Microwave Assisted Facile Cleavage of 2,4-Dinitrophenyl-hydrazones to The Corresponding Carbonyl Compounds with N,N^′-Dibromo-N,N^′-1,2-ethanediylbis(p-toluenesulphonamide)

Abstract

Deprotections of 2,4-dinitrophenylhydrazones to their corresponding carbonyl compounds have been carried out in good yields by using N,N^′-dibromo-N,N^′-1,2-ethanediylbis(p-toluene-sulphonamide (BNBTS, 2) under microwave irradiation.

Introduction

Carbonyl compound derivatives such as 2,4-dinitrophenylhydrazones and oximes are important intermediates in organic synthesis because of their use in the characterization and purification [1,2,3] of such compounds and the important role they play as protective groups for this functionality [4,5,6,7]. Consequently, the regeneration of carbonyl compounds from the corresponding 2,4-dinitrophenylhydrazones under mild conditions is an important process in synthetic organic chemistry. Several such procedures for regeneration of carbonyl compounds from 2,4-dinitrophenylhydrazones have been reported, for example: with Dowex-50 cation exchange resin [5], clayfen [6], potassium bromate [7], etc.

Although some are carried out under mild conditions most of these regenerations are often hazardous or use very toxic, expensive, or not readily available reagents, or reagents which need to be freshly prepared [8,9,10]. Thus, there is still a need to develop new and facile procedures for the regeneration of carbonyl compounds from 2,4-dinitrophenylhydrazones.

Result and Discussion

We have previously reported a convenient method for the deoximation of ketone and aldehyde oximes to their corresponding carbonyl compounds [11] using the new reagent BNBTS (2), prepared from N,N^′-1,2-ethanediylbis(p-toluenesulphonamide) (1) (Figure 1) [12]. Herein, we wish to report a mild and convenient method for the clean, fast and economical oxidative deprotection of 2,4-dinitrophenylhydrazones with this reagent under microwave irradiation.

Figure 1.

2,4-Dinitrophenylhydrazones 3 reacted with BNBTS (2) in CH₂Cl₂ under microwave irradiation to afford the corresponding carbonyl compounds 4 without any detectable byproducts (Scheme 1). The products of the reaction with BNBTS were isolated simply by filtering off compound 1 and evaporating the solvent from the filtrate. The results of the conversion of various 2,4-dinitrophenylhydrazones to their corresponding ketones and aldehydes are presented in Table 1.

Scheme 1.

Table 1.

Cleavage of 2,4-dinitrophenylhydrazones with BNBTS under microwave irradiation

Entry	R1	R2	Producta	Reaction Times (min)	Yield(%)
1	CH3	C6H5	Acetophenone	50	91
2	CH3	p-ClC6H5	p-Chloroacetophenone	60	92
3	CH3	p-MeOC6H5	p-Methoxyacetophenone	50	94
4	CH3	p-BrC6H5	p-Bromoacetophenone	50	93
5	C6H5	C6H5	Benzophenone	60	90
6	C6H5	p-ClC6H5	p-Chlorobenzophenone	30	92
7	H	C6H5	Benzaldehyde	50	89
8	H	p-ClC6H5	p-Chlorobenzaldehyde	40	95
9	H	p-MeC6H5	p-Tolualdehyde	50	92
10	H	o-MeOC6H5	o-Methoxybenzaldehyde	40	92
11	C6H5	C6H5CH(OH)	Benzoin	50	91

^a Products were characterized by their physical constants, IR spectra and comparison with authentic samples.

Conclusions

From the results obtained, we find that the described procedure and the reaction conditions are simple. The reagent (BNBTS) is stable and the recovered reagent can be reused.

Experimental

General

IR spectra was recorded using a Shimadzu 435-U-04 spectrophotometer (KBr pellets). The 2,4-dinitrophenylhydrazones were prepared by a standard procedure [13]. The purity of compounds was checked by TLC (silica gel 60 F₂₅₄/CHCl₃ and CCl₄/UV). A Shivaki Co. microwave (220 V, 700 W, RF output 2450 MHz) was used for the microwave irradiations.

General procedure for facile cleavage of 2,4-dinitrophenylhydrazones with BNBTS under microwave irradiation

A mixture of 2,4-dinitrophenylhydrazone (5 mmol), dichloromethane (15 mL) and BNBTS (5 mmol) was introduced in a flask and was refluxed under irradiated in a microwave oven at a power output of 700 W for the appropriate time as indicated in Table 1. After irradiation, cold water was added to hydrolyze the intermediate, and the insoluble sulfonamide 1 was removed by filtration and washed with cold dichloromethane (10 mL). The organic layer was separated from the aqueous layer, combined with the washes and dried with MgSO₄. Dichloromethane was removed under reduced pressure gave the crude product. Solid products were recrystallized from diethyl ether, oily products were dissolved in ether and the ether solution washed, dried and concentrated.