tBuXPhos

Technical Specifications

Product Description & Scientific Applications

tBuXPhos is an air-stable, electron-rich biaryl phosphine, and the only member of the Buchwald set on this shelf whose backbone carries no oxygen at all.

Why the backbone matters. Lacking oxygen atoms, its efficient palladium complex must be the P,C-bound isomer. The methoxylated relative tBuBrettPhos forms the P,O-bound isomer instead, from which reductive elimination proceeds slowly. That single structural difference is the basis for choosing between two ligands that look near-identical on a structure drawing.

Trifluoromethylation, carboxylation and cyanation. On vinyl sulfonates, BrettPhos — optimal for aryl chlorides — gave poor results, where this ligand gave the best reactivity at 72% yield; vinyl triflates and nonaflates from cyclohexanone precursors were trifluoromethylated in moderate to good yields. In reductive carboxylation of aryl bromides with carbon dioxide using diethylzinc as reductant it showed unique reactivity. In cyanation of aryl chlorides, substrates with pronounced electron deficiency required this ligand for efficient reductive elimination.

Hydroxylation and alkoxylation. It supported the first reported hydroxylation of aryl bromides and chlorides with a hydroxide salt, at 80 to 100 °C. Methoxylation and deuteriomethoxylation of bromochalcones finish in 5 to 40 minutes; a RockPhos system also works there but takes considerably longer, and in a separate comparison tBuXPhos gave a higher number of alkoxylated derivatives in shorter reaction time at similar yields. The advantage narrows as the nucleophile grows — lengthening the alcohol chain increases reaction time and decreases yield.

Fluorinated alcohols and loadings. It and RockPhos were both efficient in 2-fluoroethoxylation of bromochalcones, complete in 5 to 100 minutes. A palladium precatalyst built on it required 4 mol% for above 95% product formation in an aqueous surfactant system; at 6 mol% with palladium acetate it was one of three ligands sufficient for excellent yields within 48 hours.

Gold catalysis. Its cationic gold complex gives cyclobutenes regioselectively from alkynes and alkenes in moderate to good yields. The tetraarylborate salt gives 95% against 80% for the alternative anion, and mixing the gold chloride with the sodium salt in situ costs no yield.

Further applications

• Palladium-catalysed Tsuji–Trost substitution and cross-coupling of benzylic fluorides
• Palladium-catalysed C–N cross-coupling of sulfinamides with aryl halides
• Palladium-catalysed Suzuki–Miyaura coupling of allylboronates with aryl halides
• Palladium-catalysed arylation of pyrazoles, indazoles and amino heterocycles
• Palladium-catalysed arylation of nitroacetates and cyanamides

Further reading: For phosphine-ligand selection, palladium sources and precatalysts, and Suzuki–Miyaura and Buchwald–Hartwig reagent choice, see NorrChemica's Lab Journal guide: Phosphine Ligands for Cross-Coupling.

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