Isoxazol-4-ylboronic Acid | CAS 1008139-25-0 | ≥98%

Isoxazol-4-ylboronic acid (isoxazole-4-boronic acid; (1,2-oxazol-4-yl)boronic acid; 4-isoxazolylboronic acid) is a heteroaromatic boronic acid bearing boron at the 4-position of the isoxazole (1,2-oxazole) ring.

May contain small amounts of the cyclic anhydride isoxazol-4-ylboroxine. Under aqueous or basic coupling conditions the two forms re-equilibrate and the impact on yield is minor.

Stability and protodeboronation: with boron at C-4 — remote from the ring O and N — it is not one of the fast-decomposing heteroatom-adjacent heteroaryl boronic acids (such as 2-pyridyl or 2-thienyl). It is used directly in DNA-compatible cyanomethylation, where productive chemistry proceeds through Suzuki coupling followed by base-promoted isoxazole fragmentation. As for boronic acids generally, protodeboronation is promoted by strong base, elevated temperature, and prolonged aqueous exposure. Supplied for cold storage under inert atmosphere; a pinacol ester or trifluoroborate is the usual surrogate where a sensitive boronic acid needs extra stability.

Applications and Reactions

• Suzuki–Miyaura cross-coupling: under milder bases and lower temperatures the intact 4-isoxazolyl group transfers to aryl, heteroaryl, and alkenyl halides or triflates to give 4-substituted isoxazoles; stronger base diverts the reagent into the cyanomethylation fragmentation below.
• Tandem cyanomethylation: a one-pot Pd-catalysed domino sequence — Suzuki coupling, base-promoted isoxazole fragmentation, then deformylation — converts (hetero)aryl halides into arylacetonitriles (ArCH₂CN), the isoxazole serving as a masked cyanomethyl equivalent.
• Two-step cyanomethylation variant: the coupling and the fragmentation are run as separate steps, for substrates incompatible with the high-temperature one-pot conditions.
• DNA-compatible cyanomethylation: the tandem process runs on DNA-tagged (hetero)aryl halides and triflates without significant DNA damage, supplying arylacetonitriles and derived arylacetic acids for DNA-encoded library (DEL) screening collections.
• Chan–Lam C–N and C–O coupling: as an arylboronic acid it can transfer the 4-isoxazolyl group to amine, amide, and phenol nucleophiles under mild copper-mediated aerobic conditions, without a halide partner.
• Rhodium-catalysed 1,4-addition: as an arylboronic acid it can add the 4-isoxazolyl group to α,β-unsaturated carbonyl acceptors, with chiral ligands giving enantioenriched products.
• Oxidative ipso-hydroxylation: as a heteroarylboronic acid, the C–B bond can be oxidised to C–OH under peroxide, persulfate, N-oxide, or photoredox conditions to give isoxazol-4-ol.
• Pinacol ester format: the corresponding pinacol boronate is the form used in the original one-pot arylacetonitrile work and serves as a masked, bench-stable boronate when a less reactive format is preferred.
• Covalent fragment for screening: boronic acid coupling reagents are repurposed as directed covalent-fragment libraries for serine hydrolases and related enzymes (autotaxin among the documented examples), the boron forming a reversible covalent adduct with the active-site serine or threonine.
• Isoxazole building block and bioisostere: installs the isoxazole ring — an established amide and ester bioisostere — at C-4 for pharmaceutical, agrochemical, and heteroaryl-analogue synthesis.

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.