Polyphosphoric acid reactant

Considerable efforts have centered on carrying out the synthesis of polybenzimidazoles at more moderate temperatures. Polymerization of the isophthalic acid or its diphenyl ester have been successfully carried out in polyphosphoric acid or methanesulfonic acid-phosphorous pentoxide at 140-180°C, but the reaction is limited by the very low solubilities (<5%) of the reactants in that solvent. The lower reaction temperature is a consequence of activation of the carboxyl reactant via phosphorylation. Lower reaction temperatures are also achieved in hot molten nonsolvents such as sulfolane and diphenyl sulfone, but the need to remove such solvents by a filtration or solvent extraction is a disadvantage. 

A variety of both protic and Lewis acids have been used to effect Fischer cyclizations. Hydrochloric acid or sulfuric acid in aqueous, alcohol or acetic acid solution are frequently used. Polyphosphoric acid and BF3 in acetic acid have also been employed[10]. Zinc chloride is the most frequently used of the common Lewis acids. This choice is supported by comparative studies with FeCI3, AICI3, CoCI2 and NiCl2, which found ZnCl2 to be the most effective catalyst[l 1]. Zinc chloride can be used either as a solid mixture with the hydrazone reactant or in ethanol or acetic acid solution[12]. 

Chloro-3-methylbenzothiophene. Heat 10 g (0.05 mol) of p-chlorophenyl-thio)acetone and 100 g of polyphosphoric acid (Section 4.2.58, p. 458) in a 250-ml flask on an oil bath maintained at 120-140 °C. When the contents of the flask initially reach the reaction temperature swirl vigorously to ensure thorough mixing of the reactants. After 2 hours allow the reaction mixture to cool and add 100 ml of water. Extract the organic product with ether, wash the combined extracts with water and dry over magnesium sulphate. Distil the residue under reduced pressure and collect 5-chloro-3-methylbenzothio-phene as a fraction of b.p. 98-100 °C/0.1 mmHg. The yield is 4.7 g (52%). 

The hybrid reagent can be added to the reactant alcohol, the alcohol can be added to the reagent, or the reagent can be selectively generated (in situ) in the reactant alcohol, provided temperatures are maintained at which polyphosphoric acid and phosphoric anhydride do not react significantly... 

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