1. Introduction

Previous few decades, transition metal mediated cross-coupling reactions have become an essential implementation in organic synthesis. Among the transition metals, palladium is the utmost metal in recent organic synthesis and broadly used for the extensive quantity of synthetic transformations mostly carbon-carbon cross -coupling reactions [1-4]. The enormous significance of C-C bond forming reactions has fortified the chemical community to search for immensely active and strong palladium catalysts, which should be flexible and efficient. In addition, Metallodendrimers are the unique class of synthetic macromolecules having extraordinarily branched, three-dimensional, nano scale-shaped with very low polydispersity and immoderate functionality [5]. The metallodendrimer generally contains three superb areas which show active center in the different catalytic system: (i) metal atom as the dendrimer center, (ii) metal atoms inside the dendrimer branches (iii) metal atoms within the periphery. As for example, metals able to coordinate with the poly(amido)amine(PAMAM) shape encompass amongst others Cu [6, 7], Au [8], Pd [9], Pt [10], Ag [11], Co [12] as well as bimetallic structures such as Pd-Au [13] and Pt-Ru [14]. Lately, metallodendrimers were broadly researched in various fields, which includes molecular light harvesting, catalysts, liquid crystals, molecular encapsulation, and drug delivery [15]. Most of the investigation was succeeded in the arena of catalysis where metals, for example, Cu (II), Rh (III), Ru (II), Pd (II), Fe (I) and Co (III) are utilized in the production of metallodendrimers. Moreover, chemoselective reactions [17], and azide-alkyne reactions are also carried out and catalyzed by metallodendrimer compounds [18, 19].

Despite the fact that the ordinary Pd catalyzed and Cu co-catalyzed Sonogashira reactions has some disadvantages. These include the usage of extremely overpriced Pd catalysts (occasionally essential in high loading), difficulties in convalescing these catalysts, excessive reaction temperatures, air sensitivity of transition metal complexes and the usage of phosphine ligand which is air sensitive. In addition, the formation of homocoupled products due to contact of the alkynes to oxidizing agents, Cu salts or air as for example Glaser coupling [20] or Hay coupling [21] are also weaknesses. However, to minimize these drawbacks we designed as well as synthesized solid crystalline air stable diazine based dendrimer assisted Pd-metallodendrimer catalyst. It has shown efficient catalytic activity under mild and phosphine-free conditions for the heck and Sonogashira reactions.

2. Experimental Sections

All procedures of air- and moisture-sensitive compounds were performed by the use of standard Schlenk techniques under an atmosphere of argon or nitrogen. The reagents were bought from Aldrich as high-purity products and usually used as received. Dehydrated DMF, DMSO, CH₃CN, and THF were used as reaction solvent. The IR spectra was taken on a Shimadzu FTIR 8400S Fourier remodel Infrared Spectrophotometer (400-4000 cm^-1) with KBr pellets. ¹H NMR and ¹³C NMR spectra were recorded at 300 MHz and 75 MHz, respectively, on a JEOL AL 300/BZ tool in addition to BRUKER DPX-400 MHz & 100 MHz spectrophoto meters respectively. Chemical shifts was taken relative to TMS. Mass spectra (MS) was measured with the aid of the usage of AXIMA-CFR, Shimadzu/Kratos TOF Mass spectrometer. Scanning Electron Microscope (SEM) and power Dispersive X-ray (EDX) was taken on a Hitachi S-4800. Analytical thin layer chromatography (TLC) became silica gel 60 F 254 covered on 25 TCC aluminum sheets (20 × 20 cm). 2, 4, 6-triamino-1, 3, 5-diazine, 4-methyl benzoyl chloride and (Ph₃P)₂PdCl₂ were purchased from Sigma Aldrich and had been directly used without further purification. The thermal behavior of metallodendrimer was determined by a thermogravimetric analyzer (NETZSCH STA 449F3) from 26 to 600°C. Elemental analyses were carried out with a Fisons EA 1108 CHNS-O apparatus. All TG and DSC facts had been received under a nitrogen environment by the use of aluminum oxide crucible at the heating rate of 10 °k/min and at a flow rate of 40 and 60 ml/min.

