Polyaniline Decorated TiO2 nanocomposite as Efficient Catalyst for Ultrasound Synthesis of Dihydropyrimidinones.

doi:10.21203/rs.3.rs-3398909/v1

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Polyaniline Decorated TiO2 nanocomposite as Efficient Catalyst for Ultrasound Synthesis of Dihydropyrimidinones.

https://doi.org/10.21203/rs.3.rs-3398909/v1

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The paper describes the synthesis of conducting polyaniline - TiO₂ nanocomposite by using chemical oxidative polymerization method using aniline as a monomer. TiO₂ nanoparticles were synthesized by sol gel method using TiCl₄ and ethanol. Synthesized TiO₂ nanoparticles and nanocomposite were characterized by various spectroscopic techniques UV, FT-IR, XRD, SEM and TGA. The XRD pattern confirms the appearance of sharp diffraction patterns indicates the small size with high purity TiO₂ nano particles are highly agglomerated with spherical morphology. The polyaniline-TiO₂ nanocomposite was employed as a promising heterocyclic, reusable catalyst for most of the organic synthesis. Nanocomposite of polyaniline -TiO₂ is an efficient, solvent free, ultrasound assisted synthesis of biologically active dihydropyrimidin-2(1H)-one/thiones (DHPM) derivatives. The advantages of this methodology are mild reaction conditions with short reaction time, excellent yields, low loading and reusability of catalyst.

Polyaniline -TiO2

Sol gel method

Nanocomposite

Biologically active

Ultrasound

DHPM

Organic synthesis focusses on the development of multiple bonds forming transformation allowing the creation of several covalent bonds in a single operation. Hence, the number of steps required for obtaining the target molecules is reduced which addresses the economy and efficiency criteria of green route chemistry. The Biginelli reaction is a well-known multi-component reaction involving one pot cyclo-condensation of aldehydes, β-keto ester and urea under strong acidic conditions [1, 2]. A major drawback of Biginelli’s reaction is low yields in the case of substituted aromatic and aliphatic aldehydes. Dihydropyrimidinones (DHPM’s) and its derivatives, which are well known for their wide range of pharmacological profiles [3, 4] like antiviral, alpha antagonists, neuropeptide (NPY) antagonists, antihypertensive, anti-inflammatory, antibacterial, calcium channel blocker [5, 6] etc. and various other applications in the field of drug research have stimulated the invention of a wide range of developed synthetic routes. Several alkaloids containing a dihydropyrimidinone and thiones core have been isolated from marine sources, which show strong biological activity. Solvent-free organic reactions have been applied as a useful protocol in organic synthesis. Solvent-free conditions often lead to shorter reaction times, increased yields, easier workup that matches with green chemistry protocols and may enhance the regio-and stereo selectivity of reactions [7, 8].

Among the conducting polymers, polyaniline and polyindole are often used as an organic part to prepare nanocomposites because of its low cost, easy preparation, controllable unique properties by oxidation and protonation state, excellent environmental stability and potential application in electronic devices. A number of different metal oxide particles have so far been encapsulated into the shell of polyaniline giving rise to a host of nanocomposites. Various reusable conducting polymeric material are used as a catalyst such as polyaniline [9] and polyaniline nanocomposite for electrochemical water splitting and dyes degradation [10]. Polyindole and nanocomposite shows best catalytic, antibacterial activity [12–17] and anticorrosive properties [18–19]. Polymer nanocomposites can also be used as textiles [20].

Titanium dioxide (TiO₂) is attractive owing to its superior properties such as high photocatalytic activity, excellent stability, super-hydrophilicity, non-toxicity, biological and chemical inertness, semiconductor and optoelectronic properties [21–22]. Electrical and optical properties, titania (TiO₂) have been extensively employed in many fields such as solar cells, sensors [24], photo catalysis [25] photo voltaic cells and photo electrolysis as well as antibacterial [26], photochemical water purification [27], and self-cleaning applications.

