Polythene glycol (PEG) as a reusable solvent system for the synthesis of 1,3,5-triazines via aerobic oxidative tandem cyclization of benzylamines and N-substituted benzylamines with amidines under transition metal-free conditions

Abstract

A green and highly efficient protocol for the synthesis of 1,3,5-triazines from benzylamines and N-substituted benzylamines with amidines in PEG-600 has been developed. This protocol is transition-metal free, phosphine ligand free and uses inexpensive, easily available molecular oxygen (O₂) as an oxidant. A series of 1,3,5-triazines derivatives were synthesized in good to excellent yields in a shorter reaction time. The ease of the product separation and reusability of PEG-600 makes it more environmentally benign and economically affordable for gram-scale synthesis.

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Green Chemistry emphasizes the development of economically feasible, synthetic procedures that avoid the use of toxic transition-metals and the utilization of environmentally benign substances and non-toxic solvents.¹ Recently, transition-metal free reactions have gained prominence in organic synthesis.² Not only does it avoid the use of toxic transition-metals, but it also reduces the cost of the developed procedure. Oxidation reactions are very important transformations in organic chemistry. Molecular oxygen (O₂) is an ideal oxidant as it is inexpensive and easily available.³ In addition, having no toxic by-products make it a highly attractive reagent from the viewpoint of green sustainable chemistry.⁴ Most organic reactions are affected by a solvent as it plays an important role in mixing the ingredients to make the system homogeneous and allows molecular interactions to be more efficient.⁵ Thus, selection of a solvent is crucial. The solvent should be inexpensive, non-toxic, non-volatile and have easy recyclability. In this context, environmentally benign solvents such as water⁶ and ionic-liquids⁷ have been employed in various organic reactions. However, most organic moieties have a very low solubility in water which results in low yields of products. Further, the toxicity and environmental burden data for most of the ionic-liquids are still unknown. Over the past decade, polythene glycol (PEG) has been used in several organic reactions.⁸ PEG and its monomethyl ethers possess unique properties such as being thermally stable, commercially available, recyclable, immiscible with various organic solvents and non-toxic media for phase transfer catalysts.⁹ Further, PEG is inexpensive, completely non-halogenated and complete toxicity profiles are available for a range of polyethylene glycol (PEG) molecular weights. Many are approved for internal utilization by the US FDA.¹⁰

1,3,5-Triazine chemistry is of great importance due to their wide applications in biological and medicinal activities such as antimicrobial,^11a antimalarial,^11b antitumor agents^11c antituberculosis^11d and inhibition of photosynthetic electron transport (PET) and binding.^11e In addition, they are used as chelating ligands for the preparation of organometallic materials,¹² transition-metal catalysts,¹³ liquid crystals¹⁴ and fluorescent brighteners.¹⁵ Although 1,3,5-trianzines possess extensive functions, only a few methods for the synthesis of 1,3,5-triazines have been reported. Traditionally, they were synthesized from halogenated 1,3,5-triazines in the presence of transition metal-catalysts,¹⁶ and from the cyclotrimerization reaction of nitriles.¹⁷ However, the former requires transition-metal palladium (Pd), less-environmentally benign halogenated substrates, and it produces stoichiometric amounts of undesirable waste, and the latter usually needs an excess of amines as the co-catalysts. Alternatively, these compounds were also obtained by the cyclization of aromatic aldehydes with amidines.¹⁸ However, the use of aldehydes has several disadvantages such as the aldehydes could undergo a decarbonylation reaction under harsh reaction conditions¹⁹ and oxidation of active aldehyde groups, leading to the formation of unwanted by-products, hence they require inert conditions.²⁰ Recently, two methods for the synthesis of 1,3,5-triazines from benzyl alcohol and amidines have been reported (Scheme 1). Nevertheless, these methods require a costly ruthenium-complex²¹ or less environmentally begin Cu(OAc)₂ ²² as a catalyst. Moreover, the difficult preparation step of the Ru–phosphine complex, the use of toxic solvents such as DMSO and toluene, a longer reaction time, reflux conditions and an aqueous work up has imposed limitations on the applicability of these methods.

Scheme 1 Synthesis of 1,3,5-triazines.

In continuation with our ongoing work on transition-metal free approaches in organic synthesis,^2h herein, we report a transition-metal free method for the synthesis of 1,3,5-triazines. Notably, this reaction proceeds via aerobic oxidative tandem cyclization of benzylamines with amidines at 130 °C for 3h.

