An efficient one-pot access to N-(pyridin-2-ylmethyl) substituent biphenyl-4-sulfonamides through water-promoted, palladium-catalyzed, microwave-assisted reactions

Abstract

An efficient one-pot, Pd(PPh₃)₄ catalyzed, water-promoted method for the synthesis of N-(pyridin-2-ylmethyl) biphenyl-4-sulfonamides was developed under microwave irradiation. This methodology is acid free, has good substrate scope, excellent functional group compatibility, and excellent product yields, and is superior to the existing procedures for the synthesis of biphenyl-4-sulfonamides bearing a pyridin-2-ylmethyl group.

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The biphenyl-4-sulfonamides have been recognized as having significant biological and pharmacological activities such as anticancer,¹ anti-tumor,² antibacterial,³ anti-allergic,⁴ antiviral,⁵ and anti-inflammatory activities.⁶ Two derivatives in this family, valdecoxib and celecoxib, have been introduced to the market as anti-inflammatory drugs.⁷

The biphenyl-4-sulfonamide derivatives, N-(pyridin-2-ylmethyl) biphenyl-4-sulfonamides also exhibit crucial biological activity.⁸ In particular, agricultural chemists have focused on the core structure to produce an abscisic acid (ABA) agonist. For example, pyrabactin (Py)⁹ has been successfully designed to simulate the function of ABA which plays a key role in overcoming abiotic stresses such as drought, cold and soil salinity, as well as in plant development.¹⁰ Due to the attractive biological properties of biphenyl-4-sulfonamides, a variety of strategies to synthesize the core structure have been developed.^1–7,11 And, the palladium-catalyzed reactions are confirmed to be the most efficient approaches.¹¹ Recently, Covel et al.⁸ reported a Suzuki coupling reaction to construct N-(pyridin-2-ylmethyl) biphenyl-4-sulfonamides (Scheme 1, method a). However, the yield is near 60%. This could be the result of forming a complex between the N-(pyridin-2-ylmethyl) group and palladium.¹² Recent research also showed that the Boc protection of the sulfonamino group could greatly promote the coupling reaction (method b), while a strong acid was required for deprotection.¹³ Otherwise, the sulfonylation reaction required very stable reactants.¹⁴ Moreover, a boiling water-catalyzed neutral and selective N-Boc deprotection from aromatic heterocycles, aromatic amines, aliphatic amines, and amides has been developed.¹⁵ So, we proposed that the Suzuki coupling of N-Boc biphenyl-4-sulfonamides followed by deprotection in one pot could produce the N-(pyridin-2-ylmethyl) biphenyl-4-sulfonamides conveniently. To the best of our knowledge, there has never been a report on a one-pot palladium-catalyzed, water-promoted reaction to afford this scaffold under microwave irradiation before.

Scheme 1 Selected methods to synthesize biphenyl-4-sulfonamides.

Our recent success in the application of microwave irradiation¹⁶ prompted us to try the deprotection of 1a under microwave irradiation. As shown in Table 1, enhancing the reaction temperature could accelerate the deprotection and increase the reaction yields (Table 1, entry 1–5). Subsequently, our study focused on the screening of solvents (Table 1, entry 5–8). We found the water-solubility of the solvent was important for the reaction. The better the water solubility the higher the product yield. As a result, dioxane–water was proved to be the most suitable solvent.

Table 1 The water-promoted SO₂N-Boc deprotection^a

Entry	Solvent (4 : 1)	T (°C)	t (min)	Yield^b (%)
a Reaction conditions: 1a 0.5 mmol, solvent 5 mL.b Isolated yield.c Conventional heating method.
1	Dioxane : H2O	85	6	<5
2	Dioxane : H2O	100	6	<5
3	Dioxane : H2O	110	6	76
4	Dioxane : H2O	120	8	92
5	Dioxane : H2O	130	8	96
6	DME : H2O	130	8	93
7	DMF : H2O	130	8	90
8	Toluene : H2O	130	15	<5
9^c	Dioxane : H2O	Reflux	600	<5

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However, we found that the deprotection of 1a under conventional heating conditions was very difficult. Hence, we chose 1a and 2a as a model to optimize the one-pot reaction conditions. As shown in Table 2, 64% of the coupling and deprotection product (3aa) was obtained with a temperature of 120 °C (Table 2, entry 1) under microwave irradiation. Process monitoring indicated that 1a could be removed directly and rapidly before the Suzuki coupling to produce 1aa at a high temperature (120 °C). Therefore, in order to improve the yield of 3aa, we should carry out the coupling reaction at a low temperature and the subsequent Boc-deprotection reaction at a high temperature. Under this assumption, we optimized the temperature for the Suzuki coupling to be 85 °C (Table 2, entry 2–4) and the temperature for the Boc-deprotection reaction to be 130 °C, respectively. Finally, the yield increased to 94% by prolonging the reaction time to 10 min (Table 2, entry 5). We next explored the effect of bases on this reaction, and among the bases we investigated (Na₂CO₃, K₂CO₃, K₃PO₄ and Cs₂CO₃), Na₂CO₃ was identified as the best one (Table 2, entry 5–8).