Consequently, there is a scope for modifications of reaction conditions and improving yields. From the literature survey, it is evident that there is a need to carry out this reaction with higher yield, low chemical loading and shorter reaction times under milder reaction conditions. The replacement of environmental hazardous solvents by solvent free conditions and catalysts (by the use of solid catalysts) are the innovative trends. In a continuation of our previous work on heterogeneous catalysts, we report a cheap, recoverable and efficient catalytic system for the synthesis of dihydropyrimidinones and thiones from aldehyde in the presence of polyaniline – TiO₂ nano catalyst.

The present study reports an efficient method for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones prepared via the condensation of aromatic aldehydes with β ketoester and urea in the presence of catalytic amount of polyaniline-TiO₂ nano catalyst under ultrasound irradiation and solvent-free conditions (Scheme. 3)

Materials and methods

Melting points of all compounds were determined using Electro thermal 9100. ¹H NMR spectra were recorded on a Bruker DRX-400 Advance instrument with DMSO-d₆ as solvents. All reagents and solvents were purchased from Merck, Fluka and across chemical companies and used without further purification.

2.1. Synthesis of TiO₂ nanoparticles

TiO₂ nanoparticles were synthesized by hydrolysis of titanium tetrachloride (TiCl₄) (0.01 mol) in ethanol under nitrogen atmosphere and oxalic acid (0.001 mol) as additive. Solution was kept at 0°C in an ice-water bath with vigorous stirring as the reaction is exothermic. During the mixing process, white fumes, presumably HCl, were released as a consequence of the hydrolysis of TiCl₄ and pH of solution is strongly acidic. The reaction mixture was stirred for 10 minutes and then irradiated for 40 minutes under ultrasound (Ultrasonic frequency: 33 KHz, Ultrasonic power: 100 Watts) to avoid aggregation of nanoparticles and stirred further for 60 minutes at room temperature till a yellow-colored solution of hydrous TiO₂ particles obtained [16]. Nano particles were then washed with distilled water filtered and dried in an oven at 80°C and calcined at 350°C for four hours in a furnace (Scheme1).

2.2. Synthesis of polyaniline - TiO₂ nanocomposite

Polymerization reactions were carried out in a well stoppered round bottomed flask containing monomer aniline (0.001mol) under inert atmosphere. To this solution synthesized TiO₂ nanoparticles (0.001mol) were added. Then the flask was sealed with a rubber septum. The solution was degassed with nitrogen for 1 hour. Dopant acrylic acid was added, the oxidizing agent, ammonium persulfate (0.01mol) was dissolved separately in10 mL de-ionized water and was added drop wise at a rate of approximately 1 mL/min to the above monomer solution containing TiO₂ nano particles under constant stirring at a temperature between 0-5^oC. After complete addition of the oxidizing agent, the reaction mixture was kept in a ultra-sonication for 45 min. The green precipitate of the polymer nanocomposite was isolated by filtration and washed with distilled water until the washing liquid became completely colorless. Finally, the polyaniline -TiO₂ nano composite was dried under vacuum at 60^oC in an oven until a constant mass was achieved. The polyaniline -TiO₂ nano composite was characterized by FTIR, XRD and SEM. [10–13].

2.3. Synthesis of Dihydropyrimidinones /thiones

In a round bottom flask a mixture of the β-dicarbonyl compound (1 mmol), aldehyde (1 mmol), urea (1.2 mmol) and polyaniline -TiO₂ nano catalyst (0.001) was taken, under ultrasound irradiation (230 V, Ultrasonic frequency: 33 KHz, Ultrasonic power: 100 Watts) at 45^oC. The reaction was monitored by TLC. After completion of the reaction, water was added to the reaction mixture to remove excess of urea and then filtered and washed to get pure product. Isolated products have almost 99% purity. The purified products were characterized by NMR, IR and mass spectral analysis.

3.1. UV- Vis characterization

UV-Vis absorption spectrum of synthesized TiO₂ nanoparticles is shown in Fig. 1a, The spectrum reveals a characteristic sharp absorption peak at wavelength 365 nm which indicates charge transfer from the valence band (mainly formed by the 2p orbital of oxide anions) to the conduction band (mainly formed by 3d t₂g orbitals of the Ti^+ 4 cations) formation of TiO₂ nanoparticles. [11].

