4-Methylphenylboronic Acid | CAS 5720-05-8 | ≥98%

4-Methylphenylboronic Acid (4-tolylboronic acid, p-tolylboronic acid, 4-methylbenzeneboronic acid) contains a para-methyl group, which confers a mildly electron-rich character on the molecule without adding coordination sites, ionisable functionality, or significant steric bulk.

Like many arylboronic acids, 4-methylphenylboronic acid may contain small amounts of the corresponding cyclic anhydride, 4-methylphenylboroxine, in the solid state. Under aqueous/basic coupling conditions, boronic acid and boroxine forms usually re-equilibrate, but even minor boroxine content can affect assay, effective stoichiometry, and reproducibility.

Applications and Reactions

• Suzuki–Miyaura coupling: Used as a p-tolyl-transfer reagent for installing the p-tolyl group into aryl, heteroaryl, and alkenyl scaffolds. The methyl substituent gives a compact lipophilic aryl group useful in SAR, materials, and synthetic-methodology work.
• Chan–Lam coupling: Common partner in copper-mediated C–N, C–O, and selected C–S arylation. It transfers the p-tolyl group to amines, amides, sulfonamides, carbamates, N–H heterocycles, compatible phenols, selected alcohols, and selected sulfur nucleophiles.
• Rhodium-catalysed conjugate addition: Participates in 1,4-addition to α,β-unsaturated carbonyl substrates, transferring the p-tolyl group to the β-position of enones, enoates, enamides, nitroalkenes, and related Michael acceptors. Under suitable chiral ligand systems, the reaction can deliver enantioenriched β-(p-tolyl) carbonyl and β-(p-tolyl)nitro products.
• Petasis borono-Mannich reaction: Combines with an amine and a carbonyl component in a multicomponent reaction to give substituted amines bearing the p-tolyl group at the α-position to nitrogen. Glyoxylic acid/glyoxylates and suitable α-hydroxy aldehydes give access to α-(p-tolyl)-α-amino acid derivatives and β-amino-alcohol scaffolds.
• Materials and functional-molecule synthesis: Practical p-tolyl-transfer building block for biaryl and arylated small-molecule intermediates used in materials, ligand, dye, and functional-molecule R&D.
• Protected boronate ester preparation: Precursor for protected 4-methylphenylboronate esters such as pinacol, neopentyl glycol, and MIDA derivatives when a more stable or chromatographically tractable p-tolyl-boron building block is required.
• Ipso-halodeboronation: Can be converted to 4-bromotoluene, 4-chlorotoluene, or 4-iodotoluene under suitable electrophilic halogenation conditions where the boronic acid has served as a directing or coupling handle in earlier steps.
• Oxidative hydroxylation: Substrate for conversion to p-cresol under aerobic photoredox, peroxide-mediated, or copper-catalysed boronic acid → phenol conditions.
• Diol recognition studies: Secondary cis-diol recognition motif in boronate-ester binding studies, with weaker practical sensing value than more acidic or amine-appended boronic acid receptors.

Further Reading

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