Facile and Mild Deprotection of Semicarbazones Under Solvent-Free Conditions with N , N , N ’ , N ’-Tetrabromo-benzene-1 , 3-disulfonylamide

Regeneration of carbonyl compounds from semicarbazones is an important reaction and has assumed added importance not least because such derivatives of carbonyl compounds can serve as protecting groups for aldehydes and ketones in organic synthesis. Extensive studies on the deprotection of these derivatives have been carried out using various catalysts. Most of the reported procedures used for this purpose involve hydrolysis under acidic and frequently stringent conditions or oxidation. These methods invariably require reflux conditions, longer reaction times, toxic metal ions as catalysts which are detrimental to the environment. There has therefore been considerable interest in the development of novel techniques for the conversion of semicarbazones back into carbonyl compounds, which work under mild conditions. In recent years, the organic reactions on solid supports and under solventless conditions, have attracted attention because of their enhanced selectivity, milder reaction conditions and associated ease of manipulation.


Introduction
5][16][17] Most of the reported procedures used for this purpose involve hydrolysis under acidic and frequently stringent conditions 5,18 or oxidation. 3,6These methods invariably require reflux conditions, 1,5,6,8 longer reaction times, 7,8 toxic metal ions 17 as catalysts which are detrimental to the environment.There has therefore been considerable interest in the development of novel techniques for the conversion of semicarbazones back into carbonyl compounds, which work under mild conditions.

Results and Discussion
Herein, we now report a convenient method for rapid regeneration of carbonyl compounds from their semicarbazones using a new reagent [TBBDA] (2) 34 in high yields under mild and solvent-free conditions (Scheme 1).
The reaction of semicarbazones 3 with TBBDA under solvent-free conditions afforded carbonyl compounds 4 without the formation of side products (Scheme 2).
The results of the conversion of various semicarbazones are presented in Table 1.
The products of the reaction with TBBDA were isolated simply by filtering off and evaporating the solvent from the filtrate.This solvent-free deprotection methodology of semicarbazones is superior to conventional deprotection methodology using a solvent.The most remarkable advantage to this methodology is that no trace of acid was formed awing to over oxidation of regenerated aldehyde.The recovered starting material 1 was brominated and used many times without any reduction in this efficiency.Ghorbani-Vaghei and Shahbazee J. Braz.Chem.Soc.
Since TBBDA contain four bromine atoms which are attached to nitrogen atoms it is very possible that this reagent releases Br + in situ which can act as an electrophilic species.An evidence for this claim has been reported in our previous work 35 in which Br + was generated in situ from N , N ' -d i b r o m o -N , N ' -1 , 2 -e t h a n e d i y l b i s ( ptoluenesulphonamide).Therefore, the following mechanism can be suggested for the convertion semicarbazones to their corresponding carbonyl compounds (Scheme 3).
In conclusion, the method has advantage in terms of yield, simplicity of reaction conditions, short reaction times, solvent-free conditions and no side products.
IR and NMR spectra were recorded using a Shimadzu 435-U-04 spectrophotometer (KBr pellets) and a 90 MHz Jeol FT-NMR spectrometer, respectively.

Table 1 .
Regeneration of carbonyl compounds from semicarbazones with TBBDAProducts were characterized by their physical properties, comparision with authentic samples and by spectroscopic methods. a