The spectral features observed in Fig. 1b, reveal peak at 320 nm which corresponds to the π - π * transition of the benzenoid rings while the sharp intense peak around 400–420 nm can be assigned to the localized polarons which are characteristic of the protonated polyaniline. A sharp peak is observed at 620 nm representing the emeraldine salt form of the polymer.

3.2. FTIR characterization

Figure 2a shows FTIR spectrum of nano which reveals the broad absorption band at ~ 3405 cm^− 1 corresponding to stretching vibration of O-H bond and peak at ~ 1638 cm^− 1 which indicates bending vibration of –O-H bond. A sharp peak at 1402 cm^− 1 is due to the stretching vibrations of Ti-O-Ti and Vibration around 720 cm^− 1 corresponds to the anti-symmetric Ti-O-Ti mode of TiO₂.

The FTIR spectra of the polyaniline doped acrylic acid are given in Fig. 2b. The broad and intense peak at 3300 − 3100 cm^− 1 -NH stretching and 1151 cm^− 1 band due to dopant anion. Since polyaniline exist in conducting polymer. The strong and intense peak at 1579 cm^− 1 in polymer is observed indicating quinoid ring stretching frequency along with this peak another peak is observed at 1500 cm^− 1 indicating benzenoid ring stretching vibration. Benzenoid and quinoid bands are observed at 1500 cm^− 1 and 1579 cm^− 1. They are comparatively broad, weak and shifted to the lower wave number. Spectral intensity of these bands is comparatively typical of highly doped emeraldine salt form of polymer. The presence of a weak peak at 1700 cm^− 1 indicates non protonated –COOH group.

FTIR of polyaniline -TiO₂ shows in Fig. 2c, some bands of PANI have shifted due to interactions with anatase TiO₂ nanoparticles. For example, the bands at 1576 cm^− 1 corresponding to the stretching mode of C = N, 1496 cm^− 1 C = C stretching, and 1310 cm^− 1 observed due to C–N stretching mode. These changes suggest that C = C, C–N C = N bands become weaker in polyaniline-TiO₂ nanocomposite, while the N–H band becomes stronger. This is probably because of the action of hydrogen bonding between the surfaces of anatase TiO₂ nanoparticles and the N–H groups in polyaniline chain. The results confirm that there is strong interaction between the polyaniline and nanocrystalline TiO₂.

3.3. XRD analysis

XRD pattern of TiO₂ and polyaniline-TiO₂ nanocomposite is shown in Fig. 3a and Fig. 3b. The diffraction pattern of TiO₂ matches well with literature and is in good agreement with the standard spectrum (JCPDS no. 84-1286). The appearance of sharp diffraction patterns indicates the small size with high purity and crystallinity of the synthesized nano TiO₂. Presence of sharp peaks at 25˚ and 48˚ indicates TiO₂ is present in anatase phase. The XRD analysis shows the major reflections between 25˚ to 70˚ (2θ values) which indicate TiO₂ nanoparticles on average size of 25–30 nm. XRD pattern of polyaniline-TiO₂ shows diffraction peak at 25˚, 40˚, 48˚, 53˚ and 68˚ are due to (110), (101), (111) and (211) are present in nanocomposite which indicates the presence of TiO₂ in Polyaniline [JCPDS file no. 21-1276]. Peaks shows the crystalline structure of nanocomposite and obtained results are in good agreement with the literature. The intensity of peaks in nanocomposite is lower.

3.4. SEM analysis

The surface morphology of TiO₂ analyzed by SEM and is represented in Fig. 4a. It shows that TiO₂ nano particles are highly agglomerated with spherical morphology to form a cluster as observed in SEM micrograph. The PANI exhibit spherical irregular morphology like cauliflower type structure (Fig. 4b). While the morphology of polyaniline-TiO₂ nanoparticles approaches spherical with particle size of about 70 nm (Fig. 4c).

