Ultrasound promoted deoximation by FeCl 3 : A highly efficient, mild and expeditious approach

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1 Indian Journal of Chemistry Vol. 44B, April 2005, pp Ultrasound promoted deoximation by FeCl 3 : A highly efficient, mild and expeditious approach Ramesh aik & M A Pasha* Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore , India. m_af_pasha@yahoo.co.in Received 10 February 2004; accepted (revised) 11 ctober 2004 The deoximation of aldoximes and ketoximes to afford the corresponding aldehydes and ketones by anhyd. ferric (ΙΙΙ) chloride in dry CH 3 C under ultrasound irradiation at 40 KHz has been reported. The method is highly efficient and applicable for both aromatic and aliphatic substrates. The conditions avoid the Beckmann rearrangement and the yields are excellent. IPC: Int.Cl. 7 C 07 C 251/32 Since carbonyl compounds are very useful starting materials for various transformations in organic synthesis, it is necessary to protect them. ne of the simplest potential routes for the protection is by converting them to oximes, 1 which in turn represents a classical method of identification of carbonyl compounds. From the synthetic point of view, deoximation has received considerable attention in reconversion of oximes into carbonyl compounds without any side reactions. However, there is still need for cheaper and more efficient methods of deoximation. Although a plethora of reagents are available to achieve this goal, only three major methods are generally employed, namely hydrolytic, oxidative and reductive conditions. A number of oxidative reagents such as periodic acid, 2 t-buh, 3 PCC/H 2 4 2, Des-Martin periodinane, 5 and wet ai 4 -silica under microwave conditions 6 are available in the literature. The methods using reductive conditions are very few, which include Raney nickel, 7 (H 4 ) 2 S 2 8 /silica, 8 Zn/AcH, 9 and WCl 6 or MoCl 5 in the presence of zinc powder. 10 Most of the reported procedures for deoximation have several limitations because of over oxidized products, use of expensive and hazardous reagents or harsh reaction conditions. These drawbacks made us attempt to develop a new, efficient, simple and cost-effective procedure using readily available reagents. Results and Discussion Ultrasound has gained prominence as a tool in the recent past to accelerate chemical reactions. 11 In continuation of our work on the use of ultrasound to accelerate reactions, we have studied the deoximation reaction by anhydrous FeCl 3 in dry CH 3 C. In this paper, we wish to report results on the deoximation reactions of aromatic and aliphatic ketoximes, as well as aldoximes, by the expeditious and readily available reagent FeCl 3 under sonochemical condition at 40 KHz in an ultrasonicbath maintained at 25 C (Scheme I). n the basis of the well-known capability of FeCl 3 as Lewis acid for accelerating the reaction, and taking account of the advantage of operating in organic solvents, 15 a set of solvents like CH 2 Cl 2, CHCl 3, THF, CH 3 2, CH 3 C, toluene and MeH were dried and selected to study the deoximation reaction. We found that CH 3 C is suitable in terms of yields. Initially, we set out to establish reasonably good conditions for the deoximation of benzaldoxime 1a with anhyd. FeCl 3 at H FeCl 3, CH 3 C )))), r.t = alkyl, aryl, H = alkyl, aryl, pyridyl Scheme I

2 AIK et al.: ULTRASUD PRMTED DEXIMATI F ALDXIMES AD KETXIMES BY FeCL room temperature in dry CH 3 C and stirring for 3 days, we obtained % of benzaldehyde. In order to improve the yield and to reduce reaction time, we thought of evaluating the reaction under reflux. Accordingly, the mixture was refluxed for 42 hr and recorded % conversion with other side products, as observed on TLC, which were however not isolated. Ultimately, we decided to accelerate the reaction under sonochemical conditions at 25 C under nitrogen atmosphere. To a solution of 1a in dry CH 3 C under cold conditions we added anhyd. FeCl 3 (equiv. Table I) and sonicated the whole for 10 min in a sonic bath working at 40 KHz. The reaction was monitored by TLC which indicated the formation of benzaldehyde without any by-products. The process was extended to a variety of aldoximes and ketoximes as substrates for this transformation. The results are summarized in Tables I and II. Both ketoximes and aldoximes with different substituents (aryl, aryl-alkyl, diaryl, alkyl and dialkyl) are converted into the respective carbonyl compounds efficiently within min in good to excellent yields. An additional equivalent of FeCl 3 is required in certain cases (Tables I and II), hence, is a useful strategy in the case of sensitive substrates containing other functional groups. A noteworthy feature of the above transformation is that the presence of electron withdrawing groups, such as 2, -Cl, (entries 1b, 1e, 1j), or electrondonating groups such as -Me, -H (entries 1c, 1d, 1g) on the aromatic ring, does not effect the yield or the efficiency of the reaction. Another noteworthy feature is that the unsaturated aliphatic ketoximes (Table II) are converted to the corresponding carbonyl compound as the sole product in good to excellent yields. For instance, α,β-unsaturated cyclic ketone, entry 3d is successfully deoximated without any byproduct and affords the cyclohexenone in high yield. The deoximation of camphor oxime under reflux conditions by the earlier methods results in the loss of camphor by sublimation. However, by using our method, it can be successfully deoximated without loss of the product. Mechanistically, the Beckmann-rearranged product could be a candidate by-product with acidic catalyst in this process. evertheless, even trace amounts of amides were not observed. This fact is supported by the sonication of benzophenone oxime with anhyd. FeCl 3 for 3 hr (Scheme II), the product obtained was deoximated benzophenone b. H Scheme II In conclusion, we have developed a mild and convenient method for the deoximation of aldoximes and ketoximes in dry CH 3 C into the respective carbonyl compounds in min by use of anhyd. FeCl 3 under sonic condition at 40 KHz. We believe that this procedure is a better, cost effective and more practical alternative to the existing methods. Experimental Section General Melting points/boiling points were obtained with Buchi B-540 apparatus. 13 C and 1 H MR spectra were recorded on a 300 MHz Jeol Lambda-300 spectrometer using CDCl 3 with TMS as internal standard. IR spectra were recorded on Hewlett Packard spectrometer. Sonication was done in a Buchi sonic-bath working at 40 KHz (constant frequency) maintained at 25 C by constant circulation of water. All the solvents were dried by standard methods where necessary. All reactions were monitored by thin layer chromatography with precoated (silica gel 60F-254) plates (Sigma Aldrich). Commercially available reagents were used without further purification, and the products were purified by flash chromatography on Merck silica gel ( mesh). General procedure for the deoximation of oximes. To a well-dissolved solution of oxime 1a (500 mg, 3.8 mmole) in dry CH 3 C (10 ml) under cold condition was added anhyd. FeCl 3 (1.23 g, 7.6 mmole) and the solution was sonicated at 40 KHz under argon atmosphere. The reaction was monitored by TLC (ethyl acetate-pet. ether 10:90). When all the oxime disappeared completely (Tables I and II), the solution was treated with potassium sodium tartrate (200 mg, to prevent jelly formation with FeCl 3 in water). It was then diluted with water, neutralized with saturated ahc 3 solution (10 ml) and extracted with CH 2 Cl 2 (3 10 ml). The organic layer was washed with saturated acl solution (3 5 ml) and dried over anhyd. a 2 S 4. The solvent was removed under reduced pressure and the residue H a b

