Electrochemically dehydrogenative C(sp²)–H/S–H cross-coupling: efficient synthesis of ortho-aminophenyl thioglycoside derivatives

Abstract

We have developed an electro-mediated method for synthesizing aryl thioglycosides through intermolecular anodic oxidative cross-dehydrogenative C(sp²)–S bond coupling reactions involving (un)protected 1-thiosugars and anilines. This protocol is sustainable without the use of external transition-metal catalysts or additional oxidants employed in previous methods. It demonstrates a broader substrate scope concerning both 1-thiosugars and anilines. Furthermore, the reaction is applicable to the late-stage functionalization of drugs. Mechanistic studies using cyclic voltammetry and control experiments reveal that a radical cross coupling process is implicated in this transformation.

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Introduction

Glycosides are widely present in the structural framework of many natural products and drugs.¹S-Glycosides, as an important class of glycosides, have received comparatively less attention than O-glycosides. These glycomimetics possess significant attributes, including resistance to chemical hydrolysis or enzymatic degradation, while maintaining comparable biological activity to their O-glycosides counterparts.² Consequently, S-glycosides have become the focus of extensive investigation as potential pharmaceutical agents in the field of medicinal research, serving as mimetics of natural O-glycosides (Fig. 1).³ Furthermore, S-glycosides are frequently employed as glycosylation donors to establish a diverse range of glycosidic linkages due to their ease of handling and rapid activation under various conditions (Fig. 1).⁴ The broad biological and synthetic applicability has triggered tremendous interest in the development of efficient methods for constructing S-glycosides. Traditional methods for synthesizing aryl thioglycosides primarily depend on nucleophilic substitution reactions between glycosyl donors and aryl thiophenols. However, these procedures involve harsh reaction conditions, and the resulting products often exhibit poor stereoselectivity.⁵ In addressing this issue, Zhu's group and Walczak's group independently reported methods involving the reversal of polarity at the anomeric carbon. However, the use of alkyl tin/lithium regents restricts the scope to saccharides, although this elegant strategy enables stereoselective control.⁶ In recent years, a synthetic strategy of aryl thioglycosides via transition metal catalysis has been developed, in which 1-thiosugars or their precursors reacts with an aryl halide or arylboronic acid under the catalysis of Cu, Pd, or Ni, resulting in the formation of the desired thioglycoside derivative (Scheme 1a).⁷ Although the synthesis of aryl thioglycosides has made remarkable progress, certain limitations persist, such as high catalyst loadings, extended reaction times, and the requirement for specialized phosphine ligands. In 2019, Messaoudi and co-workers reported a method for synthesizing aryl thioglycosides through a single-electron Ni/photoredox dual-catalyzed cross coupling reaction.⁸ This method is suitable for protected thiosugars and aryl iodides, although some aryl bromides are not compatible with this protocol. Additionally, the Messaoudi group developed the first electrochemical method for coupling 1-thiosugars with aryl, alkenyl, and alkynyl bromides under nickel-catalyzed conditions.⁹ This method requires the addition of expensive catalysts and ligands, although the substrate scope has been broadened to a certain extent (Scheme 1b).

Fig. 1 Selected bioactive compounds and thioglycoside donors.

Scheme 1 Previous methods for the synthesis of aryl thioglycosides and this work.

In recent years, electrochemical dehydrogenative cross-coupling reactions have shown new possibilities for green organic synthesis due to their ability to avoid substrate prefunctionalization, ensuring highly atom-economic construction of C–C and C–heteroatom bonds.¹⁰ As early as 2019, Lei and co-workers discovered an electrooxidative para-selective C–H/N–H cross-coupling between arenes and diarylamine derivatives.¹¹ This transformation required no external oxidants with hydrogen gas as the sole byproduct (Scheme 1c). To the best of our knowledge, the strategy of employing direct C–H/S–H dehydrogenative cross-coupling for constructing C–S bonds has been rarely reported.^10c–e,12 We sought to investigate whether the synthesis of aryl thioglycosides could be achieved through anodic oxidative C(sp²)–H/S–H dehydrogenative cross-coupling under redox-catalyst-free conditions. The present study is outlined herein (Scheme 1). The reaction conditions are mild enough to tolerate free hydroxyl groups and complex molecules.

