Me4tBuXPhos

Technical Specifications

Product Description & Scientific Applications

Me4tBuXPhos is tBuXPhos carrying four additional methyl groups on the phosphorus-bearing ring, and in practice the two ligands divide the substrate scope between them.

It is supplied and stored under inert gas at room temperature.

Applications and Reactions

• Phenols from aryl halides: the founding application, and a demanding one. The catalyst has to overcome inhibition by free hydroxide, a difficult carbon–oxygen reductive elimination given how small the hydroxide group is, and over-arylation of the product phenol to the diaryl ether. The first hydroxylation of aryl bromides and chlorides with a hydroxide salt used a palladium catalyst supported by this ligand or by tBuXPhos, at 80–100 °C, with the optimal ligand chosen substrate by substrate. Palladium systems for this transformation offer wider scope, lower catalyst loadings and milder conditions.
• Why the bulk matters: ligand bulk raises the rate of carbon–oxygen reductive elimination, and hydroxide attack on the arylpalladium intermediate outpaces phenoxide transmetalation.
• Choosing between the two ligands: this one performed best on aryl chlorides without an ortho substituent, while tBuXPhos gave consistently excellent yields above 90% on ortho-substituted substrates. The source states this as a substrate-dependent optimum rather than one ligand being generally better.
• Onward chemistry: the phenol need not be isolated. Tandem sequences convert it in the same pot to benzofurans, or alkylate it to alkyl aryl ethers.
• Documented limits: in gold catalysis one cyclisation was sluggish on its gold chloride with silver triflate, where a triphenylphosphine gold chloride succeeded instead. In one solvent screen another ligand outperformed it alongside RockPhos, tBuBrettPhos and AdBrettPhos.

Further Reading

Practical guidance on choosing a phosphine ligand for palladium- and nickel-catalysed cross-coupling in NorrChemica's Lab Journal: Phosphine Ligands for Cross-Coupling.

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