as-obtained products. To gain a mechanistic insight into the reaction cascade, a set of control experiments were carried out. The results demonstrate that the dienamide is one of the key reaction intermediates. NMR experiments confirmed that the fast dehydrogenation process occurs during the early stage of the reaction. The alkyne insertion is believed to be the rate-determining step in the reaction cascade, as suggested by competition experiments.