Formamidines are derivatives of the unstable imidic acid having a unique and fascinating spectrum of biological and pharmacological activity. The aim of the present study id to evaluate the phenylphosphonic acid (PPA) as a catalyst for synthesis of symmetry and asymmetry formamidines. Method: A solution of the aniline (0.094 g, 1.0 mmol) and trimethyl orthoformate (0.318 g, 3.0 mmol) in 2.5 mL water as solvent and phenylphosphonic acid (0.015 g, 10 mol%) was heat to 55 oC with natural condition

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... for 15 min. After an appropriate time (TLC) the reaction mixture was washed with cold water, and the product was isolated by filtering. A solution of the aniline (0.094 g, 1.0 mmol, 1 equiv) and DMF-DMA (0.360 g, 3 mmol) in solvent-free condition and phenylphosphonic acid (0.015 g, 10 mol%) was heat to 55 oC with natural condition until completion (TLC). The reaction was cooled to room temperature, and after addition water to the reaction, the product as colorless oil was extracted by CH2Cl2. Result: We had found that both Amberlist-15 and phenylphosphonic acid (PPA) are effective in reaction progress in 45 and 10 min, respectively. But, based on quick reaction time and also easy water separable of the formamidine from water soluble phenylphosphonic acid (PPA), we select PPA as ideal catalyst in synthesis of N, N-diarylformamidine. Conclusion: The reaction of alkyl orthoformates with aromatic amines in the presence of phenylphosphonic Acid (PPA) as a catalyst to produce alkyl N, N-disubstituted Formamidines has been shown to be general. Reaction of orthoformates with an aromatic primary-amine, preferably in the presence of PPA in solvent-free or water-medium condition, gives formamidine in excellent yields.