Professor Qilong Shen's research group at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, developed an extremely mild and efficient new synthetic strategy. Under transition metal-free conditions, various α-heteroaryl amides and esters can be constructed in high yields simply by mixing zinc powder, α-bromoamides (or esters), and heteroaryl halides at room temperature. This not only greatly simplifies the reaction process but also provides a practical and green new method for the synthesis of related drug molecules and their skeletons. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.
Background information:
The heteroaryl C(sp2)-C(sp3) bond is the core framework of many drug, pesticide, and functional material molecules. Traditional synthetic methods often rely on aromatic nucleophilic substitution (SNAr) under strong base conditions or on expensive transition metal catalysts such as palladium and nickel. However, strong base conditions often destroy the fragile functional groups in the substrate, and transition metal catalysis is not only costly but also suffers from the problem of "difficulty in removing heavy metal residues" in the later stages of drug synthesis.
Reformatsky reagents are a class of classic organozinc reagents with a history of over 130 years. They exhibit excellent functional group tolerance but weak nucleophilicity, traditionally reacting only with highly reactive electrophilic reagents such as aldehydes and ketones. How to "awaken" their reactivity and achieve direct coupling with unactivated heteroaryl halides has always been a highly challenging problem.
Highlights of this article:
1. Extremely simple reaction conditions: Say goodbye to expensive catalysts and demanding ligands. Simply add zinc powder at room temperature to trigger the reaction, and there is no need to prepare Reformatsky reagent in advance ("one-pot" mixing is sufficient).
2. Excellent functional group compatibility and practicality: The reaction is compatible with various heteroaromatic rings containing fluorine, chlorine, bromine, iodine and other functional groups. It has good compatibility with various functional groups and can be safely used even if the substrate contains sensitive groups such as alkyl iodine and aryl iodine.
Furthermore, this reaction can be easily scaled up to the 100-gram scale and has been successfully applied to the post-modification of various drug analogues and natural product derivatives.
3. Substrate-directed activation mechanism: Mechanistic studies have revealed that the nitrogen atom on the heteroaryl ring can coordinate with the electron-deficient zinc center in the Reformatsky reagent. This interaction not only increases the electrophilicity of the heteroaryl halide but also induces the Reformatsky reagent to isomerize from the carboenol (C-enolate) to the more nucleophilic oxoenol (O-enolate).
Summary and Outlook:
This work breaks the traditional dependence of intermolecular SnNAr reactions on strong bases and highly electron-deficient aromatic rings, significantly reducing the reaction energy barrier through a unique coordination-inducible mechanism. The reaction is simple and efficient, enriching the reaction chemistry of Reformatsky reagents and providing a new paradigm for the construction of C(sp3)-C(sp2) bonds without transition metal involvement.
This research was published as a Research Article in CCS Chemistry. The first author is Yangxiao Li, a doctoral student at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, and the corresponding author is Professor Qilong Shen.
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About the journal: CCS Chemistry is the Chinese Chemical Society's flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem .
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman's Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/ .