3.5. Thermal stability

The thermal gravimetric analysis (TGA) curves for PANI and its nano-composite are presented in Fig. 5. TiO₂ nanoparticles are stable in air and no decomposition takes place in the range 50 to 900°C. On the other hand, polyaniline decomposes sharply at 390°C (Fig. 5). On examining the TGA curves, it can be observed that the polyaniline nano-composite has higher thermal stability than pure polyaniline.

Polyaniline decorated TiO₂ nanoparticles were successfully synthesized in situ by chemical oxidative polymerization. The FT-IR and XRD results proved the strong interfacial interaction between the TiO₂ nanoparticles and the polyaniline chain. It is confirmed that the diameter of the resulted PANI-TiO₂ nanoparticles is about 70 nm and the interactions between the two components are strong.

An efficient and environment friendly methodology for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones/thiones using polyaniline-TiO₂ nanocomposite as a heterogeneous catalyst. The present protocol is superior in terms of scope for the synthesis of dihydropyrimidinones and thiones, offers several notable advantages over the existing methods, including ease in handling, reusability, thermal stability, environmentally friendly, short reaction times, and operational simplicity.

Table 1

Polyaniline – TiO₂ promoted synthesis of dihydropyrimidinones and thiones under solvent free condition
Entry	Product	Aldehyde R	X	Ultrasound		M.P.
Entry	Product	Aldehyde R	X	Time (min)	Yield (%)	M.P.
1	4a	C₆H₅	O	45	92 (78)^b	202–205
2	4b	Ph CH = CH	O	35	95	230–236
3	4c	4-NO₂-C₆H₅	O	60	87 (58)^b	234–236
4	4d	4 NMe₂C6H₄	O	35	92 (86)^b	256–258
5	4e	4 Cl-C₆H₄	O	47	87 (56)^b	211–213
6	4f	C₆H₅	S	42	88	203–205
7	4g	PhCH = CH	S	43	86	227–230
8	4h	4-NO₂-C₆H₅	S	65	89	153–155
9	4i	4 NMe₂C6H₄	S	40	90	230–233
10	4j	4 Cl-C₆H₄	S	50	86	193–194

All products were characterized by IR, ¹H NMR and LCMS

^a Polyaniline -TiO₂ promoted synthesis of DHPM

^b Conventional methods of synthesis of DHPM (cat HCl in EtOH, reflux, 18 h)

Spectral data of selected compounds:

1) 5-Ethoxycarbonyl-6-methyl-4-phenyl-3, 4-dihydropyrimidin-2(1H)-one (4a):

¹H NMR (400 MHz, CDCl₃) δ: 1.10 (t, 3H, J=6.88 Hz, OCH₂CH₃), 2.24 (s, 3H, CH₃), 4.06 (q, 2H, J=6.88 Hz, OCH₂CH₃), 5.12 (s, 1H, C(4)-H), 7.26-7.30 (m, 5H, C₆H₅), 7.82 (s, 1H, NH), 9.11 (s, 1H, NH); MS (m/e): 261(M+1), 260 (M⁺); IR (KBr) cm^-1: 3242, 3115, 1726, 1647, 1600, 1419, 1311, 1091, 783.

2) 5-Ethoxycarbonyl-6-methyl-4-cinnamyl-3,4-dihydropyrimidin-2(1H)-one (4b):

¹H NMR (400 MHz, DMSO-d₆) δ: 1.20 (t, 3H, OCH₂CH₃), 2.23 (s, 3H, vinyl methyl), 4.10 (q, 2H, OCH₂CH₃), 4.85 (d, 1H, C(4) -H), 6.2 (d, 1H, CH=CH), 6.4 (d, 1H, CH=CH), 7.2 (m, 5H, Ar-H), 9.5 (bs, 1H, NH), 10.2 (bs, 1H, NH). MS (m/e): 287(M+1); IR (KBr) cm^-1: 3115, 3039, 1710, 1678, 1615, 1250, 750.