3 780 IDIA J. CHEM., SEC B, APRIL 2005 Table I Deoximation of aromatic oximes by FeCl 3 under ultrasound irradiation FeCl 3, CH 3 C ))) r.t. 1 2 (1) (equiv) (min) (2) Literature Found (cm 1 ) 1a m 1695 (C=) 1b (C=),1537 ( 2 ) n 1c Me H (H), 1641(C=) 1284 (Me) n 1d m 1698 (C=),1267 (Me) Me 1e (C=), 1542 ( 2 ) n 1f H (H), 1685 (C=) 1g H m 3450 (H), 1698 (C=) 1h m 1678 (C=), 1284 (--) 1i m 1688 (C=) 1j Cl (C=) n Contd

4 AIK et al.: ULTRASUD PRMTED DEXIMATI F ALDXIMES AD KETXIMES BY FeCL Table I Deoximation of aromatic oximes by FeCl 3 under ultrasound irradiation Contd FeCl 3, CH 3 C ))) r.t. 1 2 (1) (equiv) (min) (2) Literature Found (cm 1 ) 1k (C=) n 1l (C=) n z- refers to isolated yields and all the products exhibited physical (m.p & b.p) and spectral (MR and IR) properties in accord with the assigned structures. m-refers to b.p., n- refers to IR recorded as KBr pellet. Table II Deoximation of aliphatic oximes by FeCl 3 under ultrasound irradiation FeCl 3, CH 3 C R ))) r.t (3) (equiv) (min) (4) Literature Found (cm 1 ) (C=) 3a b c d e (C=) 1471, f x 1670 (C=) z- refers to isolated yield and all products exhibited physical (m.p & b.p) and spectral (MR and IR) properties in accord with the assigned structures. x- refers to m.p. & IR recorded as KBr pellet.

5 782 IDIA J. CHEM., SEC B, APRIL 2005 was purified by flash chromatography to give benzaldehyde 2a (369 mg, 90%) as a colorless oily liquid using ethyl acetate/pet. ether (10:90) as eluent. Acknowledgement Ramesh aik thanks Mr P R Sridhar of the IISc, Bangalore for useful discussions and the Bangalore University for financial assistance. References 1 Greene T G & Wuts P G M in Protective Groups in rganic Synthesis, 2 nd ed (John Wiley & Sons, ew York) Maloney J R, Lyle R E, Saavedra J E & Lyle G G, Synthesis, 1978, Barhate B, Gajare A S, Wakharkar R D & Sudalai A, Tetrahedron Lett, 38, 1997, Drabowicz J, Synthesis, 1980, Chaudhuri S S & Akamanchi K G, Tetrahedron Lett, 39, 1998, Varma R S, Dahiya R & Saini R K, Tetrahedron Lett, 38, 1997, Curran D P, Brill J F & Rakiewicz D M, J rg Chem, 49, 1984, Varma R S & Mesharam H M, Tetrahedron Lett, 38, 1997, th Int Elect Conf on Synth rg Chem (ECSC-4), September 2000, Firouzabadi H, Jamalian A & Karimi B, Bull Chem Soc Jpn, 75, 2002, Mason T J & Lorime J P, In Applied Sonochemistry: Uses of Power Ultrasound in Chemistry and Processing (Wiley, ew York), 2002, Review, Synthesis, 1989, Rama K & Pasha M A, Indian J Chem, 41B, 2002, agaraja D & Pasha M A, Tetrahedron Lett, 40, 1999, Rama K, agendra E & Pasha M A, Indian J Chem, 39B, 2000, Rajendra P T, Mahalingam A K & Sharma G V M, Tetrahedron Lett, 42, 2001,759.