Quinolin-5-ylboronic Acid CAS 355386-94-6 | ≥97%

Quinolin-5-ylboronic acid (5-quinolineboronic acid, 5-boronoquinoline) is a heteroaryl boronic acid bearing the quinoline scaffold, with boron at the 5-position on the carbocyclic ring.

May contain small amounts of the cyclic anhydride tris(quinolin-5-yl)boroxine. Under aqueous or basic coupling conditions the two forms re-equilibrate and the impact on yield is minor.

Position and stability: C-5 lies on the carbocyclic (benzene) ring of quinoline, not the pyridine ring. Boron there sits peri to C-4 across the ring junction — a naphthalene-like relationship — while the ring nitrogen lies on the far pyridine ring, remote from boron. Well separated from the nitrogen, the reagent behaves as a carbocyclic-ring arylboronic acid rather than a labile 2-pyridyl-type boron, coupling robustly without the protodeboronation and palladium-coordination liabilities of boron sited on or beside the pyridine ring. The C-4 peri-hydrogen imposes a steric demand on the C-5 biaryl bond, influencing the dihedral of the 5-arylquinoline product.

Applications and Reactions

• Suzuki–Miyaura cross-coupling: Installs the 5-quinolyl fragment onto aryl, heteroaryl, and vinyl halides to give 5-arylquinolines and related heterobiaryls. Couples under standard conditions — Pd(PPh₃)₄ or Pd(dppf)Cl₂ with K₂CO₃ in aqueous dioxane — and is compatible with nickel-catalysed variants. The benzannulated quinoline contributes a rigid, nitrogen-containing aryl fragment for analogue and materials synthesis.
• Heterocyclic building block for SAR campaigns: Builds 5-arylquinoline and bis(heterocyclic) scaffolds for structure–activity studies. The 5-quinolyl motif is a rigid, planar, nitrogen-containing biaryl whose ring nitrogen adds a hydrogen-bond acceptor and raises polarity and aqueous solubility relative to naphthalene, making it a favoured aza-naphthalene bioisostere for naphthyl and other fused carbocyclic aromatics in lead optimisation.
• OLED and optoelectronic materials: The electron-poor, pyridine-type nitrogen makes quinoline a π-deficient, electron-accepting ring with a low-lying LUMO — the property behind its long record in organic electronics, exemplified by the archetypal OLED material tris(8-hydroxyquinolinato)aluminium (Alq₃), itself a quinoline metal chelate. 5-Arylquinolines accessed by Suzuki coupling form a distinct, covalently linked class of quinoline π-systems explored as electron-transport and hole-blocking layers, where the low LUMO promotes electron injection from the cathode and confines holes at the emissive interface. Emission tunes with the electronic character of the C-5 aryl group installed in the coupling.
• Coordination chemistry and metal complexes: The quinoline nitrogen of the coupled products is an N-donor that binds transition metals — iridium, platinum, ruthenium, zinc — to give luminescent and electroluminescent complexes and ditopic bridging ligands for metal–organic framework construction. Quinoline-type N-donor ligands recur throughout phosphorescent and emissive metal-complex design.
• Fluorescent probes and chemical sensors: The quinoline ring is an intrinsic UV/visible fluorophore with large, tunable Stokes shifts, its emission shifting with the electronic character of the C-5 substituent. The ring nitrogen acts as a proton-responsive switch — protonation shifts absorption and emission, enabling ratiometric pH sensing, a built-in advantage over carbocyclic fluorophores. Nitrogen coordination further supports colorimetric and fluorescent detection of metal cations such as Zn²⁺, Cu²⁺, and Fe³⁺, with use in biological imaging and environmental monitoring.

Further Reading

For boronic acids, boronic esters, protodeboronation, boroxine content, and Suzuki–Miyaura reagent selection, see NorrChemica's Lab Journal guide: Choosing Your Boron Source for Suzuki–Miyaura Coupling.