3) 5-Ethoxycarbonyl-4-(4-nitro-phenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4c):

¹H NMR (400 MHz, CDCl₃) δ:1.1 (t, 3H, OCH₂CH₃), 2.27 (s, 3H, CH₃), 3.98 (q, 2H, OCH₂CH₃), 5.30 (d, 1H, C(4)-H), 7.51 (d, 2H, Ar-H), 7.89 (s, 1H, NH), 8.22 (d, 2H, Ar-H), 9.37 (s, 1H, NH); MS (m/e): 306 (M+1); IR (KBr) cm^-1: 3224, 2950, 1745, 1670, 1510, 1340, 1125, 780

4) 5-Ethoxycarbonyl-4-(4-N(CH₃)₂)l-6-methyl-dihydropyrimidin-2(1H)-one (4d):

¹H NMR (400 MHz, DMSO-d₆) δ: 1.18 (t, 3H, J=7.6 Hz, CH₃), 2.4 (s, 3H, CH₃), 2.8 (s, 6H, N(CH₃)₂), 4.00 (q, 2H, J=7.6 Hz, OCH₂CH₃), 5.02 (d, 1H, C(4)-H), 6.6 (d, 2H, J=9.1 Hz, Ar-H), 7.00 (d, 2H, J=9.1 Hz, Ar-H), 7.6 and 9.16 (2 bs, 2H, NH); MS (m/e): 304 (M+1); IR (KBr) cm⁻¹: 3200, 3100, 1700, 1685.

5) 5-Ethoxycarbonyl-4-(4-chlorophenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4e):

¹H NMR (400 MHz, DMSO-d₆) δ: 1.12 (t, 3H, J=7.14 Hz, OCH₂CH₃), 2.30 (s, 3H, CH₃), 3.91 (q, 2H, J=7.16 Hz, OCH₂CH₃), 5.07 (d, 1H, J=2.28, CH), 7.21 (d, 2H, J=9.18, Ar-H), 7.69 (d, 2H, J=9.18, Ar-H), 7.94(s, 1H, NH), 9.29 (bs, 1H, NH); MS (m/e): 295 (M+1); IR (KBr) cm^-1: 3225, 1720, 1615.

6) 5-Ethoxycarbonyl-4-(4-chlorophenyl)-6-methyl-3, 4-dihydropyrimidin-2(1H)-one (4f):

¹HNMR (DMSO-d₆) δ: 1.12 (t, 3H, J = 7.14 Hz, -OCH₂CH₃), 2.30 (s, 3H, -CH₃), 3.91 (q, 2H, J=7.16 Hz,- OCH₂CH₃), 5.70 (d, 1H, J = 2.28, -CH), 7.21 (d, 2H, J = 9.18, Ar-H), 7.69 (s, 1H, -NH),7.94 (d, 2H, J = 9.18, Ar-H), 9.16 (s, 1H, -NH). MS (m/e): 297 (M+1);IR (KBr, cm-1): IR (KBr, cm-1): 3225, 1720, 1615.

7) 5-Ethoxycarbonyl-4-(4-phenyl)-6-methyl-3, 4-dihydropyrimidin-2(1H)-thione(4g):

1H NMR (DMSO-d₆): δ 1.19 (t, J=7.3 Hz, 3H, CH₃), 2.27 (s, 3H, CH₃), 4.08 (q, J=7.3Hz, 2H, -OCH2), 5.22 (s, 1H, CH), 7.23 (m, 5H, Ar), 9.25 and 9.9 (2s, 2H,2brs. NH); MS (m/e): (M+1) 293; IR (KBr, cm-1): 3244, 3115, 1727, 1647, 1600, 1419, 1311, 1091, 783.

8) 5-Ethoxycarbonyl-4-(4-nitrophenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-thione (4h):

¹H NMR (DMSOd₆): δ 1.13 (t, 3H, CH₃-CH₂O), 2.31 (s, 3H, -CH₃), 3.99 (q, 2H, -OCH₂), 5.22 (s, 1H, CH), 7.56 (d, 2H, ArH), 7.90 (s, 1H,- NH), 8.11 (d, 2H, ArH), 9.39 (s, 1H, NH)

MS: (M+1);338); IR (KBr) cm^-1: 3226, 2950, 1745, 1670, 1510, 1340, 1127, 782.

