An Efficient Oxidation of Benzoins to Benzils by Manganese(II) Schiff Base Complexes Using Green Oxidant

A simple, highly efficient andmild catalytic oxidation of benzoins to the corresponding benzils was developed usingmanganese(II) Schiff base complexes as novel and reusable catalyst in the presence of acetonitrile as solvent andH2O2 as green oxidant.is simple method affords benzil derivatives at room temperature in short reaction timeswith high yield and purity.is convenient procedure will allow a further increase of the diversity within the benzil family.


Introduction
Benzil, an alpha diketone, is one of the important organic intermediates and has received an enormous attention because of its practical applications in organic and pharmaceutical industry such as photosensitive and synthetic reagents [1][2][3].e oxidation of benzoins is one of the most efficient and practical methods for the synthesis of benzils.In general, the oxidation of benzoins to benzils has been accomplished by classical reagents such as nitric acid [4], Fehling's solution [4], thallium nitrate [5], ammonium nitrate-copper acetate [6], bismuth nitrate-copper acetate [7], and ferric nitrate [8].In addition, the oxidation of benzoins to benzils was studied using p-benzoquinone or air in the presence of (Fe II (SPh) 4 ) 2 and (Fe II (SePh) 4 ) 2 [9], Fe(II)-cysteine peptide complexes [7] and ter.butyl peroxide [10].e electrochemical oxidation of benzoins to benzils in the presence of a catalytic amount of KI in basic media [11] was also studied.Also, Paris et al. used (Mn III (pydx-en)Cl(H 2 O))-Y as catalyst and H 2 O 2 as an oxidant for oxidation of benzoin to benzil in methanol at re�ux temperature [12].
However, in spite of their potential utility, some of the reported methods suffer from drawbacks such as longer reaction time, lower yields, expensive catalysts, harsh conditions, or complexity of workup.ere still appears a need either to improve the existing oxidation methods or to introduce novel reagents to permit better selectivity under milder conditions and with easy work-up procedures [12].
Transition metal catalysts supported by Schiff base ligands have assumed a prominent role in modern synthesis.Schiff base complexes of transition metals having O and N donor atoms have shown an exponential increase as inorganic catalysts for various organic transformations.Distinct advantages of such ligands include their low cost, facile syntheses, and convenient incorporation of inexpensive, chiral 1,2-diamines into the ligand backbone.Moreover, the ligands generally afford air and moisture-stable complexes.On the other hand, coordination chemistry of manganese has been studied extensively so that that manganese center is surrounded by O-or N-donor ligands [13,14].In addition, they also act as catalysts for important reactions [15][16][17][18][19][20].
e development of manganese(II) Schiff base heterogeneous catalysts for oxidation reaction has become a major area of research recently [21][22][23], as the potential advantages of these materials over homogeneous systems can make a major impact on the environmental performance of a synthesis.While the homogeneous catalysts exhibit excellent activity and selectivity, the technical problems encountered in their use, such as the difficulty in product separation, long reaction times, self-aggregation of active sites, and undesirable side products [24].In order to overcome the problems associated with homogeneous systems, heterogeneous catalyst was used in Organic Chemistry.In the present study, we have reported a simple and convenient method for the effective oxidation of benzoin derivatives to corresponding benzils under mild conditions by manganese(ΙΙ) Schiff base complex and H 2 O 2 at room temperature (Scheme 1).H 2 O 2 is inexpensive and environmental friendly oxidant.It is observed that the homogeneous catalyst is very active in high conversion rate of benzoin to benzil.However, the homogeneous catalysts have the difficulty in separating the catalysts from the reaction mixture at the end of the process.But, this simple method aimed to overcome the limitations.e easy removal of the catalyst makes them better compared to other homogeneous catalyst.Moreover, the catalytic oxidation can be carried out at room temperature in short reaction time with high yield and purity.

Experimental
2.1.Materials.Chemicals were purchased from the Merck and Fluka Chemical Companies in high purity.All of the materials were of commercial reagent grade.e amines and carbonyl compounds were puri�ed by standard procedures [25].