Table 2 Optimization of the reaction conditions^a

Entry	R	Solvent (4 : 1)	Base	Pd(PPh3)4	T1 (°C), t1 (min)	T2 (°C), t2 (min)	Yield^b (3aa)	Yield^b (3a)	Yield^b (1aa)
a Reaction conditions: 1 (0.5 mmol), 2a (0.5 mmol), base (1.0 mmol), solvent (5 mL), protected by N2.b Isolated yield.c Conventional heating, t2 = 600 min.
1	Boc	Dioxane : H2O	Na2CO3	2 mol%	120, 10	—, —	64	0	29
2	Boc	Dioxane : H2O	Na2CO3	2 mol%	80, 5	130, 8	35	0	58
3	Boc	Dioxane : H2O	Na2CO3	2 mol%	85, 5	130, 8	53	0	42
4	Boc	Dioxane : H2O	Na2CO3	2 mol%	90, 5	130, 8	48	0	36
5	Boc	Dioxane : H2O	Na2CO3	2 mol%	85, 10	130, 8	94	0	0
6	Boc	Dioxane : H2O	K2CO3	2 mol%	85, 10	130, 8	91	0	0
7	Boc	Dioxane : H2O	K3PO3	2 mol%	85, 10	130, 8	89	0	<5
8	Boc	Dioxane : H2O	Cs2CO3	2 mol%	85, 10	130, 8	86	0	<5
9	Boc	Dioxane : H2O	—	2 mol%	85, 10	130, 8	0	0	96
10	Boc	Dioxane : H2O	Na2CO3	—	85, 10	130, 8	0	0	95
11	Boc	Toluene : H2O	Na2CO3	2 mol%	85, 10	130, 8	<5	86	<5
12	Boc	DME : H2O	Na2CO3	2 mol%	85, 10	130, 8	92	0	<5
13	Boc	DMF : H2O	Na2CO3	2 mol%	85, 10	130, 8	87	0	<5
14	H	Dioxane : H2O	Na2CO3	2 mol%	85, 10	130, 8	63	0	29
15^c	Boc	Dioxane : H2O	Na2CO3	2 mol%	85, 90	Reflux, 600	<5	81	<5

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In addition, we noted that the coupling reaction will not take place in the absence of catalyst or base, but the deprotection reaction can still go smoothly (Table 2, entry 9 and 10). Subsequently, we screened the reaction solvents, such as water mixed with dioxane, toluene, DMF, and DME (Table 2, entry 5, 11–13). The mixture of dioxane and water (v/v = 4 : 1) was proved to produce the best results. On the contrary, the coupling reaction between 1aa and 2a under the same conditions produced much lower yield of 3aa (Table 2, entry 14). Compared with conventional heating (Table 2, entry 15), microwave irradiation can significantly accelerate the reaction and notably improve the yield of the product.

With the optimized reaction conditions in hand, we screened the substrate scope of the one-pot reaction of the Suzuki coupling and deprotection. Various aryl and heteroarylboronic acids were tested, and the results were summarized in Table 3. The reactions between 1a and arylboronic acids always went smoothly. Both electron-donating and electron-withdrawing groups, such as methoxy, t-butyl, halogen (F, Cl), and trifluoromethyl groups, afforded the corresponding products in excellent yields (Table 3, entry 2–8, 12 and 13). The reaction yields of brominated or nitrated phenylboronic acids were moderate due to the self-coupling or other unexpected side reactions (Table 3, entry 9–11). More importantly, the reaction was proved to be well tolerant of valuable but unstable groups, such as hydroxyl, acetyl and formyl (Table 3, entry 14–16). In addition, disubstituted and heteroarylboronic acids were also investigated and afforded the desired products in good to excellent yields (Table 3, entry 17–24). To further examine the efficiency of this one-pot reaction and rapidly expand our unique compound collection, we also carried out the reaction between 4-bromo-N-Boc-N-(pyridin-2-ylmethyl)-naphthalene-1-sulfonamide (1b) and phenylboronic acids (2), which produced the desired products in good to excellent yield (Table 4, entry 1–9).

