Zn(OCOCH3)2·2H2O Catalysed Efficient Preparation of 2-Phenyl-4-Arylmethylidene-5-Oxazolinones under Ultrasonic Condition
Sadeq Hamood Saleh Azzam 1, Amreen Khanum 2, Mohamed Afzal Pasha 2,*
Department of Chemistry, Faculty of Science, Sana’a University, YEMEN
Department of Studies in Chemistry, Jnanabharathi Campus, Bangalore University, Bengaluru-560 056, INDIA
* Correspondence: Mohamed Afzal Pasha
Academic Editor: Antonio Monopoli
Special Issue: Applied Catalysis for a Circular Economy
Received: September 30, 2022 | Accepted: January 30, 2023 | Published: February 15, 2023
Catalysis Research 2023, Volume 3, Issue 1, doi:10.21926/cr.2301007
Recommended citation: Azzam SHS, Khanum A, Pasha MA. Zn(OCOCH3)2·2H2O Catalysed Efficient Preparation of 2-Phenyl-4-Arylmethylidene-5-Oxazolinones under Ultrasonic Condition. Catalysis Research 2023;3(1):12; doi:10.21926/cr.2301007.
© 2023 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.
2-Phenyl-4-arylmethylidene-5-oxazolinones were synthesized in very high yield by subjecting a mixture of an aryl aldehyde/heterocyclic aldehyde/cinnamaldehyde and hippuric acid in anhydrous acetic anhydride and catalytic Zn(OCOCH3)2·2H2O to ultrasonication at 35 KHz for 4 to 8 min. This method has many advantages: use of a green catalyst; yields are high, involves easy workup procedure, is energy efficient and economically feasible.
2-phenyl-4-arylmethylidene-5-oxazolinones; Zn(OCOCH3)2•2H2O; hippuric acid; araldehydes; acetic anhydride; ultrasonication
2-Phenyl-4-aryl methylidene-5-oxazolinones are important in synthesizing fine chemicals and serve as precursors for biologically active molecules such as: unsaturated amino acids, peptides and biosensors . Oxazolin-5-ones are multifunctional due to the presence of C=C, C=N, and C=O bonds. Hence, they are important in the synthesis of natural products and pharmaceutical chemicals [2,3].
Substituted oxazoles are important as they are compatible in the preparation of biologically active molecules such as: anti-bacterial, antifungal, anti-inflammatory [2,3,4], analgesic , anticancer [6,7], anti-diabetic  and antiobesity  agents. Further, 2-phenyl-4-aryl methylidene-5-oxazolinones are useful for the synthesis of thiadiazoles and triazoles  which also exhibit antimycobacterial [11,12], antimycotic  and antidepressive  activities. Oxazoles are further used as fluorescent whitening agents, dyes, pigments and lubricants [15,16,17,18].
The development of methods for azlactones is still an active field of investigation. Erlenmeyer Plöchl's reaction of an aldehyde, hippuric acid in acetic anhydride by using sodium acetate as a catalyst is well known . Erlenmeyer Plöchl reaction has remained unchanged since the last century with some modification of catalysts such as: zwitter ionic imidazolium salt , [Yb(OTf)3] , [TCT,PPh3] , [TsCl/DMF] , nano-sphere SiO2–AP , AcOK , Montmorillonite K-10 , MgO/Al2O3 , KF-Al2O3 , silica–alumina supported heteropolyacids , Nano Fe2O3 , Ca(OAc)2 , Bi(III) Salts , [CellFemImi]OH , [bmim]3PW12O40 , [bmim]PF6 or [bmim]BF4 . However, some of these methods have their excellence, but, suffer from drawbacks such as: high temperature, low yields, generation of toxic substances, and use of stoichiometric catalysts.
Sonochemistry has received attention in the past two decades. The driving force of ultrasonic reactions has many facets: The method is energy efficient, hence, is economical, the reactions can be scaled up using ultrasonic reactors; there is a need for a technology that minimizes the waste .
2. Results & Discussion
In continuation of our work on the preparation of oxazolin-5-ones using iodine as a catalyst under microwave irradiation , and the use of nano MgO under sonic condition , which have got their own advantages; and in continuation of our work on ultrasound-assisted reactions [39,40,41,42,43,44,45,46,47] we, herein, report another energy efficient synthesis of oxazolin-5-ones from hippuric acid, aryl aldehydes/heterocyclic aldehyde (furfural)/cinnamaldehyde and acetic anhydride in the presence of readily available, green and highly economical catalyst Zn(OCOCH3)2·2H2O under the sonic condition as shown in the Scheme1.