9) 5-Ethoxycarbonyl-4-(4-N(CH₃)₂)l-6-methyl-3,4-dihydropyrimidin-2(1H)-thione (4i):

¹H NMR (400 MHz, DMSO-d₆) δ: 1.20 (t, 3H, J=7.6 Hz, CH₃), 2.25 (s, 3H, CH₃), 2.87 (s, 6H, N(CH₃)₂), 4.00 (q, 2H, J=7.6 Hz, OCH₂), 5.18 (d, 1H, C(4)-H), 6.62 (d, 2H, J=9.1 Hz, Ar-H), 7.19 (d, 2H, J=9.1 Hz, Ar-H), 9.80 (bs, 1H, NH), 10.2 (bs, 1H, NH); MS (m/e): 320 (M+1); IR (KBr) cm^-1: 3196, 1640, 1552, 1475, 1191.

10) 5-Ethoxycarbonyl-4-(4-chlorophenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-thione (4j)

¹HNMR (DMSO-d₆) δ: 1.10 (t,3H, J =7.01 Hz, -OCH₂CH₃), 2.29(s, 3H, -CH₃), 2.26 (3H,s, -CH₃) 4.00 (q, 2H, J=7.01 Hz,- OCH₂CH₃), 5.16 (d, 1H, -C(4)H), 7.22(d, 2H, J = 7.04 Hz, Ar-H), 7.43 (d, 2H, J = 7.04 Hz, Ar-H), 10.40 (br s, 1H, -NH),9.68 (br s, 1H, -NH).MS:m/e (M+1) 311 ((M+1) IR (KBr) cm^-1: 3225, 1720, 1615.

Acknowledgement:

The authors are grateful to the Savitribai Phule Pune University, C-MET and NCL, Pune for characterization and activity. We also thank Nowrosjee Wadia College, Pune for providing lab facility.

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Schemes 1 and 4 are available in the Supplementary Files section

GraphicalAbstract.docx
Schem1.png
Scheme 1. Synthesis of TiO₂ nanoparticles.
Schem2.png
Scheme 2: Synthesis of Polyaniline -TiO₂ nanocomposite.
Schem3.png
Scheme 3. Synthesis of DHPM’s using polyaniline - TiO₂ nanoparticles.
Schem4.png
Scheme 4. Probable mechanism for DHPM synthesis using polyaniline - TiO₂ as a catalyst.
supplementaryData.docx

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Polyaniline Decorated TiO2 nanocomposite as Efficient Catalyst for Ultrasound Synthesis of Dihydropyrimidinones.

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Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Synthesis of TiO₂ nanoparticles

2.2. Synthesis of polyaniline - TiO₂ nanocomposite

2.3. Synthesis of Dihydropyrimidinones /thiones

3. Results and discussion

3.1. UV- Vis characterization

3.2. FTIR characterization

3.3. XRD analysis

3.4. SEM analysis

3.5. Thermal stability

4. Conclusion

Declarations

Acknowledgement:

References

Schemes

Supplementary Files

Status:

Version 1

Polyaniline Decorated TiO2 nanocomposite as Efficient Catalyst for Ultrasound Synthesis of Dihydropyrimidinones.

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Synthesis of TiO2 nanoparticles

2.2. Synthesis of polyaniline - TiO2 nanocomposite

2.3. Synthesis of Dihydropyrimidinones /thiones

3. Results and discussion

3.1. UV- Vis characterization

3.2. FTIR characterization

3.3. XRD analysis

3.4. SEM analysis

3.5. Thermal stability

4. Conclusion

Declarations

Acknowledgement:

References

Schemes

Supplementary Files

Status:

Version 1

2.1. Synthesis of TiO₂ nanoparticles

2.2. Synthesis of polyaniline - TiO₂ nanocomposite