Use of a Droplet Platform To Optimize Pd-Catalyzed C–N Coupling Reactions Promoted by Organic Bases

• Published in: Organic process research & development 2019-08, Vol.23 (8), p.1594-1601
• Main Authors: Baumgartner, Lorenz M, Dennis, Joseph M, White, Nicholas A, Buchwald, Stephen L, Jensen, Klavs F
• Format: Article
• Language: English
• ISSN: 1083-6160; 1520-586X
• DOI: 10.1021/acs.oprd.9b00236

Recent advances in Pd-catalyzed carbon–nitrogen cross-coupling have enabled the use of soluble organic bases instead of insoluble or strong inorganic bases that are traditionally employed. The single-phase nature of these reaction conditions facilitates their implementation in continuous flow systems, high-throughput optimization platforms, and large-scale applications. In this work, we utilized an automated microfluidic optimization platform to determine optimal reaction conditions for the couplings of an aryl triflate with four types of commonly employed amine nucleophiles: anilines, amides, primary aliphatic amines, and secondary aliphatic amines. By analyzing trends in catalyst reactivity across different reaction temperatures, base strengths, and base concentrations, we have developed a set of general recommendations for Pd-catalyzed cross-coupling reactions involving organic bases. The optimization algorithm determined that the catalyst supported by the dialkyltriarylmonophosphine ligand AlPhos was the most active in the coupling of each amine nucleophile. Furthermore, our automated optimization revealed that the phosphazene base BTTP can be used to facilitate the coupling of secondary alkylamines and aryl triflates.