Initially, the direct reaction between benzylamine (1a) and benzamidine hydrochloride (2a) was selected as a model reaction to evaluate the feasibility of our system in an environmentally benign solvent at 100 °C for 24 h. When the reaction was carried out in H₂O, ethanol and glycerol, no formation of the desired product was noted (Table 1, entries 1–3). Surprisingly, changing the solvent system to PEG-600 resulted in the formation of the desired product 3a in a 70% yield (Table 2, entry 4). This surprise result encouraged us to choose PEG-600 as the solvent. To our delight, increasing the temperature has a significant effect on the yield of 3a, providing a 95% yield at 130 °C (Table 1, entries 5–7). Increasing the reaction temperature above 130 °C didn't have a significant effect on the yield of 3a (Table 1, entry 8). The reaction time could be reduced to 3 h from 24 h (Table 1, entries 9–11). Decreasing the reaction time beyond 3 h leads to a significant decrease in the yield of the desired product 3a (Table 1, entry 12). Before heating the reaction mixture was transparent, but after heating for 3 h it turned yellow in colour. The addition of DMSO resulted in a decrease in the yield of product 3a (Table 2, entry 13). When the reaction was carried out in the absence of a base, no formation of the desired product was observed, even after running the reaction for 24 h (Table 1, entry 14). Bases such as Na₂CO₃ and K₂CO₃ did not give good results for the formation of the desired product 3a (Table 1, entry 15). Further, decreasing the amount of base from 1.0 mmol to 0.5 mmol resulted in the formation of product 3a with only a 51% yield (Table 1, entry 15). Thus, the optimized reaction conditions are: benzylamine (1a, 0.5 mmol), amidine hydrochloride (2a, 1.0 mmol), Cs₂CO₃ (1.0 mmol), PEG-600 (2.5 mL) for 3 h under O₂.

Table 1 Optimization of reaction condition^a

Entry	Solvent	Temp (°C)	Time (h)	Yield^b (%)
a Reaction conditions: benzylamine (1a, 0.5 mmol), benzamidine hydrochloride (2a, 1.0 mmol), Cs2CO3 (1.0 mmol) and solvent (2.5 mL). b Isolated yield. c Na2CO3 was used instead of Cs2CO3. d K2CO3 was used instead of Cs2CO3. e Cs2CO3 (0.5 mmol).
1	H2O	100	24	—
2	EtOH	100	24	—
3	Glycerol	100	24	—
4	PEG-600	100	24	70
5	PEG-600	110	24	82
6	PEG-600	120	24	90
7	PEG-600	130	24	95
8	PEG-600	140	24	96
9	PEG-600	130	12	95
10	PEG-600	130	6	95
11	PEG-600	130	3	95
12	PEG-600	130	2	71
13	PEG-600 : DMSO (1 : 1)	130	3	64
14	PEG-600	130	3, 24	—, —
15	PEG-600	130	3	57^c, 78^d, 51^e

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Table 2 Aerobic oxidative cyclization of benzylamines with amidine hydrochlorides^a

Entry	Amines (1)	Amidines (2)	Products (3)	Yield^b (%)
a Reaction conditions: benzylamine (1, 0.5 mmol), amidine hydrochlorides (2, 1.0 mmol), Cs2CO3 (1.0 mmol), solvent (2.5 mL) for 3 h. b Isolated yield.
1				95
2		2a		96
3		2a		96
4		2a		71
5		2a		75
6		2a		83
7		2a		92
8		2a		89
9		2a		97
10		2a		96
11		2a		96
12	1a			82
13	1c	2b		85
14		2b		87
15	1c			80

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Encouraged by these results, we used various benzylamines and amidines to establish the scope and limitations of this protocol. A series of 1,3,5-triazines were synthesized in good to excellent yields under the optimized reaction conditions and representative results are listed in Table 2. Firstly, the effect of an electron donating group and an electron withdrawing group on benzylamine with benzamidine was studied (Table 2, entries 1–8). It was found that electron donating substituents such as –Me, –OMe, proceed smoothly and provided the desired products (3b–3c) in excellent isolated yields (Table 2, entries 2 and 3). Subsequently, the reaction of benzylamines bearing strong electron withdrawing groups such as –CN and –NO₂ at different positions provided corresponding products (3d, 3e and 3f) in good yields (Table 2, entries 4–6). Interestingly, halogen substituents such as –Cl and –F could also be transformed in an efficient manner, providing respective products in very good yields (Table 2, entries 7 and 8). Next, heteroatom containing benzylamines such as pyridine-2-yl-2-methanamine, pyridin-3-ylmethanamine (1i and 1j) and furan-2-ylmethanamine (1k) were tested. Delightfully, all the reactions progressed efficiently affording products 3i–3k in very good yields (Table 2, entries 9–11). These obtained heteroaryl substituted 1,3,5-triazines have potential to be used as C^N or C^N^C ligands in pincer complexes.²³ Next, an apparent substituent effect on amidine was also explored (Table 2, entries 12–15). Reaction of 1a, 1c and 1m with para-methylbenzamidine (2b) progressed very well affording 1,3,5-triazines (3l–3n) in excellent yields (Table 2, entries 12–14). Also, the reaction of 1c with para-bromobenzamidine resulted in the formation of the desired product (3o) with a good yield (Table 2, entry 15). Unfortunately, no formation of corresponding products could be observed when the reaction was carried out with both aliphatic amines and aliphatic amidines (results are not shown).

