NixantPhos

Technical Specifications

Product Description & Scientific Applications

Nixantphos (4,6-bis(diphenylphosphino)phenoxazine) is a wide-bite-angle diphosphine carrying an N–H bridge where Xantphos has a gem-dimethyl group, and a natural bite angle of 114°. The phenoxazine core has a pKa near 22, so that N–H can be removed by base — and the deprotonated ligand behaves like a different compound.

It is more air- and oxidation-stable than the trialkylphosphines used for the same substrates, several of which are pyrophoric. It is not bench-stable in absolute terms: the phenoxazine core oxidises, and the material is supplied air- and heat-sensitive, stored refrigerated under inert gas.

Applications and Reactions

• Room-temperature coupling of unactivated aryl chlorides: with an alkali-metal silylamide base the ligand is deprotonated to a heterobimetallic alkali-metal–ligand–palladium system that oxidatively adds unactivated aryl chlorides at 24 °C — something chelating diphosphines were not expected to do, since they react through the less favourable bis-ligated pathway and normally require heating. In deprotonative cross-coupling of diarylmethanes to triarylmethanes, palladium acetate with this ligand gave 56–99% yields across eleven aryl chlorides at 2.5–10 mol% loading in tetrahydrofuran at room temperature, with cyano, keto, acetyl, phenol, acetamide, phenothiazine and indole groups all tolerated. In the same screen the ligand gave 91% assay yield, its N-benzylated analogue — which cannot be deprotonated — under 2%, and Xantphos and DPEphos none at all.
• Linear-selective hydroformylation: at 114° it sits among the high-bite-angle diphosphines that give the best linear-to-branched ratios, up to 96:4. Substituted for a binaphthyl diphosphine in one system, it raised product yield to 95% with no detectable loss of linear selectivity.
• Carbonylation: across a diphosphine series yields improved as the bite angle increased, and at 114° it gave 89% of the carbamate product.
• Decarboxylative coupling: the reaction requires this diphosphine to support the palladium. Ligand-free conditions and two other phosphines gave only traces.
• Iridium emitters: with its xanthene relative it forms luminescent iridium(III) complexes, solution emission efficiency falling slightly as the ligand bulk widens the bite angle.
• Where the nitrogen costs: in one bis-alkoxycarbonylation it gave 55% of the diester against 82% for Xantphos, competitive coordination of its nitrogen to palladium the proposed cause. In one linear-aldehyde synthesis both yield and regioselectivity were only moderate, and in the deprotonative coupling 3-chloropyridine and the 2- and 3-chlorothiophenes fail.

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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