Microwave Promoted Regeneration of Carbonyl Compounds from Oximes Using N, N -Dichloro Poly(Styrene-co-divinylbenzene)Sulphonamide Resin

: An efficient, economically viable and operationally simple method was developed for deoximation of oximes (of ketones and aldehydes) to their corresponding carbonyl compounds using polymer beads of N, N -dichloro poly(styrene-co-divinylbenzene)sulphonamide resin. Polymeric reagent offered speedy conversion and substantial yields of products under mild condition and is recyclable. Deoximation was monitored by the use of 13 C NMR.

A wide variety of deoximation reagents have been developed so far such as manganese triacetate 18 , dinitrogen tetraoxide 19 , trimethylsilyl chlorochromate 20 , titanium silicalite 21 , pyridinium chlorochromate 22 , bismuth trichloride 23 , ammonium persulfate-silica gel 24 , sodium periodate 25 . Among them some require strong acidic condition; some takes long reaction time and give low product yield while some are poisonous and oxidative reagents posses serious isolation difficulties.
Heterogenised reagents [26][27][28] has evinced the attention of researchers because of their handling ease, lower toxicity, non explosive and malodorous nature. Another important motive of supported reagents is facile isolation of the pure product than their solution phase equivalents. Few reports are available on the solid supported deoximating agents under mild condition 29,30 .
N-halo reagents are widely used in fine organic synthesis because of its highly reactive N-X bond and various mode of its splitting. This polymeric resin proved to be a very strong chlorinating agent and found to be easily regenerable and reusable. In earlier work N,N-dichloro poly(styrene-co-divinylbenzene)sulphonamide has been reported as decontaminant 31 and as chlorinating reagent for synthesis of dialkyl chlorophosphates 32 . This development encouraged us to explore the utility of polymeric beads in multi-dimensions.
Here we report an organic transformation: a conventional and microwave assisted clean, rapid and efficient method for the regeneration of carbonyl compounds with the help of beads.

Experimental
IR spectra were recorded on FT-IR spectrometer Perkin-Elmer model 1720 on KBr disk. (for solids) and neat for liquids. 13 C{ 1 H}NMR spectra were recorded in CDCl 3 solvent on Bruker DPX Advance FT-NMR instrument at 400 MHz and 100 MHz respectively using trimethylsilane as an internal standard at 0.0 ppm. A house hold LG make oven was used for studies.

General procedure for deprotection of oximes
A mixture of oxime (2 mmol), carbon tetrachloride (8 mL) and N, N-dichloro poly(styrene-codivinylbenzene)sulphonamide resin (2 mmol) was refluxed or irradiated in the microwave oven at power 600 W for specific time period (Table 1). When all the oxime was converted, the intermediate was hydrolysed by adding water and washed with CCl 4 . Before adding CCl 4 beads were separated out through filtration. The organic layer was dried with MgSO 4 after separation from aqueous layer. Carbon tetrachloride was evaporated to get desired product. The yield and 13 C NMR chemical shift values are summarized in the Table 1

Results and Discussion
The microwave energy has given a new dimension and vision to organic reactions 29 . The reaction of oximes with polymeric reagent in CCl 4 provide carbonyl compound without any side product, under the microwave radiations (Scheme 1). The completion time of reaction was monitored by TLC and 13

Scheme 1
We obtained almost all quantitative yields in short reaction times. The microwave temperature for the reaction of various oximes with N, N-dichloro poly(styrene-codivinylbenzene)sulphonamide polymeric beads afforded the corresponding carbonyl compounds in 55 to 90 sec. in excellent yields (Scheme 1, Table 1).The uniqueness of this method is its simplicity and time saving. The process of a typical reaction was monitored by appearance and disappearance of 13 C NMR signal of carbonyl and oximino carbon respectively (Figure 1).

Figure 1.
Progress of a typical reaction as monitored by 13 C{ 1 H} NMR analysis at different time intervals for entry 2.
Reason behind the use of NMR is here, that it provides direct analysis of reaction mixture without the use of any tedious sample preparation method as it is required in GC-MS. The carbon chemical shift changes with significant difference (~ ∆δ = 35 ppm) on conversion of oxime (δ range of C=N is 145-170 ppm) into carbonyl (δ range of C=O is 190-227 ppm). 13 C{ 1 H} NMR chemical shifts of oximes and carbonyl compounds are presented in the Table 2.
The results of conversion of oximes to their corresponding carbonyl compounds are presented in the Table 1. In order to evaluate the generality and applicability of this method a variety of oximes were deprotected to their parent carbonyl compounds. The product of reaction with polymeric reagent was isolated simply by filtering off the solid polymer and subsequently evaporation of the solvent from filtrate. The advantages of this method are good yield, mild condition, short reaction time and no side product. The recovered polymer was again chlorinated and reused up to several times without any loss in activity.

Conclusion
In conclusion, N, N-dichloro poly(styrene-co-divinylbenzene)sulphonamide resin is a efficient reagent to fulfil different required objectives of organic synthesis. Here in this work it is proved to be an important and valuable reagent for the conversion of oximes to their corresponding carbonyl compounds. The conversion was very efficient, with excellent yield, and polymeric reagent can be recovered, activated and reused. The main advantage of the method is that reactions were very clean and operationally simple. Therefore method is attractive for organic chemist.