2.1. Synthesis of 2, 4, 6-Tris (di-4-methylbenzamido)-1, 3, 5-Diazine Palladium (II) chloride (3)

4-Methyl benzoyl chloride 2 (1.47 g, 9.54 mmol), (Ph₃P)₂PdCl₂ (0.11g, 10% mol) were sequentially added to a solution of 2, 4, 6-triamino-1, 3, 5-diazine 1 (0.2 g, 1.59 mmol) in DMF (10 mL). The solution was degassed and stirred at room temperature under a nitrogen atmosphere for 1 h and the reaction was continued for 5 h at 70°C. The progress of the reaction was monitored by TLC. At the starting of the reaction, the mixture was a clear solution and after sometimes the reactants were turned into white solid precipitated and products was formed checked by TLC. After completion of the reaction, distilled H₂O was added. After the removal of solvent, the product was washed with sodium hydrogen carbonate solution and purified by recrystallization by using ethanol and found the desired product 3. White crystalline solid; yield: 90%; IR (KBr): δ max 3052.29, 2976.31, 1710.20, 1611.69, 1418.69, 1360.20 cm^-1.¹H NMR (300 MHz, CD₃OD): δ 7.80 (m, 12 H), 7.23 (m, 12 H), 4.96 (s, 1H) 2.3 (s, 18 H) ppm.¹³C NMR (75 MHz, CDCl₃): δ 21.63, 129.28, 130.50, 131.28, 144.63 and 172.33 ppm. MALDI-TOF MS: m/z (%) = calcd. for C₅₂N₅H₄₃O₆ Pd₃Cl₆ 1365.90; found 1369.44 (100) [M]⁺, Anal. Calcd. (%) for C₅₂N₅H₄₃O₆ Pd₃Cl₆: C, 45.70; H, 3.17; N, 5.13. Found: C, 45.67; H, 3.10; N, 5.10.

2.2. Catalytic Activity Tests: Palladium Mediated Metallodendrimer 3 in the Sonogasira Reaction

Typical procedure: In a Schlenk tube equipped with a magnetic stirring bar were placed under nitrogen atmosphere with 1.0 mmol of aryl halide, (1.2 mmol) of phenylacetylene, 1 mol % of Pd-metallodendrimer 3, CuI 0.5 mol%, Et₃N (2 mL) as a base and CH₃CN (5 mL) as a solvent. The tube was degassed and the flask was immediately placed in an oil bath. The resulting mixture was stirred at 60°C temperatures for 3h and the reaction was checked by TLC. Then the solvent was vaporized under reduced pressure and the residue obtained was purified by silica gel chromatography using ethyl acetate and hexane (4:1).

Synthesis of 4-(2-phenylethynyl)phenol 11, [22]

Solid product was obtained, Yield 92%, mp: 124-126°C; ¹H NMR (CDCl₃, 400 MHz): δ 5.08 (s, 1 H, OH); 6.75 (d, 2H, J=8.8 Hz); 7.29-7.35 (m, 3 H); 7.43 (d, J= 8.8, 2H); 7.55 (d, J=8.8, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 93.93, 114.13, 115.30, 123.97, 128.47, 129.03, 133.93, 134.43, 159.51.

Synthesis of 1-(2-p-tolylethynyl)benzene 12, [22]

Solid colourless product was obtained, Yield 95%, mp: 70-72°C (lit. 71°C); ¹H NMR (400 MHz): δ 2.31(s, 3 H); 7.20 (d, 2H, J= 8.8 Hz); 7.32 (t, 3H, J= 5.6 Hz); 7.33 (d, 2H, J=8.4 Hz); 7.54 (d, 2H, J= 8.0 Hz). ¹³C NMR (100 MHz, CDCl₃): δ 92.51, 93.13, 120.20, 122.23, 128.27, 128.47, 128.97, 132.03, 133.93, 139.13.