Scheme 1 Preparation of 2-phenyl-4-substitutedmethylidene-5-oxazolinones using catalytic Zn(OCOCH3)2·2H2O under sonic condition.
To evaluate the suitability of Zn(OCOCH3)2·2H2O, and to establish its catalytic role in the synthesis of biologically important oxazoline-5-ones, we initiated the present study at room temperature using hippuric acid, aryl aldehydes/heterocyclic aldehyde/cinnamaldehyde and acetic anhydride in EtOH as a solvent.
Several catalysts were examined for oxazoline-5-ones synthesis; to find the correct condition, a control reaction was carried out without a catalyst to get 5% product as presented in Table 1 (entry 1). Furthermore, to optimize the reaction conditions, we executed the reaction of 4-chlorobenzaldehyde with hippuric acid in acetic anhydride and Zn(OCOCH3)2·2H2O as an activator under diverse reaction environments such as: mechanical stirring, grinding, heating, refluxing in Ethanol, and under sonic condition. Grinding of the reactants with Zn(OCOCH3)2·2H2O gave a low yield (entry 14). On further investigation, we found that, the product yield is low under different conditions. Delightfully, Zn(OCOCH3)2·2H2O efficiently promoted the transformation under ultrasonic condition showing the highest degree of conversion, as it gave a very high yield in a short time (Table 1, entry 16). When the reaction was carried out under ultrasonication, the product was obtained in 95% yield within 8 min. The obtained results have been compared with some of the available methods in the literature for the preparation of 5-oxazolinones and the details are presented in Table 1.
Table 1 Synthesis of 2-Phenyl-4-(4'-chlorophenylmethylidene)-5-oxazolinone under varied conditions.
After examining the various reaction parameters, we examined the substrate scope and efficiency of the reaction with aryl/het(aryl)/cinnamyl aldehydes having electron withdrawing and electron donating groups in their aromatic rings, to get the corresponding products in very good to high yields under optimized reaction conditions as shown in Table 2. The 1HNMR spectra of all the prepared 5-oxazolinones showed characteristic methylidene proton signal between δ 7.22 and 8.37 ppm. The azalactone carbonyl group showed a strong signal in the IR spectra between υ 1797 to 1785 cm-1 of all the prepared products.
Table 2 Conversion of aryl aldehydes/furfural/cinnamaldehyde into the corresponding 2-phenyl-4-substitutedmethylidene-5-oxazolinones using catalytic Zn(OCOCH3)2·2H2O under sonic condition.
3.1 Materials and Instruments
Zn(OCOCH3)2·2H2O, acetic anhydride, aryl aldehydes/furfural/cinnamaldehyde, hippuric acid and other chemicals are commercial. Silica gel G254 TLC regulated the reactions under a UV lamp. The reactions were studied in a sonic bath at 35 kHz at 25°C. The IR spectra were recorded using a SHIMADZU FT-IR-8400S instrument, and 1HNMR spectra on a 400 MHz Bruker spectrophotometer in CDCl3 as a solvent.
3.2 General Procedure
Two mmol each of aryl aldehyde/furfural/ cinnamaldehyde, hippuric acid, 4 mmol of acetic anhydride and 0.5 mmol Zn(OCOCH3)2·2H2O in 3 mL ethanol were integrated well, and shrilled in the sonic bath for 4‒8 min (Table 2). The crude product was filtered, washed with hot water and recrystallized from methanol-water (2:1) to get 90–95% product.
3.3 Spectral Data
3.3.1 2-Phenyl-4-(4'-Methoxyphenylmethylidene)-5-Oxazolinone (3a): 
IR (KBr): ν C=O 1787.89 cm-1.
1H NMR (400 MHz, CDCl3): δ 3.87 (s, 3H), 6.95–6.99 (d, 2H, J = 8.8 Hz), 7.26 (s, 1H, =C-H), 7.43–7.57 (m, 3H), 8.18–8.19 (m, 2H), 8.21 (d, 2H, J = 8.8 Hz) ppm.
