Potassium Phosphate Tribasic, Anhydrous (K3PO4)

Technical Specifications

Product Description & Scientific Applications

Potassium Phosphate Tribasic (K₃PO₄) is a strong inorganic base with a conjugate-acid pKa near 12.7. It is not interchangeable with the other potassium phosphates: in the Suzuki–Miyaura coupling of haloaryl MIDA boronic esters it gave 92% conversion under conditions where KH₂PO₄ and K₂HPO₄ each gave none, and the lithium, sodium, caesium, magnesium and calcium phosphates gave 6% or less. Those phosphates share its pKa and solution pH, so other properties of the salt decide the outcome.

Role in transmetalation. Calculations place it inside the catalytic cycle rather than outside it: the base lowers the transmetalation barrier relative to a base-free pathway by activating the carbon–boron bond, and on the most feasible route it acts as a Lewis base complexing the Lewis acidic boronate.

Base selection and water. Across surveys of potassium bases it is repeatedly optimal, with higher and lower pKaH bases giving less control and yielding poor coupling or oligomeric products. Three equivalents of base with five equivalents of water control both the coupling and the competing boron speciation. Water is a reagent here rather than a contaminant — in dry dioxane the same base gives no reaction, and the monohydrate has outperformed the anhydrous salt under otherwise identical conditions. Protodeboronation competes, and under classic coupling conditions most likely proceeds by specific base catalysis of a solvolytic reaction.

Copper catalysis and C–H functionalisation. In copper-catalysed couplings it has outperformed triethylamine, DBU, potassium carbonate, sodium hydroxide, caesium carbonate and potassium tert-butoxide in base screens. Copper(I) chloride with this base was selected for quinoxalinone synthesis on grounds of handling, cost and toxicity, and it serves as the elimination base in one-pot C–H vinylation of N-heteroarenes.

Metal-free catalysis. At 5 mol% it catalyses the chemoselective reduction of keto amides to hydroxy amides with polymethylhydrosiloxane, a transition-metal-free protocol tolerating ketone, nitro, halide, nitrile and amide functionality.

Where it underperforms. In a benzyl halide coupling both caesium carbonate and caesium fluoride outperformed it, with only a small amount of product after two hours.

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