Synthesis of 1-(2-(4-methoxyphenyl)ethynyl)benzene 13, [23]

Solid white product was obtained, Yield 96%, mp: 54-56°C (lit. 57°C); ¹H NMR (400 MHz): δ 3.71 (s, 3H); 6.88 (d, 2H, J=8.8 Hz); 7.33 (t, J=10.8 Hz, 3H); 7.36 (d, 2 H, J=8.8 Hz); 7.44 (d, 2H, J=8.0 Hz), ¹³C NMR (100 MHz, CDCl₃): δ 55.51, 93.23, 113.23, 115.20, 123.37, 128.47, 129.47, 132.03, 133.93, 160.97.

2.3. Catalytic Activity Tests: Palladium Mediated Metallodendrimer 3 in the Heck Reaction

Typical procedure: A combination of 4-iododophenol (1 mmol) with styrene (1.2 mmol), Pd-metallodendrimer 3 (1.0 mol%) and triethylamine (1.2 mL) was stirred in DMF (5 mL) in an R.B flask under nitrogen environment. The solution was heated at 85°C for 24 hrs. The advancement of the reaction was observed with the aid of TLC (n-hexane/ethyl acetate 1:1). After the achievement of the desired conversion of the reaction, the reaction mixture was evaporated to dryness under reduced pressure and the residue was turned into extracted with chloroform.

The chloroform extract was washed with distilled water and dried over anhydrous Na₂SO₄, filtered and concentrated under reduced pressure. Then it was refined by silica gel column chromatography with n-hexane / ethyl acetate (3:1).

Synthesis of trans stilbene, 22 [24]

Colourless Solid was found and melting point was 74-76°C, Yield 94%, IR (KBr): δ max 3027, 1600.35, 1496, 1452.24, 1319.36, 1267.27, 962.35, 909.25, 733.74. ¹H NMR (400 MHz, CDCl₃), δ 7.02 (s, 2 H), 7.35 (dd, 1 H, J=1.2 Hz, 7.2 Hz), 7.43 (dd, 4 H, J=9.2 Hz, 7.2 Hz), 7.63 (dd, 4H, J=1.2 Hz, 8.8 Hz). ¹³C NMR (100 MHz, CDCl₃): δ 126.56, 127.61, 128.11, 128.85, 137.46.

Synthesis of (E)-Methyl 3-o-tolylacrylate 23, [24]

Yield 95%, IR (KBr): δ max 2950.32, 1722.32, 1639.35, 1267.27, 1220.20, 1172.21, 980.30, 764.27. ¹H NMR (400 MHz, CDCl₃), δ 2.48 (s, 3H); 3.84 (s, 3H); 6.46 (d, 1H, J=16.0 Hz); 7.35-7.68 (m, 3H); 7.83-7.98 (m, 1H); 8.33 (d, J=8.0 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 19.66, 51.56, 118.46, 126.17, 126.61, 130.11, 130.75, 133.56, 137.66, 142.46, 167.36.

Synthesis of 1-Methoxy-4-styrylbenzene 24, [24]

White solid product was found, Yield 96%, mp. 135-137°C, IR (KBr): δ max 2960.74, 1601.35, 1511.24, 1319.35, 1251.27, 1179.75, 1031.15, 966.36, 812.25. ¹H NMR (400 MHz, CDCl₃), δ 3.88 (s, 3H); 6.37 (d, 2H, J=8.0 Hz); 6.57 (d, 1H, J=16 Hz); 6.86 (d, 1 H, J=16 Hz); 6.39-6.29 (m, 1H); 7.58 (t, 2H, J=7.2 Hz); 7.94 (t, 4 H, J=9.2 Hz). ¹³C NMR (100 MHz, CDCl₃): δ 55.31, 114.29, 126.32, 126.76, 127.26, 127.86, 128.33, 128.77, 130.22, 137.34, 159.40.