Results and discussion

To assess the feasibility of this study, we selected the coupling of tetra-O-acetylated 1-thio-β-D-glucopyranose 1a (1.0 equiv.) with 1-(4-methylphenyl)pyrrolidine 2a (1.0 equiv.) as a model study under various reaction conditions (Table 1 and Tables S1–S3†). 30% of 3a could be obtained as a single β-anomer by using ⁿBu₄NBF₄ as the electrolyte, and dry CH₃CN as the solvent under a constant electric current of 9.0 mA at room temperature in an undivided cell equipped with a graphite plate as the anode and a platinum plate as the cathode (Table 1, entry 1). ⁿBu₄NBF₄ was identified as the optimal electrolyte, with other electrolytes like ⁿBu₄NPF₆ and TBAB proving less effective (Table S1†). When the graphite plate anode was replaced with reticulated vitreous carbon (RVC), 2a can be consumed completely, despite 3a was afforded with similar yield (Table 1, entry 5). This suggests that the efficiency of the RVC electrode in this reaction is higher than that of the graphite electrode, to the extent that an unwanted side reaction of aniline occurred. To our delight, by reducing the electric current to 3.0 mA and employing RVC as the anode, a 70% isolated yield of the desired product 3a was observed (Table 1, entry 8). These experimental results indicated that the product 3a is unstable under a high current. For instance, it may undergo further oxidation at the anode. Moreover, a comparable reaction efficiency was achieved by increasing the amount of the 2a from 1.0 equiv. to 2.0 equiv. (Table 1, entry 9). The optimization of the reaction conditions was extended to include different solvents (Table 1, entries 10–12). However, no significant improvement in the yield of 3a was observed with DMF, MeOH, and DMSO. Further research showed that the reaction in an air atmosphere was less efficient and gave product 3a in only 15% yield (Table 1, entry 13). This may due to the fact that 1-thiosugars are easily oxidized by oxygen. The presence of electricity was critical for the transformation (Table 1, entry 14).

Table 1 Reaction optimization^a

Entry	Deviation from above	Yields^b (%)
a Conditions A: reticulated vitreous carbon RVC anode, platinum plate cathode, constant current = 3.0 mA, 1a (0.3 mmol, 1.0 equiv.), 2a (0.3 mmol, 1.0 equiv.), ^nBu4NBF4 (1.0 mmol), CH3CN (6.0 mL), Ar, room temperature, 4 h. b Isolated yield.
1	C(+)|Pt(−), 9.0 mA, 2 h	30
2	C(+)|C(−), 9.0 mA, 2 h	7
3	GC(+)|CF(−), 9.0 mA, 2 h	Trace
4	Pt(+)|Pt(−), 9.0 mA, 2 h	15
5	RVC(+)|Pt(−), 9.0 mA, 2 h	28
6	C(+)|Pt(−), 60 °C	60
7	Pt(+)|C(−),60 °C	15
8	RVC(+)|Pt(−)	70
9	RVC(+)|Pt(−), 2 equiv. of 2a	68
10	RVC(+)|Pt(−), DMF as solvent	Trace
11	RVC(+)|Pt(−), MeOH as solvent	20
12	RVC(+)|Pt(−), DMSO as solvent	18
13	RVC(+)|Pt(−), open-air	15
14	No electric current	N.R.