13C NMR (100 MHz, CDCl3): δ 55.56, 113.74, 114.53, 127.06, 128.1, 128.8, 131.8, 132.31, 134.58, 162.2, 167.9 ppm.
3.3.2 2-Phenyl-4-Benzylidene-5-Oxazolinone (3b): 
IR (KBr): ν C=O 1790 cm-1.
1H NMR (400 MHz, CDCl3): δ 7.26 (s, 1H, =C-H), 7.46–7.64 (m, 5H, ArH), 8.18–8.23 (m, 5H, ArH) ppm.
13C NMR (100 MHz, CDCl3): δ125.63, 128.40, 128.91, 131.20, 131.80, 132.47, 133.36, 133.55 ppm.
3.3.3 4-(4'-Nitropheylmethylidene)-5-Oxazolinone (3c): 
IR (KBr): ν C=O 1785.53 cm-1.
1H NMR (400 MHz, CDCl3): δ 7.36 (s, 1H, =C-H), 7.62–8.05 (m, 5H, ArH), 7.96 (dd, 2H, H-2′ and H-6′), 8.20 (dd, 2H, H-3′ and H-5′) ppm.
13C NMR (100 MHz, CDCl3): δ 118.5, 126.9, 128.3, 129.2, 131.1, 132.4, 134.8, 138.2, 142.6, 147.5, 161.7, 173.2 ppm
MS (ESI) m/z: 294.06 [M+H]+•.
3.3.4 2-Phenyl-4-(3′,4′-Dimethoxyphenylmethylidene)-5-Oxazolinone (3d):
IR (KBr): ν C=O 1785.96 cm-1.
1HMR (400 MHz, CDCl3): δ 3.98 (s, 3H), 4.04 (s, 3H), 6.95 (d, 1H, J = 9 .2) 7.22 (s, 1H, =C-H), 7.55 (m, 4H, J = 8.4 Hz), 8.15 (s, 1H), 8.19 (d, 2H, J = 9.6 Hz) ppm.
13C NMR (100 MHz, CDCl3): δ 110.91, 113.98, 126.88, 127.96, 128.05, 128.97, 131.21, 132.10, 133.06, 149.22, 152.10, 162.48, 167.84 ppm.
3.3.5 2-Phenyl-4-(3'-Nitrophenylmethyidene)-5-Oxazolinone (3e): 
IR (KBr): ν C=O 1787.89 cm-1.
1H NMR (400 MHz, CDCl3): δ 7.41 (s, 1H, =C-H), 7.50–7.95 (m, 5H, ArH), 7.62 (t, 1H, H-5′), 7.84 (dd, 1H, H-6′), 8.19 (dd, 1H, H-4′), 8.54 (s, 1H, H-2′) ppm.
13C NMR (100 MHz, CDCl3): δ 121.7, 125.4, 127.6, 128.8, 129.1, 129.9, 131.8, 132.8, 134.3, 135.1, 141.8, 148.5, 160.9, 171.5 ppm.
MS (ESI) m/z: 294.06 [M+H] +•.
3.3.6 2-Phenyl-4-(2'-Nitrophenylmethylidene)-5-Oxazolinone (3f): 
IR (KBr): ν C=O 1793.68 cm-1.
1H NMR (300 MHz, CDCl3): δ 7.53–7.96 (m, 5H, Ar-H), 7.80–8.01 (m, 4H, Ar-H), 8.37 (s, 1H, =C-H) ppm.
13C NMR (100 MHz, CDCl3): δ 112.2, 123.2, 126.1, 127.1, 127.3, 128.0, 128.4, 128.5, 128.7, 128.9, 131.0, 131.1, 134.2, 147.3, 160.8, 165.9 ppm.
MS (ESI) m/z: 295.06 [M+H]; HRMS-EI: found: 294.064, calculated: 294.071.
3.3.7 2-Phenyl-4-(2'-Chlorophenylmethylidene)-5-Oxazolinone (3g): 
IR (KBr): ν C=O 1793.68 cm-1.
1H NMR (300 MHz, CDCl3): δ 7.51‒7.94 (m, 5H, Ar-H), 7.31‒7.43 (m, 4H, Ar-H), 7.63 (s, 1H, C=C-H) ppm.
