Reduction of Carbonyl Compounds with Zn(BH4)2 Under Microwave Irradiation

Introduction

Alcohols are important substrates in organic synthesis.So much synthetic methods have been reported by researchers. But, using of borohydride such as LiBH₄, NaBH₄ and Zn(BH₄)₄are more common. These borohydrides have been used for the reduction purpose in reducing system such as NaBH₄/C¹, NaBH₄/M.W², NaBH₄/Al₂O₃³, NaBH₄/TiO₂⁴, NaBH₄/(NH₄)₂C₂O₄⁵, NaBH₄/Ba(OAc)₂⁶,NaBH₄/DOWEX(R)50WX4⁷, Zn(BH₄)₂/H₂O⁸, Zn(BH₄)₂/Al₂O₃⁹, Zn(BH₄)₂/C¹⁰, Zn(BH₄)₂/2NaCl¹¹, Zn(BH₄)₂/ZrCl₄¹², Zn(BH₄)₂/U.S¹³and so on. In this context, we now wish to report an efficient and facile preparation of alcohols using aldehydes and ketones by Zn(BH₄)₂/Microwave system in H₂O as green solvent.

Results and Discussions

We have chosen the microwave irradiation because it drives chemical reactions effectively and quickly¹⁴. The model reaction has been selected by reduction of benzaldehyde. This reaction was carried out in H₂O (5 mL) as green solvent, different amounts of Zn(BH₄)₂ and different power amplitude of microwave oven for the selection of appropriate conditions. The optimization reaction conditions showed that using 1 molar equivalents of Zn(BH₄)₂ and 30% power amplitude of microwave oven (300 W) was the best for reduction reaction.The reaction was completed in 60 sec and benzyl alcohol was obtained in 95% yield as shown in scheme 1.

Scheme 1  Click here to View scheme

 

The efficiency of this protocol was further examined by using various structurally different aldehydes. In this approach, the corresponding alcohols were obtained in excellent yields (91-96%) within 60 sec.as shown in Table 1(entries 1-7).In the next attempt, the reduction of ketones has been investigated. The reduction of ketones, because of their less reactivity needs the use of 2 molar equivalents of Zn(BH₄)₂as shown in scheme 2.

Scheme 2  Click here to View scheme

 

A variety of ketones were subjected toZn(BH₄)₂ in water(5 mL) under microwave irradiation (300 W).The results showed that the corresponding secondary alcohols were obtained in excellent yields (80-97%) within 120 sec as shown in Table 1 (entries 8-14). Addition of distilled water to the reaction mixture and then extracting with CH₂Cl₂ afforded the crude corresponding alcohol.

Experimental

All microwave assisted reactions were carried out in aYusch household microwave oven (1000 W). The instrumentwas modified for laboratory applications with an external reflux condenser.IR and ¹H NMR spectra were recorded on PerkinElmer FT-IR RXI and 400 MHz Bruker spectrometers, respectively. The products were characterized by their ¹H NMR or IR spectra 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.

Reduction of benzaldehyde with Zn(BH₄)₂/Microwave Irradiation, A typical procedure

Zn(BH₄)₂ was prepared from ZnCl₄ (5.452 g, 0.04 mol) and NaBH₄ (3.177 g, 0.084 mol) according to an available procedure in the literature ¹¹.In a round-bottomed flask (10 mL) charged with distilled water (5 mL), Zn(BH₄)₂ (0.095 g, 1mmol) and benzaldehyde (0.106 g, 1 mmol) was added. After fitting the flask to the external condenser at the inside of the oven, the mixture was irradiated with a microwave oven (30% power amplitude, 300 W) for60 sec. The progress of the reaction was monitored by TLC(eluent; CH₂Cl₂). At the end of the reduction, distilledwater (5 mL) was added to the reaction mixture and it was then extracted with CH₂Cl₂(2×10 mL). The combined extracts were dried over anhydrous sodium sulfate. Evaporation ofthe solvent afforded the pure liquid benzyl alcohol (0.102 g, 95%).

Conclusion

In this research, we have shown that a variety of carbonyl compounds such as aldehydes and ketones have been reduced to their corresponding alcohols with zincborohydride under microwave irradiation. The reductions were completed within 60-120 seconds with excellent yields of the corresponding alcohols (80-97). Therefore, this protocol with the easy work-up procedure could be a usefuladdition to the present methodologies.

Acknowledgements

The authors gratefully appreciated the financial support of this, work by the research council of Islamic Azad University branch of Mahabad.

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