Highly efficient method for oximation of aldehydes in the presence of bis-thiourea complexes of cobalt, nickel, copper and zinc chlorides

Article history: Received June 21, 2019 Received in revised form December 8, 2019 Accepted December 8, 2019 Available online December 8, 2019 In this study, the selective oximation of structurally diverse aromatic aldehydes (versus ketones) to the corresponding aldoxime derivatives was investigated using the combination system of NH2OH·HCl and bis-thiourea complexes of cobalt, nickel, copper and zinc chlorides, MII(tu)2Cl2, in a mixture of CH3CN-H2O (1:1). All reactions were carried out successfully at room temperature within the immediate time up to 130 min giving the products in high yields. Investigation of the results exhibited that the applied bis-thiourea metal complexes represented the catalytic activity in order of Co(tu)2Cl2> Ni(tu)2Cl2> Cu(tu)2Cl2> Zn(tu)2Cl2 in their oximation reactions. © 2020 Growing Science Ltd. All rights reserved.

Among the documented catalyst systems for the formation of oximes, most studies are focused on the ammoximation of cyclohexanone and therefore a very limited range of substrates have been investigated. In this context, Sloboda-Rozner reported a sandwich-type polyoxometalate (POM) cluster, Na12[WZn3(H2O)2(ZnW9O34)2], which catalyzes the reaction of NH3 and H2O2 to afford the in situ preparation of hydroxyl amine. 34 As well, the titled POM catalyst activates the nucleophilic surfaces of the resulting hydroxylamine to promote the oximation reaction. The bare Lewis base nucleophilic surfaces are resulted from the external oxygen atoms of W-O-W and W=O species. They act as nucleophilic sites as well as stabilizers of cationic intermediates. [35][36][37][38] In a case for using NaZn5W19, however, the oximation reaction was led to low yields of the corresponding aromatic aldoximes due formation of byproducts (amides and nitriles) and carboxylic acids while aliphatic aldehydes were used as substrates. In addition, the inherent acidity of the catalyst can causes the further transformation of the oximation products. [39][40] Therefore, improving of the selectivity in the oximation of aromatic aldehydes is a subject of more interests. From the industrial aspects, this method suffers from two major drawbacks: relatively high cost of hydroxylamine and the derived serious problems via disposing large amounts of inorganic salts which are co-produced in oximation reactions. Therefore, the requirement for decreasing the use of hydroxylamine in more than stoichio-metric amounts demands the environmental friendly and waste-free procedures as well as the in situ preparation of hydroxylamine for the oximation of aldehydes and ketones. Moreover, how to suppress the formation of by-products and increase the selectivity of oximation protocols are of the great significances. Consequently, the short lifetime, insufficient thermal stability and difficulty in recovery of the applied catalyst systems (because of their high solubility in water and polar organic solvents) are the issues which should be taken into account in the development and introduction of new oximation procedures.
In line with the outlined strategies and continuation of our research program directed to the application of bis-thiourea metal complexes of cobalt, nickel, copper and zinc chlorides, M II (tu)2Cl2, as catalysts for reduction of nitro compounds 41 and silylation of alcohols 42 , herein, we wish to introduce a new and highly efficient method for the selective oximation of structurally diverse aromatic and aliphatic aldehydes versus ketones using the combination system of M II (tu)2Cl2/ NH2OH·HCl in a mixture of CH3CN-H2O (1:1) at room temperature (Scheme 1). Scheme 1. Oximation of aldehydes with M II (tu)2Cl2/NH2OH·HCl system

Results and Discussion
The study was started by the preliminary preparation of bis-thiourea metal complexes of CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O and ZnCl2 as bivalent transition metal leaders of groups 9, 10, 11 and 12 (or VIII, IB and IIB) from Periodic Table (Scheme 2). The complexes were characterized by their physical data and then authorized with the reported data in the literature. 43