Apparatus.
IR spectra were recorded as KBr pellets on a Perkin-Elmer 781 spectrophotometer and on an Impact 400 Nicolet FTIR spectrophotometer. 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded on a Bruker DPX-400 Avance spectrometer.Tetramethyl silane (TMS) was used as an internal reference.UV spectra were recorded on a Hitachi 200-20 spectrometer using spectrophotometric grade ethanol (Baker).e melting points were determined by a Yanagimoto micromelting point apparatus.e purity determination of the substrates and reaction monitoring were accomplished by thin layer chromatography (TLC) on silica gel polygram SILG/UV 254 plates.

General Procedure for the Preparation of the Schiff Base
Ligands.To a mixture of salicylaldehyde (0.4 g, 3.27 mmol) in MeOH was added the diamine (1.65 mmol) with stirring in one portion.e stirring was continued to complete the reaction.e progress of the reaction was monitored by thin layer chromatography (TLC).Aer the completion of the reaction, a colored substance was obtained.e solid product was �ltered off and washed with cold MeOH.e crude product was puri�ed by recrystallization from ethanol and the pure Schiff base was obtained in high yield aer leaving for the appropriate time.e Schiff base products were identi�ed by physical and spectroscopic data and by a comparison with authentic samples prepared in accordance to the literature procedures (Scheme 2) [26].

General Procedure for the Preparation of the Schiff Base Complexes of Mn(II).
To a solution of the Schiff base ligand (1 mmol) in MeOH (10 mL) was added Mn(O 2 CCH 3 ) 2 (1 mmol) dropwise under re�uxing conditions.e reaction mixture was stirred to complete the reaction.e progress of the reaction was monitored by TLC.Aer the completion of complex formation, a colored substance was obtained.e solid product was �ltered off and washed with MeOH.e crude product was puri�ed by recrystallization from methanol and the pure Schiff base complex was obtained.e complex products were identi�ed by comparison with authentic samples prepared in accordance to the literature procedures (Scheme 2) [26].

General Procedure for Oxidation of Benzoin to Benzil.
Oxidation of benzoin was carried out using Mn(II) Schiff base complexes as a catalyst in a 50 mL �ask.In a typical reaction, 1 mmol of benzoin was dissolved in 10 mL acetonitrile in the presence of KOH.Oxidation of reaction mixture was done with H 2 O 2 with catalytic amount manganese(II) Schiff base.e reaction mixture was stirred to completion at room temperature and gave excellent yields under the mild oxidation conditions.e completion of reaction was monitored by TLC (petroleum ether: ethyl acetate, 4 : 1 v/v).Aer the completion of the reaction, the catalyst was removed by addition of absolute methanol to the mixture and Schiff base complex was recrystallized from petroleum ether.e solvent was evaporated, and the pure benzil derivatives were obtained.e structure of these compounds has been investigated using different methods of spectroscopy and spectrometry: UV, 1 H NMR, 13 C NMR, IR, and MS.

