Boronic acids

In organic synthesis, boronic acids are indispensable reaction partners and their chemistry is anchored by several foundational named reactions. The Suzuki-Miyaura coupling, the palladium-catalysed cross-coupling of aryl or vinyl boronic acids with aryl halides or pseudohalides, is one of the most widely applied C-C bond-forming reactions in pharmaceutical synthesis, valued for its broad functional group tolerance and mild conditions. The Chan-Lam coupling uses copper catalysis to form C-N, C-O, and C-S bonds from boronic acids and heteroatom nucleophiles at ambient temperature. The Petasis reaction, a multicomponent condensation of boronic acids, amines, and aldehydes, provides direct access to alpha-amino acid derivatives and other densely functionalised amines of high relevance to medicinal chemistry. The Matteson homologation iteratively elaborates alkylboronic esters to homologated products with control over the carbon skeleton and stereochemistry.

Emerging synthetic techniques are making significant use of boronic acids and their derivatives. In photochemistry, visible-light photoredox catalysis has established boronic acids as convenient alkyl radical precursors through oxidative C-B bond homolysis, enabling Minisci-type alkylation of heteroarenes and metallaphotoredox cross-coupling of unactivated sp3 carbons, transformations of direct relevance to the diversification of drug scaffolds. Photochemical C-H borylation strategies, including transition-metal-free approaches driven by hydrogen atom transfer, now enable the introduction of boronate ester groups into unactivated C(sp3)-H bonds under mild, selective conditions. Electrochemically, direct anodic oxidation has enabled C(sp3)-H borylation of alkanes without the need for directing groups or stoichiometric oxidants, while photoelectrochemical iron-catalysed methods achieve site-selective primary C-H borylation under conditions inaccessible to either technique alone. The convergence of photochemistry and electrochemistry in this area exemplifies how boronic acid chemistry continues to evolve alongside the broader renaissance in sustainable synthesis.

Boronic acids occupy a unique position in both medicinal chemistry and organic synthesis, distinguished by boron's ability to adopt both trigonal planar (sp2) and tetrahedral (sp3) hybridisation states, a property that underpins their biological activity and synthetic versatility. In drug discovery, the boronic acid group is exploited primarily as a reversible covalent warhead. The Lewis acidic boron atom reacts with the nucleophilic hydroxyl group of active-site serine or threonine residues to form tetrahedral boronate adducts that mimic the transition state of amide bond hydrolysis, conferring exceptional potency and selectivity against serine proteases and related enzyme classes. This mechanism is well-suited to reversible covalent inhibitor design, avoiding the toxicological liabilities associated with irreversible target engagement. Beyond direct pharmacological use, boronic acids form reversible covalent complexes with diol-containing biomolecules, enabling applications in glycan sensing, carbohydrate-directed drug delivery, and the recognition of saccharide biomarkers relevant to oncology and metabolic disease. The functional group is bench-stable, of low general toxicity relative to other electrophilic warheads, and amenable to diverse late-stage synthetic transformations, making boronic acid-containing building blocks highly attractive throughout hit-to-lead and lead optimisation programmes.

Two first-in-class FDA-approved drugs demonstrate the therapeutic power of the boronic acid pharmacophore. Bortezomib (Velcade, Millennium Pharmaceuticals/Janssen), approved in 2003, is a dipeptidyl boronic acid and the first-in-class inhibitor of the 26S proteasome, approved for the treatment of multiple myeloma and mantle cell lymphoma. Its boronic acid group reversibly engages the N-terminal threonine of the proteasome's catalytic beta subunit, forming the tetrahedral adduct that blocks ubiquitin-mediated protein degradation and triggers apoptosis in malignant plasma cells. Vaborbactam (as meropenem/vaborbactam, Vabomere, Rempex/Melinta), approved in 2017, is the first boronic acid-based beta-lactamase inhibitor approved by the FDA. A cyclic boronate, it restores meropenem activity against carbapenem-resistant Enterobacteriaceae by forming a reversible covalent adduct with the active-site serine of KPC carbapenemases, with no intrinsic antibacterial activity of its own.