A Facile Procedure for the Conversion of Oximes to Ketones and Aldehydes with Potassium Dichromate in Dimethylformamide under Homogeneous Conditions

Oxidative cleavage of oximes to their parent corresponding carbonyl compounds using potassium dichromate-dimethylformamide system under homogeneous conditions in good yields is described. The main advantage of the present method is no need of preparation of hexavalent chromium-based reagents due to using a polar aprotic solvent.


Experimental
Oxidative cleavage of benzophenone oxime to benzophenone; Typical Procedure: Benzophenone oxime (197 mg, 1 mmol) in DMF (10 mL) was placed in a flask, which was stirred with a magnetic stirrer.And a solution of potassium dichromate (588 mg, 2 mmol) in DMF (10 mL) was added dropwise.Care was taken to maintain the temperature of the reaction mixture at 65-70 o C. The progress of the reaction was monitored by TLC (plates: aluminum-backed silica gel Merck 60 GF 254 ) using hexane:ethyl acetate (7:3) as an eluent.After 1 h the crude product was obtained, which crude product was then purified by preparative TLC with hexane:ethyl acetate (7:3) to afford 169 mg (93%) benzophenone.

Results and Discussion
Previously, we found that potassium dichromate (K 2 Cr 2 O 7 ) is easily dissolved in a polar aprotic solvent, N,N-dimethylformamide (DMF), and it is a good system for selective oxidation of alcohols into corresponding aldehydes and ketones [24], and oxidation of benzoins into corresponding benzils [25], respectively, under homogeneous conditions.As a part of our ongoing program related to develop new oxidation methods we now report here the oxidative cleavage of oximes (1) to their corresponding carbonyl compounds (2) using our reported K 2 Cr 2 O 7 -DMF system (Scheme 1), which is an extension of application for K 2 Cr 2 O 7 -DMF oxidation.

Scheme 1
In the present procedure, a 1 to 2 molar ratio of the substrate to the oxidant is employed.After potassium dichromate is stirred with oximes in DMF at around 65-70 o C, the corresponding carbonyl compounds are obtained in good yields.Of this oxidation procedure, both potassium dichromate and N,N-dimethyl formamide are readily commercial available and inexpensive, and they can be used directly for the reaction without further purification.The progress of the oxidation is monitored with TLC, and the product is purified by preparative TLC.The oxidized products are all known compounds and identified by spectroscopic comparison with authentic samples.Our results are reported in the Table 1.
Comparing with most of the reported procedures for the oxidative cleavage of oximes to their corresponding carbonyl compounds with hexavalent chromium derivatives, the main advantages of the present method is no need of preparation of hexavalent chromium-based reagent.In addition, because the reaction is carried out in polar aprotic solvent, it can be performed under homogeneous conditions.Furthermore, the present method offers additional advantages in term of simplicity of performance, comparatively mild reaction conditions, and good yields.

Conclusion
Efficient oxidative cleavage of oximes to their parent carbonyl compounds using potassium dichromate-dimethylformamide system under homogeneous conditions in the yield between 82 and 93% is described.The main advantage of the present method is no need of preparation of hexavalent chromium-based reagent.The present procedure is an extension of our previous work as well.

a
All products were identified by comparison of their physical and spectral data with those of authentic samples.b Yield of isolated pure product.

Table 1 .
Oxidative cleavage of oximes to their corresponding carbonyl compounds with potassium dichromate-dimethyl formamide under homogeneous conditions.