Scheme 2. Reaction of bivalent metal chlorides with thiourea
The promoter activity of the prepared complexes on the oximation of aldehyde was then investigated by the reaction of 4-chlorobenzaldehyde as a model compound with hydroxylamine hydrochloride in the absence and presence of M II (tu)2Cl2 complexes at different conditions ( Table 1). Observation of the results shows that in the absence of metal complexes, the oximation reactions did not has a reasonable efficiency. Whereas by using any of bis-thiourea metal complexes, the model reaction was carried out perfectly to afford 4-chlorobenzaldoxime as a sole product. Entries 6, 13, 20 and 27 (Table 1) exhibited that using a molar equivalent of M II (tu)2Cl2/NH2OH·HCl (0.2:1.2) per 1 mmol of 4-chlorobenzaldehyde was sufficient to complete the reaction in a perfect efficiency within the immediate time up to 15 sec. In addition, a mixture of CH3CN-H2O (1:1) was the best solvent of choice to progress of the reaction at room temperature. The results also represented that although all of the complexes influenced the oximation of 4-chlorobenz-aldehyde with hydroxylamine hydro-chloride, however, the rate enhancement and promoter activity of Co(tu)2Cl2 was greater than the other metal complexes. It is also notable that the oximation of 4-chlorobenzaldehyde with NH2OH ·HCl, in the presence of CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O and ZnCl2 did not has any impressive results. The capability of M II (tu)2Cl2/NH2OH·HCl system for oximation of structurally diverse aromatic aldehydes was studied at the optimized reaction conditions. The results of this investigation are illustrated in Table 2. As seen, all reactions were carried out successfully at room temperature within the immediate time up to 65 min to afford aromatic aldoximes in high to excellent yields. The result shows that benzaldehyde can be converted to benzaldoxime in 96% yield (Table 2, entry 1). In the case of electron-releasing substitutions on aromatic rings such as methoxy, methyl and hydroxyl groups, the corresponding aldoximes can be also obtained in high yields. As well, aromatic aldehydes with electron-withdrawing functionalities including 2-Cl, 4-Cl, 4-F, 3-NO2 and 4-NO2 were also successfully converted to the corresponding aldoximes in 82-98% yields using M II (tu)2Cl2/NH2OH ·HCl system. Entry 17 represents that this synthetic method is also efficient for the oximation of aliphatic aldehydes via the transformation of citral to citral oxime. It is noteworthy that under the examined reaction conditions, all attempts for the oximation of acetophenone and 4-methoxy acetophenone as ketonic materials with M II (tu)2Cl2/NH2OH·HCl system were unsuccessful.
Investigation of the results (Table 2) exhibited that among the examined bis-thiourea metal complexes, cobalt chloride showed a higher catalytic activity than the other metal chlorides as Co(tu)2Cl2> Ni(tu)2Cl2> Cu(tu)2Cl2> Zn(tu)2Cl2. It was proposed that Lewis acid susceptibility of bivalent transition metal cations of first row of Periodic Table and relative stability of the prepared bisthiourea complexes according to Irving-Williams series 44,45 maybe play a role in their catalytic activities. Co 2+ with less stable bis-thiourea complex and more Lewis acidity can release thiourea and thus accept NH2OH as a new ligand for participation in the formation of oximes. In this promotion, however, Zn 2+ with more d-electrons behaves as less reactive bis-thiourea metal complex for thiourea/NH2OH ligand displacement.
In order to highlight the promoter activity of M II (tu)2Cl2/NH2OH·HCl system, we therefore compared the oximation of 4-methoxybenzaldehyed with the current protocol and other reported methods. Investigation of the results (Table 3) shows that in view points of the short reaction times, mild reaction conditions, high yields, low loading amounts of NH2OH·HCl and catalysts, cheapness and easy availability of the catalysts, the present method shows more or comparable efficiency than the other documented protocols.

Conclusions
In this study, bis-thiourea metal complexes of cobalt, nickel, copper and zinc chlorides were prepared and then utilized for the oximation of structurally diverse aromatic and aliphatic aldehydes with hydroxylamine hydrochloride successfully. All reactions were carried out in a mixture of CH3CN-H2O (1:1) at room temperature within the immediate time up to 65 min to afford aldoximes in high to excellent yields. The metal complexes showed a prominent catalytic activity as Co(tu)2Cl2> Ni(tu)2Cl2> Cu(tu)2Cl2> Zn(tu)2Cl2 in their oximation reactions. Short reaction times, high to excellent yield of the products, easy workup procedure as well as using the commercially available materials are the advantages which make this protocol a synthetically useful addition to the present methodologies.

General
All reagents and substrates were purchased from commercial sources with high quality and they were used without further purification. FT-IR and 1 H NMR spectra were recorded on Thermo Nicolet Nexus 670 and 300 MHz Bruker spectrometers, respectively. The products were characterized by their 1 H NMR and FT-IR spectra followed by comparison with the authentic ones. All yields refer to isolated pure products. TLC was applied for the purity determination of substrates, products and reaction monitoring over silica gel 60 F254 aluminum sheet.

Preparation of bis-thiourea metal chloride complexes
To a round-bottom flask (100 mL) containing a magnetic stirrer and the solution of metal chloride (CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O, or ZnCl2) (0.01 mol, in 20 mL EtOH), an ethanolic solution of thiourea (0.02 mol, 1.52 g in 20 mL) was added. The mixture was stirred under reflux conditions for 4 h. During the progress of the reaction, bis-thiourea metal complex was precipitated. The content of flask was transferred to a Petri-dish for evaporation of the solvent. The residue was washed with absolute ethanol to remove any contaminant. Drying the residue under air atmosphere affords M II (tu)2Cl2 complex. It is notable that for dissolving thiourea in ethanol, slightly warming was required.

Typical procedure for oximation of 4-chlorobenzaldehyde with Co(tu)2Cl2/NH2OH·HCl system
In a round-bottom flask (10 mL) equipped with a magnetic stirrer, a solution of 4-chlorobenzaldehyde (1 mmol, 0.141 g) in a mixture of CH3CN-H2O (1:1) (1.5 mL) was prepared. After one min, hydroxylamine hydrochloride (1.2 mmol, 0.083 g) was added and the resulting solution was stirred at room temperature for 30 sec. To the prepared solution, Co(tu)2Cl2 (0.2 mmol, 0.0563 g) was added and stirring of the reaction mixture was continued for 5 sec at room temperature. Progress of the reaction was monitored by TLC (n-hexane/EtOAc: 5/2). After completion of the reaction, H2O (3 mL) was added and the mixture was stirred for 5 min. The aldoxim product was extracted with EtOAc (2 × 4 mL) and the organic layer was then dried over anhydrous Na2SO4. Evaporation of the solvent afforded the pure 4-chlorobenzaldoxime in 95% yield ( Table 2, entry 2).