Efforts were also made to expand the scope of the method to N-mono and N,N-di substituted benzylamines and the results are summarized in Table 3. The reaction proceeded very well for N-methylbenzylamines (1aa) and N,N-dimethylbenzylamine (1aaa) providing the corresponding products in very good yields (Table 3, entries 1–6 and 8). Also, the reactions of N-ethylbenzylamine (1ab) and N-ethanolbenzylamine (1ab′) were found to be effective (Table 3, entries 7 and 10). However, the reaction of N,N-diethylbenzylamine (1abb) gave a relatively low yield of 3a compared to 1aaa (Table 3, entries 8 and 11).

Table 3 Aerobic oxidative cyclization of N-substituted amines with amidine hydrochlorides^a

Entry	Amine (1)	Amidine (2)	Product (3)	Yield^b (%)
a Reaction conditions: benzylamine (1, 0.5 mmol), amidine hydrochlorides (2, 1.0 mmol), Cs2CO3 (1.0 mmol), solvent (2.5 mL) for 3 h. b Isolated yield. c Reaction was carried out for 8 h.
1		2a	3a	94
2		2a	3c	95
3		2a		89
4	1aa	2b	3m	83
5	1ca	2b	3n	84
6	1ma	2b	3o	86
7		2a	3a	90
8		2a	3a	78, 82^c
9	1aaa	2b	3m	81
10		2a	3a	86
11		2a	3a	70^c

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To show the synthetic utility of this protocol, gram-scale reactions were carried out by using substrates 1a (2 g, 18.87 mmol) with 2a (5.89 g, 37.74 mmol) and 1c (2.0 g, 16.53 mmol) with 2a (5.16 g, 33.06 mmol) under the optimized reaction conditions. As per our expectation, the reaction preceded well by providing 3c and 3g in 91% and 85% isolated yields, respectively.

In order to understand the mechanism of these reactions, some control experiments were carried out (Scheme 2). When the reaction was carried out in the absence of amidines and Cs₂CO₃ formation of the imine (5a) was noted by self coupling of benzylamine. On the other hand, N-methyl and N,N-dimethylbenzylamine remain unchanged. This is in agreement with the previous report on the synthesis of imines.²⁴ However, in the presence of para-methoxyaniline (4a) formation of the imine (5aa) was observed from 1a, 1aa and 1aaa. The yields of these imines are very low even after running the reaction for 16 h. This is possibly because 4a is much less basic than the amidines. The formation of these imines (5a and 5aa) was confirmed by GCMS. This proves that the presence of another amine influences the reaction and leads to the formation of an aldehyde via oxidative cleavage of the benzylic carbon and nitrogen bond. Based on the observations of control experiments, a plausible reaction mechanism has been illustrated (Scheme 3). The reaction proceeds with the in situ generation of an aldehyde (A). Meanwhile, the amidine salt (2) is neutralized by Cs₂CO₃ from its hydrochloride salt. Consequentially, reacting with A to give 1,3,5-tirazine (3a) via dehydrogenative aromatisation of C. This proposed mechanism is consistent with the previous reports.^21,22 To the best of our knowledge, there is no report on the synthesis of imines from N-mono substituted benzylamine under transition-metal free conditions. This is the first time we have shown the formation of an imine from N-mono and di-substituted benzylamine under transition-metal free conditions. Further, unlike the previous reports,^24,25 the self coupling reaction of benzylamine does not involve the use of any activating agents such as a catalyst or acid.

Scheme 2 Control experiments.

Scheme 3 A plausible reaction mechanism.

At last, we attempted to reuse the PEG-600. After completion of the reaction, PEG was recovered and subjected to another run, affording the product in almost the same yield. This process was repeated three more times, affording the product in excellent yields (Table 4). It is important to note that weight loss of ∼10% of PEG was observed for every run due to handling loss. The simple experimental and ease of product separation combined with the easy recovery and reuse of PEG is expected to contribute to the development of a green methodology for the synthesis of 1,3,5-triazines.

Table 4 Recyclability study of PEG^a

Run	1	2	3
a Reaction conditions: benzylamine (1, 0.5 mmol), amidine hydrochlorides (2, 1.0 mmol), Cs2CO3 (1.0 mmol), PEG (2.5 mL) for 3 h. b Isolated yield.
Yield^b (%)	95	95	94

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Conclusions

In conclusion, a highly efficient, transition-metal free synthesis of 1,3,5-triazines has been developed by employing molecular oxygen as a green oxidant in PEG-600. The use of molecular oxygen and PEG-600 as a non toxic solvent has made this protocol economical, environmentally benign and potentially viable for commercial and academic applications. Various 1,3,5-triazines were synthesized in good to excellent yields. The developed methodology is suitable for gram-scale synthesis, owing to its simple non-aqueous work up, shorter reaction time and higher yield. This protocol offers an excellent chance to avoid toxic solvents, catalysts and use of natural resources compared to previous reports. Noteworthily, for the first time we have demonstrated the synthesis of imines from N-mono and N,N-di substituted benzylamines. Moreover, after extraction with ether, PEG-600 was recovered and could be reused for up to three consecutive cycles.

Acknowledgements

The author (ART) is greatly thankful to UGC (University Grant Commission) India, New Delhi for providing BSR fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR, HRMS and GCMS. See DOI: 10.1039/c5gc01884f

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