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With the optimal conditions in hand (standard conditions A), the substrate scope with respect to the sugar component was investigated. As shown in Table 2, a variety of mono-, di- and trisaccharides-derived 1-thiosugars were initially subjected to the optimal conditions. D-Galactose, N-acetyl-D-glucosamine, 2-deoxy-D-glucose, L-arabinose, and L-fucose-derived 1-thiosugars underwent this reaction smoothly to give aryl thioglycosides 3f–j in 65–75% yields, which all maintain the configuration of glycosyl thiols. In cases involving D-glucose containing common protecting groups, such as benzoyl, benzyl, pivaloyl and silyl, the reaction proceeded smoothly, affording 3b–e in good yields. To further demonstrate the synthetic utility of our protocol, we evaluated whether the S-arylation of unprotected thiosugars or cysteine amino acid could be realized using this electrochemical method. To our delight, the reaction of unprotected 1-thio-β-D-glucopyranose and L-cysteine yielded the respective products 3k and 3l in moderate yields. In addition, adamantane (3m, 68% yield), lithocholic acid (3n, 65% yield), and ibuprofen (3o, 60%) could also be introduced into aryl thioglycosides. Moreover, this coupling reaction exhibited good compatibility with disaccharides, such as lactose and cellobiose, leading to the formation of disaccharide products 3p (55% yield) and 3q (59% yield). The trisaccharide maltotriose was also examined, and the desired product 3r was obtained with 65% yield.

Table 2 Substrate scope of thiosugars^a

a Conditions A: reticulated vitreous carbon RVC anode, platinum plate cathode, constant current = 3.0 mA, 1 (0.3 mmol, 1.0 equiv.), 2a (0.3 mmol, 1.0 equiv.), ⁿBu₄NBF₄ (1.0 mmol), CH₃CN (6.0 mL), Ar, room temperature, 4 h.

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The electro-oxidative dehydrogenative reaction was further investigated with various substituted tertiary arylamines, affording the corresponding aryl thioglycosides 3s–aj in reasonable yields (Table 3). We were pleased to find that anilines 2s–u, bearing morpholine, piperidine, and acyclic amine on the aryl rings, were applicable under the optimal reaction conditions. The desired products 3s–u could be obtained with moderate yields from 35 to 50%. Gratifyingly, it is found that anilines with N-protecting groups could react with thiosugar 1a to generate the corresponding products 3v–w in good yields (62–67%). A variety of tertiary arylamines bearing electron-donating groups, such as tertiary butyl and methoxyl, were examined, yielding moderate yields (Table 3, 3x–y, 60–67%). The interaction between 1a and anilines substituted with weakly electron-withdrawing groups (F, Cl, and Br) resulted in the desired products in yields ranging from 65% to 67% under the optimal conditions (Table 3, 3z–ab). However, when aniline substrates with strongly electron-withdrawing groups (CN, CF₃, and COOCH₃) were used, the target compounds were not detected, primarily due to their poor electronic effect on oxidation potential (2adE_p = 1.23 V; 2aeE_p = 1.18 V).^13,14 In contrast, 3af–ag could be obtained when the strongly electron-withdrawing groups were not directly linked to the phenyl group (2afE_p = 1.09 V; 2agE_p = 0.91 V). Additionally, phenyl-substituted tertiary arylamine was tolerated in this reaction, and the desired product 3ah could be obtained in 65% yield. Similarly, tertiary arylamine bearing another pyrrolidine was also suitable for the reaction, giving product 3ai in 56% yield. Notably, α-naphthylamine was also compatible with the electrolysis protocol (Table 3, 3aj, 50%). It should be noted that unsubstituted tertiary arylamines or tertiary arylamines with ortho-substitution (2ak, E_p = 0.84 V, see the Supporting Information for details) do not participate in the reaction under this system. This may be attributed to the lower oxidation potential of tertiary arylamines with para-substitution (2a, E_p = 0.79 V).

Table 3 Substrate scope of tertiary arylamines^a

a Conditions A: reticulated vitreous carbon RVC anode, platinum plate cathode, constant current = 3.0 mA, 1a (0.3 mmol, 1.0 equiv.), 2 (0.3 mmol, 1.0 equiv.), ⁿBu₄NBF₄ (1.0 mmol), CH₃CN (6.0 mL), Ar, room temperature, 4 h.

