Reduction of Aldehydes and Ketones with LiBH4 Under Microwave Irradiation

Introduction

Alcohols are important in organic synthesis.We have reported some reducing systems for the preparation of alcohols from the corresponding carbonyl compounds 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(NH₄)₂/ZrCl₄¹², and reagents such as [Zn(BH₄)₂.py]¹³, [Zn(BH₄)(nmi)]¹⁴, [Zn(BH₄)₂.nic)]¹⁵. In this context, we now wish to report an efficient, facile preparation of alcohols using aldehydes and ketones by LiBH₄/Microwave system in H₂Oas green solvent.

Results and Discussions

Microwave irradiation as an unconventional energy source has been used to carry out many kinds of chemical reactions. The microwave irradiation drives chemical reactions effectively and quickly^16-17. 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 LiBH₄ and different power amplitude of microwave ovenfor the selection of appropriate conditions. The optimization reaction conditions showed that using 1 molar equivalents of LiBH₄and 30% power amplitude of microwave oven (300 W) was the best for reduction reaction.The reaction was completed in 30 sec and benzyl alcohol was obtained in 95% yield as shown in scheme 1.

 

Scheme 1 Click here to View table

 

The efficiency of this protocol was further examined by using various structurally different aldehydes. In this approach, the correspondingalcohols were obtained in excellent yields (92-97%) within 30 sec.as shown in Table 1(entries 1-8).In the next attempt, the reduction of ketones has been investigated. The reductionof ketones, because of their less reactivity needs the use of 2 molar equivalents of LiBH₄. A variety of ketoneswere subjected to LiBH₄ inwater(5 mL) under microwave irradiation (300 W).The results showedthat the corresponding secondary alcohols were obtained inexcellent yields (94-97%) within 80-240 sec as shown in Table 1 (entries 9-13).Addition of distilledwater to the reaction mixture and then extracting with CH₂Cl₂afforded the crude corresponding alcohol.

 

TABLE 1:Reduction of Aldehydes (1 mmol) and Ketones (1 mmol) by LiBH4(1-2 mmol) in H2O (5 mL) Under Microwave Irradiation (300 W). Click here to View table

 

Experimental

All microwave assisted reactions were carried out in aYusch household microwave oven (1000W). 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 benzaldehydewith LiBH₄/Microwave Irradiation, A typical procedure:

In a round-bottomed flask (10 mL) charged with distilled water (5 mL), LiBH₄ (0.022 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 irradiatedwith a microwave oven (30% power amplitude, 300 W) for30 sec. The progress of the reaction was monitored by TLC(eluent; CCl₄/Et₂O: 5/2). At the end of the reduction, distilledwater (5 mL) was added to the reaction mixture and it wasthen extracted with CH₂Cl₂(2×10 mL). The combined extractswere dried over anhydrous sodium sulfate. Evaporation ofthe solvent afforded the pure liquid benzyl alcohol (0.102 g, 95%).

Conclusion

In this context, we have shown that a variety of aldehydes and ketoneshave been reduced to their corresponding alcohols with lithiumborohydride under microwave irradiation. Thereductions were completed within 30-240 sec with excellent yields of the corresponding alcohols. 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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