3. Results and Discussion

Metallodendrimer 3 was synthesized by the reaction of 2,4,6-Triaminopyrimidine 1(1.59 mmol) with 4-methyl benzoyl chloride 2 (9.54 mmol) in the presence of (Ph₃P)₂PdCl₂ (10 mol%) in anhydrous DMF at room temperature to 70°C for 6 h under a nitrogen atmosphere (Scheme 1). The progress of the reaction was monitored by thin layer chromatography (TLC) and after complete conversion of the reaction, purification of the solid reaction mixture by recrystallization gave the dendrimerized product 3 in 92% yield (Table 1, entry 1). PdCl₂ was also effective and provided 3 in 65% (entry 2). Since (Ph₃P)₂PdCl₂ was given more yield than PdCl₂, we selected (Ph₃P)₂PdCl₂ for the later optimization. The use of tetrahydrofuran as the solvent caused in low yields of 3, because of the poor solubility of 3 (entry 3). DMSO also good solvent and gave 3 in 87% (entry 4). Increasing the amount of (Ph₃P)₂PdCl₂ from 10 to 20 mol% did not progress the final yield of 3, the yield reduced due to the creation of a complex as a byproduct instead of the desired product (entry 5). The product was found to be soluble in all common organic solvents and was characterized by IR, ¹H NMR, ¹³C NMR, and mass spectrometry.

Scheme 1

Table 1. Optimization for Preparation of Metallodendrimer 3

The assignments of IR spectral bands most useful in establishing the structural identity of metal complexes. The strong IR peak of 3 confirmed the presence of required groups of the compound. The appearance of the signal in the range of 3000-3100 cm^-1 and 1680-1760 cm^-1 represented the C-H, C=O group respectively whereas a strong absorption band at 1580-1620 cm^-1 displayed C=N. The peak at 1300-1360 and 1250-1335 cm^-1 confirmed the presence of C=C and C-N group respectively. C-Cl stretching absorption was observed at 761.8 cm^-1. Here it's far remarkable that there's no band for N-H in the place approximately 3400–3250 cm^-1 which turned into indicated that 1° amino group of triaminodiazine is transformed into 3° amido group. In ^IH NMR and ¹³C NMR spectra confirmed the preferred compound 3. Because of the formation of Pd-O coordinated bond, small adjustments within the aromatic area in comparison to starting materials indicated that metallic complexation has been achieved. There's no ¹H NMR peak for NH or NH₂ group, which confirmed the shape of compound 3. The self-assembly procedure of the compound 3 was investigated with Pd (II) as the central metal ions, and this led to a well-designated four coordinated complex that was confirmed through MALDI-TOF MS (Figures S 1- S 6 in the Supporting Information).

The images of the compound 3 were taken in a Scanning Electron Microscope (SEM) at an accelerating voltage of 10 KV with magnifications ranging from 100 µm-500 nm. SEM images of the compound 3 revealed that the formation of high component ratio randomly disbursed, entangled nanofibers. The length of the fibers is numerous µM. Coordination between the ligand and Pd (II) shaped branches of a tree without leaves and cylindrical fiber-like shape for compound 3 (Figure 1). Every chain stacked with neighboring chain by way of π-π stacking interaction and fashioned fibril morphology. The fibers are stabilized with the aid of π-stacking of the aromatic core of the ligands.

Figure 1. SEM images of the compound 3

Metal detection or evaluation of metallodendrimer has been achieved through employing the energy Dispersive X-ray (EDX) method. From EDX assessment, the existence of Palladium was well detected and it was 4.10% of weight and 0.56 % of atomic of the compound and also chlorine was 0.82% of weight and 0.33% of atomic of the compound 3. So it may be stated that palladium was encapsulated as palladium chloride in our preferred produced compound and the reaction was effective for the preparation of the required metallodendrimer 3.