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It is worth mentioning that when N, N, 4-trimethylaniline 2b was subjected to the reaction conditions, both the product 3ak (via C(sp²)–S coupling, 18% yield) and 3bk (via C(sp³)–S coupling, 18% yield) were obtained. After a series of optimizations of conditions (Tables S4 and S5†), the target product 3ak (CCDC 2324843†) was obtained in 65% isolated yield utilizing ⁿBu₄NBF₄ as the electrolyte and dry CH₃CN as the solvent under a constant electric current of 3.0 mA at 60 °C. The reaction took place in an undivided cell equipped with a platinum plate as the anode and a graphite plate as the cathode. Notably, 3bk was not produced under these conditions (Table S5,† entry 1).

With these encouraging results, we next investigated the scope and limitations of this electro-oxidative dehydrogenative reaction using a series of N, N-dimethylanilines 2 bearing different functional groups. As showed in Table 4, cross-couplings of 1-thio-β-D-glucopyranose with N, N-dimethylanilines bearing various functions (–Me, –Et, –iPr, –Br, –F, –OMe) at para positions have been successfully achieved, affording the corresponding thioglycosides (3al–av) in yields up to 72%. Besides, this coupling reaction tolerates different glycosyl thiols such as O-acetylated 1-thio-β-D-glucopyranose 1ar, O-acetylated 1-thio-β-D-galactopyranose 1as, O-acetylated 1-thio-β-D-mannopyranose 1at, O-acetylated 1-thio-β-D-fucopyranose 1au, and O-acetylated 1-thio-β-D-xylopyranose 1av. These substrates react smoothly with (4-methoxyphenyl)-dimethylamine, leading to thioglycosides 3ar–av in yields up to 70%.

Table 4 Substrate scope of tertiary arylamines and thiosugars^a

a Conditions B: platinum plate anode, graphite plate cathode, constant current = 3.0 mA, 1 (0.3 mmol, 1.0 equiv.), 2 (0.3 mmol, 1.0 equiv.), ⁿBu₄NBF₄ (1.0 mmol), CH₃CN (6.0 mL), Ar, 60 °C, 4 h. b Graphite plate anode, platinum plate cathode, constant current = 9.0 mA, 1 (0.3 mmol, 1.0 equiv.), 2 (0.3 mmol, 1.0 equiv.), ⁿBu₄NBF₄ (1.0 mmol), CH₃CN (6.0 mL), Ar, room temperature, 2 h.

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To demonstrate the practical applicability of our strategy, several synthetic transformations of aryl thioglycoside 3w were conducted (Scheme 2a). Deprotection of 3w could easily proceed with H₂ under Pd(OH)₂/C catalysis, yielding ortho-aminophenyl thioglycoside 3w-1 in 75% yield. 3w-1 could undergo a condensation reaction with ibuprofen to give 3w-2 in 80% yield. 3w-3 could be synthesized from 3w-1 and cinnamaldehyde using reductive amination. 3w-1 can also be converted into an aryl diazonium salt and then reacts with KSCSOEt to form 3w-4. The sulfoxide compound 3w-5 could be obtained through the oxidation of 3w-1 with mCPBA in 38% yield. The ortho-isocyanophenyl thioglycoside 3w-6 (ICPT donor) could be prepared from 3w-1 in a 90% overall yield in two steps (formyl protection and dehydration). Subsequently, the C-glycosylation of 3w-6 with our previously developed “Boomerang” strategy proceeded smoothly to provide C-nucleoside analogue 3w-7 in 70% yield with an α configuration (Scheme 2a).^4a The deprotection and ester condensation reaction of 3e proceed sequentially, yielding the probenecid derivative 3e-1 in useful overall yields. Furthermore, mixing 3ab and loratadine boronic ester with Pd(dppf)Cl₂ and K₂CO₃, the Pd-catalyzed Suzuki–Miyaura coupling occurred smoothly, resulting in the loratadine derivative 3ab-1 in 78% yield (Scheme 2b).

Scheme 2 Synthetic application.

