Synthesis of N-Citralbenzenaminesbynabh₄/B(Oh)₃System

Introduction

Reduction of α, β-unsaturated carbonyl compounds can follow two pathways: addition to carbonyl group (1,2-reduction) to give allylicproducts or addition to the conjugated double bond (1,4-reduction) to give saturated carbonyl compounds. NaBH₄ (common reducing agent) uses for the 1,2-reduction of conjugated carbonyl compounds under different reducing system ^1-7.On the other hands, in the synthetic project we needed some N-citralbenzenamines.Also, recently we reported a convenient system for direct reductive amination of aldehydes by NaBH₄/B(OH)₃ system⁸. Therefore we decide to use of this reducing system for the synthesis of N-citralbenzenamines with this hope,the reductive amination of citralas anα, β-unsaturated carbonyl compoundfollow addition to carbonyl group (1, 2-addition) to give the corresponding α, β-unsaturatedN-benzenamines.

Results and Discussions

We have done the reductive amination reactions based on the optimized reaction that has been reported in the literature ⁷. These reactions were carried out with molar ratio of citral (1 mmol), anilines (1 mmol), B(OH)₃ (1 mmol) and NaBH₄ (1 mmol) in CH₃CN (3 mL) at room temperature. The reactionswere completed within 5 min with 75-85% yields of product as shown in scheme 1. In this reactions N-(3,7-dimethylocta-2,6-dienyl)benzenamines(N-citralbenzenamines) as major products have been produced more than 75%;butgeraniol and unreacted anlinesas by-products less than 15%.

Scheme 1 Click here to View Scheme

 

Experimental

The products were characterized by their ¹H NMR (400 MHz Bruker) or IR (PerkinElmer FT-IR RXI) and comparison with authentic samples (melting or boiling points). TLC was applied for the purity determination of substrates, products and reaction monitoring over silica gel 60 F₂₅₄ aluminum sheet.

Reductive amination of citral and aniline with NaBH₄/Al(OH)₃, A typical procedure:

In a round-bottomed flask (10 mL) equipped with a magnetic stirrer, a solution of citral (0.152 g, 1 mmol), aniline (0.093 g, 1 mmol) and Al(OH)₃ (0.078, 1 mmol) in CH₃CN (3 mL) was prepared. The resulting mixture was stirred for 5 min at room temperature. Then the NaBH₄ (0.036 g, 1 mmol) was added to the reaction mixture and stirred at room temperature. TLC monitored the progress of the reaction (eluent; CCl₄/Ether: 5/2). The reaction was filtered after completion within 5 min. Evaporation of the solvent and short column chromatography of the resulting crude material over sil­ica gel (eluent; CCl₄/Ether: 5/2) afforded the N-(3,7-dimethylocta-2,6-dienyl)benzenamine(0.l95 g, 85%).

Conclusion

In this investigation, we have shown that the combination reducing system of NaBH₄/B(OH)₃ in CH₃CN can be used for reductive amination of citral with a variety of anilines to their corresponding N-citralbenzenaminesin good yields (75-80%) within 5 min at room temperature. Reduction reactions were carried out with 1 molar equivalents of NaBH₄ in the presence of 1molar amounts of B(OH)₃.

Acknowledgements

The authors gratefully appreciated the financial support of this work by Mohsen minaee.

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