科研之友
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摘要
Pyridine nuclei are ubiquitous structural motifs found in many bioactive molecules and pharmaceuticals. Therefore, the site-selective C-H functionalization at different position of pyridines is of great importance in synthetic chemistry. To access diversified pyridine skeletons, many methods have been developed to enable the C2-and C4-selective C-X bond（X = C, N, and O, etc.） formation by taking advantage of the coordination of active catalyst to pyridine "N" and directing groups. However, the regioselectively introducing chemical bonds into the C3-position of pyridines without an additional chelation-assistance is very difficult to achieve. Carbene transfer reaction provides a powerful tool to construct C-C bonds. Recently, liganddirected C-H carbenoid functionalizations have received intensive interests, this methodology could regioselectively install C-C bond into the particular position on the target molecules.1 In comparison, the site-selective coupling between diazo compounds and arenes without additional directing-groups remains rather challenging. Yet, significant breakthroughs have been made recently by Zhou,2 Zhang,3 and Shi,4 to realize noble metal-catalyzed direct phenyl Csp2-H carbenoid insertion of electron-rich benzenes through electrophilic aromatic substitution（SEAr）. However, the direct pyridyl Csp2-H carbenoid insertion has not been achieved due to its electron-deficient conjugated system. Nevertheless, our recent work about C-H metallene insertions5 stimulates us to envision that efficiently leveraging electronic properties of pyridine ring would possibly allow for the direct pyridine C-H carbenoid insertion. To verify our hypothesis, herein we explored a Co-catalyzed crosscoupling reaction between pyridines and diazoesters, in which C-H carbenoid insertion highregioselectively occurred at the C3-position of pyridines（Scheme 1）.6
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