Table 3 Scope of arylboronic acid (2) and 1a^a

Entry	Boronic acid (2)	t1 (min)	3	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2 (0.5 mmol), Pd(PPh3)4 (2 mol%), Na2CO3 (1.0 mmol), dioxane : H2O = 4 mL : 1 mL, T2 = 130 °C, t2 = 8 min, protected by N2.b Isolated yield.
1	2a: R^1 = R^2 = R^3 = R^4 = H	10	3aa	94
2	2b: R^1 = R^2 = R^4 = H, R^3 = OCH3	10	3ab	94
3	2c: R^1 = R^2 = R^4 = H, R^3 = ^tBu	10	3ac	95
4	2d: R^1 = R^2 = R^4 = H, R^3 = F	15	3ad	94
5	2e: R^1 = R^3 = R^4 = H, R^2 = F	15	3ae	93
6	2f: R^2 = R^3 = R^4 = H, R^1 = F	15	3af	90
7	2g: R^1 = R^2 = R^4 = H, R^3 = Cl	15	3ag	93
8	2h: R^1 = R^3 = R^4 = H, R^2 = Cl	15	3ah	92
9	2i: R^1 = R^3 = R^4 = H, R^2 = Br	15	3ai	53
10	2j: R^1 = R^2 = R^4 = H, R^3 = Br	15	3aj	51
11	2k: R^1 = R^2 = R^4 = H, R^3 = NO2	25	3ak	58
12	2l: R^1 = R^2 = R^4 = H, R^3 = CF3	20	3al	91
13	2m: R^1 = R^2 = R^4 = H, R^3 = OCF3	15	3am	93
14	2n: R^1 = R^2 = R^4 = H, R^3 = Ac	20	3an	90
15	2o: R^1 = R^3 = R^4 = H, R^2 = CHO	15	3ao	93
16	2p: R^1 = R^3 = R^4 = H, R^2 = OH	15	3ap	87
17	2q: R^2 = R^4 = H, R^1 = F, R^3 = Cl	15	3aq	91
18	2r: R^1 = R^4 = H, R^2 = Cl, R^3 = Cl	25	3ar	92
19	2s: R^1 = R^3 = H, R^2 = Cl, R^4 = Cl	25	3as	92
20	2t: R^1 = R^4 = H, R^2 = CH3, R^3 = F	15	3at	91
21	2u: [1,1′-biphenyl]-4-ylboronic acid	15	3au	92
22	2v: furan-2-ylboronic acid	20	3av	93
23	2w: dibenzo[b,d]furan-4-ylboronic acid	25	3aw	85
24	2x: pyridin-3-ylboronic acid	20	3ax	89

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Table 4 Scope of phenylboronic acid (2) and 1b^a

Entry	Boronic acid (2)	t1 (min)	3	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2 (0.5 mmol), Pd(PPh3)4 (2 mol%), Na2CO3 (1.0 mmol), dioxane : H2O = 4 mL : 1 mL, T2 = 130 °C, t2 = 8 min, protected by N2.b Isolated yield.
1	2b: R^1 = R^2 = R^4 = H, R^3 = OCH3	10	3bb	93
2	2c: R^1 = R^2 = R^4 = H, R^3 = ^tBu	10	3bc	95
3	2d: R^1 = R^2 = R^4 = H, R^3 = F	10	3bd	91
4	2e: R^1 = R^3 = R^4 = H, R^2 = F	15	3be	92
5	2h: R^1 = R^3 = R^4 = H, R^2 = Cl	15	3bh	93
6	2j: R^1 = R^3 = R^4 = H, R^2 = Br	15	3bj	52
7	2m: R^1 = R^2 = R^4 = H, R^3 = OCF3	15	3bm	91
8	2q: R^2 = R^4 = H, R^1 = F, R^3 = Cl	15	3bq	90
9	2r: R^1 = R^4 = H, R^2 = Cl, R^3 = Cl	15	3br	91

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Through a further high-throughput computational screening, compounds 3aa and 3ai have been found to be active on the ABA receptor PYL1. Very interestingly, both compounds are predicted to bind PYL1 in a similar fashion to a known ABA agonist, pyrabactin (Fig. 1). The sulfonamide functional group of 3ai and 3aa can form a hydrogen bond with residue E100. The pyridine ring can form another hydrogen bond with residue K65 and a π–π stacking interaction with residue Y126 (Fig. 1B and C). The binding differences can be found in the other side of the pocket. There is a hydrophobic interaction between the bromine atom and residue P94. However, the biphenyl group makes 3ai and 3aa bind with P94 more closely. The estimated binding free energies are −8.89 kcal mol⁻¹ and −8.20 kcal mol⁻¹ for 3ai and 3aa, respectively, which are a little lower than that of pyrabactin (−7.98 kcal mol⁻¹). The root growth inhibition experiment showed that the Arabidopsis thaliana plant is a little more sensitive to the treatment of 3ai, 3aa, and pyrabactin, than ABA (Fig. 1D). As shown in Fig. S1 (ESI†), the root growth inhibition rates of 3ai, 3aa, and pyrabactin were 82 ± 12%, 98 ± 5%, and 94 ± 7%, respectively, while that of ABA was only 68 ± 13%.

Fig. 1 Computational modeling of Py (A), 3ai (B) and 3aa (C) in PYL1 (PDB code: 3NEG). (D) The Arabidopsis plant wild type lines show sensitivity in root growth, upon a 100 μM treatment with these chemicals. (E) The chemical structures of Py, 3aa, and 3ai.

In summary, we have reported an efficient method to prepare N-(pyridin-2-ylmethyl) biphenyl-4-sulfonamides via a palladium-catalyzed, water-promoted and microwave-assisted one-pot reaction. In addition, the excellent reactivity and broad substrate scopes make the developed methodology operationally concise and facilitates rapid library construction. Further efforts to examine the bioactivity of these compounds are underway and will be reported in due course.

Acknowledgements

The research was supported in part by the National Natural Science Foundation of China (No. 21332004 and 21202055), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20201263001), and the Fok Ying-Tong Education Foundation (No. 142017).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, characterization data, and ¹H, ¹³C and HRMS. CCDC 1057970. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13302e

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