We have reported new homogeneous palladium containing metallodendrimer 3 based on diazine catalyst which is an air-stable and very much effective catalyst for the Heck, Sonoghasira coupling reactions. In order to evaluate the activities of our catalytic system, the reaction between iodobenzene and phenylacetylene was initially selected for the Sonoghasira coupling reaction in the presence of a catalytic amount of the Pd-metallodendrimer 3 as a homogeneous catalyst (Scheme 2, Table 2). The catalytic systems revealed different reactivity depending not only on the palladium precursor but also on the nature of the base and the solvent and temperature.

Figure 2. TG and DSC curves of the compound 3

Scheme 2

Table 2. Optimization of the Pd-metallodendrimer 3 catalyzed Sonogashira reaction between 4-iodophenol and phenylacetylene

t-BuOK, Na₂CO₃, and K₂CO₃ with CH₃CN or DMF as the solvent were appropriate bases to complete the Sonogashira coupling without using CuI at 70°C for 3h with a yield higher than 50% (Table 2, entries 1–3). Interestingly, when NaOH, KOH with solvent CH₃CN, DMF, C₂H₅OH, CH₃OH as the solvent using CuI 0.5 mol% as co-catalyst and catalyst 1.0 mol% at 70°C the above Sonogashira reaction was carried out for 2.0 h above upto 80% yield was found (Table 2, entries 4–10).

Copper salts are used as co-catalysts within the Sonogashira reaction, even though it makes the Sonogashira method air sensitive. Notwithstanding this drawback, copper salts in the presence of a base have been used notably for the formation of Cu-alkynyl species that transmetallate to palladium within the catalytic cycle. Due to investigate the effect of CuI as a catalyst, a blank experiment was performed in the absence of a palladium-metallodendrimer 3 catalyst revealed that no reaction happened even within the presence of CuI (2 mol%) at 24 hour (Table 2, entry 11). In the presence of Et₃N, DMF, Pd-metallodendrimer 3 (1.0 mol%) as well as CuI (0.5 mol%), the reaction was completed within two hours without any deleterious effects at the conversions with 90% yield (Table 2, entry 12).

Therefore, the optimized reaction condition was found with 90% yield when 1.0 mol% of Pd-metallodendrimer 3 as a catalyst, 0.5 mol% CuI as a co-catalyst, Et₃N, and DMF were used under the nitrogen atmosphere at 70°C at 2 hour (Table 2, Entry 12). After optimization, we achieved the catalytic activities with more electron-donating or greater sterically hindered aryl halide substrates. Good to remarkable conversions were acquired with the greater electron-donating or extra sterically hindered methyl and methoxy-substituted aryl halide substrates under the chosen reaction catalyzed by catalyst 3 after 2h (Scheme 3, Table 3, entries 1–6).

Scheme 3

Table 3. Sonogashira coupling reaction of different aryl chlorides with phenylacetylene catalyzed by Pd-metallodendrimer 3

Reaction conditions: Aryl halide (1mmol), phenylacetylene (1.2 mmol), catalyst (1.0 mol%), base (1.2 mmol), solvent (5 mL), 70°C, 2h, under nitrogen atmosphere, Yield% was calculated based on aryl halide.

Likewise, due to optimization the Heck reaction conditions, a series of reactions under various combinations of bases, solvents, and temperatures, using Pd-metallodendrimer 3 as a catalyst, was pursued. Iodobenzene and styrene were designated as the classic substrates in this coupling reaction and the results have been presented in Scheme 4 (Table 4).

Scheme 4

Table 4. Optimization of the Pd-metallodendrimer 3 catalyzed Heck reaction between 4-iodobenzene and styrene

Reaction conditions: Iodobenzene (1 mmol), styrene (1.2 mmol), base (1.2 mmol), 24 hour, Temp. 85°C, Yields% was calculated on the basis of iodobenzene.