Results and Discussion
Schiff base complexes of metal(II) have been recognized as being among the most promising catalysts for various reactions.We used Mn(II) Schiff base complexes as catalyst and H 2 O 2 as oxidant, for the oxidation of some benzoins into benzils at room temperature in high yield.
Firstly, among the various oxidants, it was concluded that the best activity and selectivity can be achieved by H 2 O 2 under optimized reaction conditions.Diluted solution of hydrogen peroxide is a universal, ecologically clean and convenient way to handle reagents for different oxidations in the liquid phase [27,28].is is an environmentally friendly oxidizing agent as its breakdown products are water and oxygen.erefore, its decomposition produces no other pollutant in the environment.As shown in Table 1, the effect of Oxidant on the reaction was studied by considering substrate-to-oxidant ratios of 1 : 1, 1 : 1.5, 1 : 2, 1 : 2.5, and 1 : 3 for the substrate (0.2 g, 1 mmol) in 10 mL of CH 3 CN at room temperature.e yield increases from 52% to 98% with increasing substrate-to-oxidant ratios from 1 : 1 to 1 : 2. At the substrate to oxidant ratio of 1 : 2, the best yield of benzil was achieved (Table 1, Entry 3).Experiments under the same conditions in the absence of catalyst were also performed and only small amounts of product were detected in the reaction mixture.
We initiated a solvent screen to explore the effect of different solvents on the oxidation of benzoin and also summarized in Table 2. e results in Table 2 show that the highest yield of benzil was achieved with acetonitrile.When other solvents were used, no signi�cant improvement in the yield was observed.In the oxidation procedure, acetonitrile was chosen because of its high polarity and solubility of catalyst in the solvent.
Aer the completion of the reaction, the catalyst was removed by addition of absolute methanol to the mixture and Schiff base complex was recrystallized from petroleum ether.en the solvent was removed by evaporation under reduced pressure to give the pure products.Under this condition, several benzoin derivatives were oxidized to the corresponding benzils.e results are summarized in Table 3. Oxidation of the benzoin substrates proceeded with decomposition of hydrogen peroxide in the presence of the Schiff base complex catalyst at room temperature.e results clearly suggest that Mn(II) Schiff base complex efficiently catalyses the conversion of benzoin to benzil in CH 3 CN.e greater activity of the (OH) 2 -salen system has clearly arisen from the existence of an electron donating ligand which facilitates the electron transfer rate, a process that has previously been observed in other oxidation reactions [29][30][31][32][33][34].A cyclic mechanism has been proposed for the oxidation of benzoin using catalyst Mn(II) Schiff base complex with hydrogen peroxide as an oxidant (Scheme 3).An oxygen molecule from decomposition of H 2 O 2 reacts with Mn(II) to form an oxomanganese complex A [35,36], which undergoes oxidative addition to benzoin and gives an intermediate � which �nally undergoes reductive elimination to give the desired product [37] and Mn(II) is regenerated back.
As shown in Table 3, the oxidation of benzoins by Mn(II) Schiff base complex was carried out in good yield at room temperature.From the results in Table 3, it seems that the benzoins containing electron-donating group were found to be more reactive and could be oxidized more easily (b and f).In contrast, the benzoins containing electron-withdrawing group have shown lower reactivity (j).is shows that the electronic effects of substituents have a signi�cant role in the oxidation.
T 1: e effect of reaction conditions on the oxidation of benzoin in the presence of Mn(II) Schiff base complex in CH 3 CN at room temperature.

Entry
Molar ratio Time Schiff base complexe (mmol) Isolated yield (%) S 3: Proposed catalytic cycle for the oxidation of benzoins to benzils using catalyst Mn(II) Schiff base complex with hydrogen peroxide as an oxidant.T 2: e effect of solvent on the yield of reaction at room temperature.

Solvent
Unreacted Aer completion of reaction, the catalyst was washed with petroleum ether (3 times) and then kept for drying at 100 ∘ C for 2 h, aer which the catalyst was reused for next cycle without any appreciable loss of its activity.Similarly, reusability for sequential reaction was also carried out and catalyst was found to be reusable for �ve cycles (Table 4).
Finally, the structure of these compounds has been investigated using different methods of spectroscopy and spectrometry: UV, 1 H NMR, 13 C NMR, IR, and MS.

Conclusions
In summary, we have found a facile and efficient method for the oxidation of symmetrical and unsymmetrical benzoins to corresponding benzils using with the environmentally friendly H 2 O 2 as a sole oxidant and of Mn(II) Schiff base complex as catalyst under milder conditions.e Schiff base complex exhibited good catalytic activity in the oxidation of various benzil derivatives with hydrogen peroxide.e present procedure has many advantages such as short reaction times, mild conditions, easy operation procedures, easy removal catalyst, and high yields.

2 2 H 2 S 2 :
O , Mn(II) Schi base complexe r.t, CH 3 CN S 1: Oxidation of symmetrical and unsymmetrical benzoin derivatives to benzil derivatives using H 2 O 2 and manganese(ΙΙ) Schiff base complex.Preparation of the Schiff base complexes of metal(II) as catalyst.