tBuBrettPhos

Technical Specifications

Product Description & Scientific Applications

tBuBrettPhos is a bulky dialkylbiaryl phosphine developed alongside RockPhos and BrettPhos for the classes of carbon–heteroatom coupling that defeat the standard ligand set. Its two methoxy groups distinguish it from RockPhos, which carries a methoxy and a methyl at the same positions, and it sits among the ligands giving low-energy access to tricoordinate palladium.

Handling. It is a white crystalline solid, air-, moisture- and thermally stable, so it can be weighed and stored without a glovebox. Beyond the transformations below it serves as a general cross-coupling ligand across Buchwald–Hartwig amination, C–O coupling, Suzuki, Negishi, Stille, Hiyama and Sonogashira couplings, and α-arylation.

What it is used for. Aryl triflates are converted to aryl fluorides on a palladium(II) catalyst built from this ligand, which is crucial there for C–F reductive elimination through a mononuclear, tricoordinate palladium(II) complex. In triflate-to-bromide exchange it outperformed BrettPhos and the XPhos ligands, though the yields in that work were low. Aryl chlorides are nitrated with sodium nitrite by oxidative addition, transmetallation and reductive elimination to the nitroarene. A single catalyst based on it drives both steps of a cascade to benzimidazoles, with complete regioselectivity across amide and arylamine substrates.

Off-cycle suppression. Ligands of this type effectively repress off-cycle species where XantPhos and XPhos give no conversion at all — the concrete reason to reach for it over a cheaper standard ligand.

Choosing the palladium source. Common precursors react sluggishly with hindered ligands like this one, and the dibenzylideneacetone released from one of them is a documented inhibitor in couplings of weak nucleophiles. Anyone pairing this ligand with the wrong palladium source will blame the ligand.

Two bond isomers. It binds palladium as either the P,O or the P,C isomer, and the two behave differently: reductive elimination proceeds slowly from the P,O-bound form. An arylpalladium complex also transfers the palladium-bound aryl group to the flanking ring in a net dearomatization — a real decomposition route rather than a curiosity.

Where other ligands win. In palladium-mediated arylation of lysine in unprotected peptides its complex showed the most pronounced reactivity at room temperature, but results improved when it was replaced with the less bulky BrettPhos, attributed to that ligand's reduced propensity to drive C–O bond-forming reductive elimination. For fluorination, later catalysts on other ligands proved more effective and extended the reaction to aryl bromides.

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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