The coupled product was now not produced, when the coupling reaction was occoured with using 1.5 mol % Pd-metallodendrimer 3 as a catalyst, with styrene (1.2 mmol), and iodobenzene (1 mmol) in CH₃CN, DMF or DMSO (5 mL), and at temperature (Room temp. to 40°C) in the presence of NaOH, Na₂CO₃ or KOH, K₂CO₃ (1.2 equivalent) at 24 h (Table 4, entries 1-5). Fascinatingly, the reaction showed the development of 50% yield with 2.0 mol% of catalyst 3 in the presence of Et₃N and DMF at 60°C at 20 h (Table 4, entry 6). Furthermore, the usage of other bases containing of K₂CO₃, KO^tBu and in the presence of the solvents, methanol, toluene or DMF at 85°C, the coupling products were found above 60% yield (Table 4, entries 7-9). The best result was observed whilst 1.0 mol% of the catalytic system 3 and Et₃N, as a base, and solvent DMF was used under the nitrogen atmosphere at 85°C at 24 h for Heck coupling reaction (Table 4, entry 10).

A wide variety of olefins and diversely substituted aryl halides were selected for cross-coupling to produce the corresponding 1, 2-disubstituted olefins. The results have been summarized in table 5. Though aryl bromides and aryl iodides performed nicely (Table 5, entries 1-6) under these optimized Heck reaction conditions. However, while compared to the iodo analogues with bromo analogues, a reduced reactivity was discovered in the case of the corresponding bromo derivatives and was found less yield % of the products.

Table 5. Heck coupling reaction of different aryl chlorides with different olefins catalyzed by Pd-metallodendrimer 3

Above all coupling products for Sonogasira and Heck reaction were analyzed by spectroscopic methods and compared with authentic spectra. (Figures S 07-S 12 and S 13-S 21 in the supporting information)

A wide form of olefins and diversely substituted aryl halides were selected for cross-coupling to produce the corresponding 1, 2-disubstituted olefins. The results have been summarized in table 5. Although aryl bromides and aryl iodides completed the conversion properly (Table 5, entries 1-6) under those optimized Heck reaction conditions. But, even as compared to the iodo analogues with bromo analogues, a reduced reactivity was observed in the case of the corresponding bromo derivatives and turned into discovered less yield % of the product.

The coupling products for Sonogasira and Heck reaction were analysed by spectroscopic methods and as compared with proper spectra. (Figures S 07-S 12 and S 13-S 21 in the supporting informations)

4. Conclusions

In precis, a unique palladium mediated metallodendrimer 3 was synthesized through the coordination reaction of 2,4,6-triaminopyrimidine (1) with 4-methyl benzoyl chloride (2). The complexation was virtually determined by IR, ¹H NMR, ¹³C NMR, elemental analysis and mass spectra. The morphological structure of the catalyst like as branches of the tree without leaves or cylindrical fiber was revealed by SEM image and good thermal stability of the compound was observed by TG and DSC investigation. This homogeneous catalytic system showed numerous advantages including low catalyst loading, substrate tolerance, excellent yields, green solvents, short reaction times and a quite simple process for synthesizing Heck and Sonogashira coupling reaction products that are biologically vital in the numerous area in the chemistry world. These special upshots of this catalyst signify a noteworthy improvement in the area of C-C bond formation reactions.

ACKNOWLEDGEMENTS

I would like to express our cordial gratefulness to the Ministry of Science and Technology, Dhaka, Bangladesh (National Science& Technology Ph.D Fellowship program 2018-2019, No- 39.00.0000.012.002.03.18.25, Code no-1260101-120005100-3821117) and Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh for funding the economic support.

Supporting Information

Md. Sayedul Islam and Md. Wahab Khan

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