【English name】Polyphosphoric Acid
[Molecular formula] (PO3H)n
【Molecular weight】79.98n
[C color paste manufacturer A registration number] 8017-16-1
[Abbreviation and alias]PPA
[Physical properties] High viscosity light yellow or colorless liquid. Solvent decomposition occurs in protic solvents and is insoluble in any non-polar solvents.
【Preparation and Products】Sold by domestic and foreign reagent companies. Polyphosphoric acid can also be obtained by mixing xmL phosphoric acid (85%, d 1.7g/cm3) and 2.2xg phosphorus pentoxide, then heating to 200°C and reacting for 30 minutes.
【Notes】It has high viscosity and is difficult to stir and pour at room temperature, but it is easy to operate above 60°C. Reaction with water is exothermic, dilution should be done under ice bath. It is less toxic but can burn cell membranes and damage the skin. Care should be taken when handling and should be done in a fume hood.
Polyphosphoric acid (PPA) has strong dehydration, low nucleophilicity and moderate acidity. Unlike sulfonic acid, polyphosphoric acid generally does not oxidize reactants and is soluble in organic solvents. Although its acidity is weaker than sulfonic acid, its dehydration properties are equivalent to 100% sulfonic acid. Therefore, it is widely used in laboratories.
Catalytic intramolecular cyclization reaction Polyphosphoric acid can catalyze the intramolecular cyclization reaction of carboxylic acids, esters, ketones, aldehydes, alcohols, alkenes and hydrazones [1], for example: the synthesis of quinones ( Formula 1)[2]. Many quinolines and quinoline derivatives are synthesized under the catalysis of PPA [3]. For example, under the catalysis of excess PPA, the yield of N-ethoxyformylethyl spirotetrahydroquinoline cyclization is about 60 %(Formula 2)[4].
Catalyze intermolecular cyclization reactions Polyphosphoric acid is generally used for intramolecular reactions, but it can also catalyze intermolecular cyclization reactions. For example: catalytic synthesis of substances such as thiophene [5] and catalytic acylation reaction. The earliest acylation reaction catalyzed by PPA was the reaction of cyclohexene and acetic acid. PPA can catalyze the reaction of carbonyl and thiol to form sulfur heterocycle (Formula 3) [6], catalyze the reaction of phenylenediamine to form benzimidazole [7], etc.
Catalyze molecular rearrangement and isomerization Because polyphosphoric acid has the characteristics of low nucleophilicity, good solubility, moderate acidity and weak oxidation, it is also an efficient catalyst for molecular rearrangement and isomerization. For example: PPA is the catalyst for the rearrangement of some amino acids (Formula 4) [8] and the rearrangement of oxime’s Be original German imported curing agent N75ckmann (Formula 5) [9].