13C NMR (100 MHz, CDCl3): δ 112.3, 126.0, 126.3, 127.3, 128.0, 128.3, 128.5, 128.8, 131.1, 131.3, 132.3, 133.5, 160.1, 168.9 ppm.
MS (ESI) m/z: 284.04 [M+H];
HRMS-EI: found: 283.040, calculated: 283.047.
3.3.8 2-Phenyl-4-(4'-Chlorophenylmethylidene)-5-Oxazolinone (3h):
IR (KBr): ν C=O 1795.6 cm-1.
1HMR (400 MHz, CDCl3): δ 7.37 (s, 1H, =C-H), 7.59–7.73 (m, 5H), 8.12 (d, 2H, J = 7.2 Hz), 8.31 (d, 2H, J = 8.2 Hz) ppm.
3.3.9 2-Phenyl-4-(2'-Furylmethylidene)-5-Oxazolinone (3i):
IR (KBr): ν C=O 1789.82 cm-1.
1HMR (400 MHz, CDCl3): δ 6.83 (d, 1H, J = 3.6 Hz), 7.27 (s, 1H, =C-H), 7.60–7.62 (m, 3H), 7.68 (d, 1H, J = 7.2 Hz), 8.08–8.11 (m, 3H) ppm.
3.3.10 2-Phenyl-4-(E-Styrylmethylidene)-5-Oxazolinone (3j):
IR (KBr): ν C=O 1785.96 cm-1.
1HMR (400 MHz, CDCl3): δ 6.709 (d, 2H, J = 12 Hz), 7.23 (d, 1H, J = 7.6 Hz, =C-H), 7.52–7.61 (m, 5H), 8.23–8.27 (m, 5H) ppm.
3.3.11 2-Phenyl-4-(4'-N,N-Dimethylaminophenylmethylidene)-5-Oxazolinone (3k): 
IR (KBr): ν C=O 1787.89 cm-1.
1H NMR (400 MHz, CDCl3): δ 2.98 (s, 6H, 2 × CH3), 6.64 (dd, 2H), 7.18 (dd, 2H), 7.36 (s, 1H, =C-H), 7.60–8.10 (m, 5H, ArH) ppm.
13C NMR (100 MHz, CDCl3): δ 44.1, 113.2, 126.8, 128.1, 128.9, 129.8, 131.2, 132.4, 134.8, 141.6, 151.7, 163.8, 175.3 ppm.
MS (ESI) m/z: 292.12 [M+H]+•.
In the present reaction, hippuric acid gets activated by Zn (OCOCH3)2 followed by conjugate addition of I across the carbonyl carbon of the aldehyde takes place to give intermediate II, which reacts with a molecule of acetic anhydride and loses two molecules of acetic acid to give the intermediate III. In the final step the loss of two more molecules of acetic acid from III in the presence of another molecule of acetic anhydride may occur to give the product IV. The last two steps are important, and we feel that, ultrasound is responsible for enhancing the rate as shown in the Scheme 2.
Scheme 2 Mechanism of formation of 2-phenyl-4-arylmethylidene-5-oxazolinones.
In conclusion, an ultrasound-assisted, highly economical, energy-efficient method for the preparation of biologically important oxazoline-5-one derivatives by the condensation of hippuric acid with aryl aldehydes/furfural/cinnamaldehyde in acetic anhydride/EtOH and Zn(OCOCH3)2·2H2O as a catalyst has been developed. The present method is efficient and simple, high yield, short duration and ease of workup make the method advantageous. Most importantly, the crude product quality is high enough that, in some cases, further purification is not required.
Dr. Sadeq Hamood Saleh Azzam sincerely thanks the Department of Chemistry, Sana’a University, Sana’a, YEMEN for providing research facility. Dr. Mohamed Afzal Pasha acknowledges the University Grants Commission, New Delhi, INDIA for BSR Faculty Fellowship: No. F.18-1/2011 (BSR); November, 2019.
Dr. Sadeq Hamood Saled Azzam prepared the oxazoline-5-ones and characterized them and prepared the draft of the manuscript. Ms. Amreen Khanum repeated the synthesis of oxazoline-5-ones as per the reviewers suggestions, characterized them and updated the references. Prof. Mohamed Afzal Pasha is the corresponding author, Research supervisor, corrected and edited the manuscript and Guided both the coauthors.
The authors have declared that no competing interests exist.
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