Lewis Acid Acceleration of C−N Bond-Forming Reductive Elimination from Heteroarylpalladium Complexes and Catalytic Amidation of Heteroaryl Bromides

A large acceleration of carbon−nitrogen bond-forming reductive elimination from heteroarylpalladium amido complexes by addition of Lewis acids is described. Several lines of data imply that this effect arises from coordination of the Lewis acid to the nitrogen of the heteroaryl group. The presence of this coordination was confirmed by isolation of a Lewis acid base complex with a borane coordinated to the pyridyl nitrogen. This adduct underwent reductive elimination faster than the complex lacking the Lewis acid, and it occurred in high yield. Control experiments showed that this acceleration of reductive elimination was not observed for the reactions of arylpalladium amide complexes. This effect of Lewis acids translated to catalytic C−N bond-forming coupling processes. The binding of Lewis acids to the pyridyl nitrogen led to an acceleration of the amidation of unactivated heteroaryl bromides catalyzed by palladium complexes of Xantphos. The rates were faster and the yields were higher for reactions of BEt3 adducts of basic heteroaryl bromides than for the free heteroaryl bromides. This phenomenon draws parallels to the beneficial effect of Lewis acids on the reductive elimination of nitriles from arylmetal cyanide complexes and the catalytic hydrocyanation of olefins.

本研究报道了向杂芳基钯氨基配合物中添加路易斯酸（Lewis acids），可显著加速其发生碳-氮键生成型还原消除反应。多组实验数据表明，该加速效应源于路易斯酸与杂芳环上氮原子的配位作用。通过分离得到硼烷与吡啶氮配位的路易斯酸碱加合物，证实了该配位作用的存在。该加合物的还原消除反应速率快于未添加路易斯酸的对应配合物，且产物收率优异。对照实验显示，芳基钯酰胺配合物的反应中并未观测到此类还原消除加速现象。路易斯酸的该加速效应可推广至催化型碳-氮键生成偶联反应中：路易斯酸与吡啶氮的配位作用，可加速Xantphos配位钯配合物催化的未活化杂芳基溴化物的酰胺化反应。碱性杂芳基溴化物的三乙基硼（BEt3）加合物参与的反应，其反应速率与产物收率均优于游离的杂芳基溴化物。该现象与路易斯酸促进芳基金属氰化物配合物还原消除生成腈类的效应，以及烯烃催化氢氰化反应中的有益效应具有相似性。