Accessing diverse azole carboxylic acid building blocks via mild C-H carboxylation: Parallel, one-pot amide couplings and ML-guided substrate scope design

This manuscript describes a mild, functional group tolerant, and metal-free C-H carboxylation that enables direct access to azole-2-carboxylic acids, followed by amide couplings in one pot. This sequence accesses a large variety of azole-2-amides, demonstrating the significant expansion of the accessible chemical space, as compared to previously known methodologies. Key to the described reactivity is the use of silyl triflate reagents, which serve as reaction mediators in C-H deprotonation and stabilizers of (otherwise unstable) azole carboxylic acid intermediates. A diverse azole substrate scope designed via ma-chine learning-guided analysis demonstrates the broad utility of the sequence. DFT calculations provide insights into the role of silyl triflates in the reaction mechanism. Transferrable applications of the protocol are successfully established: (i) A low pressure (CO2 balloon) option for synthesizing azole-2-carboxylic acids without the need for high-pressure equipment; (ii) the use of 13CO2 for the synthesis of labeled compounds; and (iii) isocyanates as alternative electrophiles for direct C-H amidation. Fundamentally, the reported protocol expands the use of heterocycle C-H functionalization from late-stage functionalization applications towards its use in library synthesis. It provides general access to densely functionalized azole-2-carboxylic acid building blocks and demonstrates their one-pot use in diversifying amide couplings.