To gain more insight into the reaction mechanism, we conducted the following experiments. The addition of TEMPO completely shut down the reaction, and the possible reactive intermediate generated from 1a was trapped by TEMPO, as monitored by HR-MS in the meantime (Scheme 3a). Next, under the standard conditions, electrolysis was carried out in the presence of 2.0 equiv. of 4, which is used as a radical acceptor. This led to the formation of 5 and 6 in 15% and 10% yield, respectively, and impeded the generation of 3a (Scheme 3b). These results indicated that this S-glycosylation reaction probably undergoes a radical pathway. Glycosyl thiol radical and aniline radical cation intermediate might be involved in the transformation. In a control experiment in the absence of aniline substrates, 1-thiosugar 1a underwent dimerization to afford disulfide 7 in 95% yield (Scheme 3c). Then the reaction between disulfide 7 and 2a was conducted. When a small amount of methanol was added as a proton source, 3a was obtained in 15% yield under the standard conditions (Scheme 3d). Cyclic voltammetry (CV) experiments of 1-thiosugar 1a, aniline 2a, coupling product 3a and the mixture of 1-thiosugar 1a and aniline 2a were performed. As shown in Fig. 2a, the oxidation peak of 2a was observed at 0.79 V, while the oxidation peak of 1a was observed at 0.58 V, indicating that 1a easily undergoes an oxidation reaction at the anode. Moreover, the oxidation peak of 3a was observed at 0.91 V, and the mixture of 1a and 2a featured two main oxidation potentials, one for aniline 2a (E_1a+2a = 0.79 V) and the other for 3a (E_1a+2a = 0.92 V). This result indicated that the oxidation of both substrates is possible under the standard conditions, and 1a seems to be more easily oxidized at the anode. As for the cathode, concurrent release of molecular hydrogen was disclosed by GC analysis (Fig. 2b).

Scheme 3 Mechanistic experiments.

Fig. 2 (a) Cyclic voltammograms of 1a, 2a, the mixture of 1a and 2a, and 3a in 0.1 M ⁿBu₄NBF₄ in MeCN, using glassy carbon as working electrode, Pt wire as counter electrode, and Ag/AgCl as reference electrode at 0.1 V s⁻¹ scan rate; (b) GC detection for H₂.

Based on the above mechanistic studies and literature reports,^10d,15 we proposed a plausible mechanism as depicted in Scheme 4. Firstly, 1-thiosugar 1a loses an electron under the oxidation of the anode to form glycosyl thiol radical A, which can undergo dimerization to generate a disulfide 7. Simultaneously, aniline 2 is oxidized to produce radical cation B, which then produces radical-cation C through tautomerization. The generated aniline radical-cation intermediate C can undergo either direct coupling with the thiyl radical A (primary path) or radical substitution (low efficiency) with the generated disulfide 7 to afford a cation intermediate D. Final successive deprotonation and aromatization of the cation intermediate afford the desired aryl thioglycoside 3. At the cathode, 1-thiosugar 1a is reduced to give hydrogen gas during the reaction.

Scheme 4 Proposed reaction mechanism.

Conclusions

We have developed an external metal-/oxidant-free, electrocatalytic method to synthesize aryl thioglycosides through the C(sp²)–H/S–H dehydrogenative cross-coupling between 1-thiosugars and anilines. The only by-product of the reaction is hydrogen released at the cathode, making the method clean and providing enormous atom-economic advantages. A variety of monosaccharides (D-galactose, N-acetyl-D-glucosamine, D-ribose, L-arabinose, D-glucose, 2-deoxy-D-glucose, D-mannose, D-glucuronide, and L-fucose), disaccharides (lactose, and cellobiose), polysaccharide (maltotriose), and L-cysteine can couple with a series of non-prefunctionalized aniline derivatives using our developed method. Interestingly, thiosugars with four hydroxy groups were compatible. Moreover, the method is amenable to late-stage modification of complex molecules, and the thioglycoside products can be used as potential glycosyl donors to construct O-glycosides or C-glycosides. We believe that this simple and effective synthetic protocol will become a useful tool for the synthesis of aryl thioglycosides.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This project is supported by the Fundamental Research Funds for the Central Universities, Nanjing Agricultural University (no. XUEKEN2022032).

References

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14. All of potentials were measured versus an Ag/AgCl reference electrode.
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Footnote

† Electronic supplementary information (ESI) available. CCDC 2324843. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo00171k

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