3-Methoxyphenylboronic Acid | CAS 10365-98-7 | ≥97%

3-Methoxyphenylboronic Acid (m-anisylboronic acid, m-methoxybenzeneboronic acid) is an arylboronic acid often used in medicinal-chemistry, agrochemical, and fine-chemical synthesis. At the meta position, the methoxy group does not donate electron density to the boronic-acid-bearing carbon by resonance; its net Hammett effect is weakly electron-withdrawing (σm ≈ +0.12), giving a boronic-acid pKa estimated near 8.5 and close to phenylboronic acid. After cross-coupling, the methoxy substituent can serve as a masked phenol, allowing later access to 3-hydroxyaryl products.

The product may contain small amounts of the cyclic anhydride 3-methoxyphenylboroxine; under aqueous or basic coupling conditions the two forms re-equilibrate and the impact on yield is minor.

Applications and Reactions

• Suzuki–Miyaura coupling: couples with aryl, heteroaryl, vinyl, and alkenyl electrophiles to give 3-methoxybiaryls and related aryl-substituted products. The meta-methoxy group is usually carried through the coupling as an intact aryl ether, which makes the reagent useful for installing a masked 3-hydroxyphenyl motif on the coupled scaffold.
• Methoxy-to-phenol unmasking: after coupling, the methoxy ether on the 3-methoxybiaryl can be cleaved by BBr3, AlCl3, or thiolate-based demethylation to give the 3-hydroxybiaryl. The free phenol can be alkylated or acylated, or converted to an aryl sulfonate such as a triflate for subsequent C–C or C–N cross-coupling.
• Rh-catalysed conjugate addition: 3-methoxyphenylboronic acid has been used as the aryl donor in rhodium-catalysed asymmetric 1,4-addition to β-aryl-α,β-unsaturated ketones and tert-butyl cinnamate-type esters. Under [Rh(nbd)2]BF4/(S,S)-chiraphos catalysis, the 3-methoxyphenyl group is transferred to the β-carbon to give enantioenriched β-diaryl carbonyl products.
• Oxidative ipso-hydroxylation: aqueous hydrogen peroxide under basic conditions can replace B(OH)2 with hydroxyl at the same carbon to give 3-methoxyphenol. This route is distinct from methoxy-to-phenol unmasking: ipso-hydroxylation cleaves the C–B bond on the boronic acid itself, while demethylation cleaves the methyl ether on a coupled product.
• Chan–Lam N/O arylation: arylboronic acids can undergo copper-mediated oxidative coupling with N- and O-nucleophiles under air. The 3-methoxyphenyl analogue may be used where the selected Chan–Lam method tolerates anisylboronic acids.
• Petasis borono-Mannich: arylboronic acids can participate in metal-free three-component coupling with amines and carbonyl partners to give α-aryl amines, α-amino acids, or β-amino alcohols. Use of the 3-methoxyphenyl analogue depends on the chosen Petasis method and substrate set.
• Ipso-halodeboronation: organoboron halodeboronation methods can replace B(OH)2 with Br, Cl, or I, providing a route to meta-haloanisole motifs (3-bromoanisole, 3-chloroanisole, 3-iodoanisole) when the substrate and halogenation method are compatible. This complements classical electrophilic aromatic substitution of anisole, where the methoxy group favours ortho/para substitution.

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.