Blue LED-driven synthesis of benzoyl-guanidine hybrids via radical–radical cross-coupling

Abstract

We describe a radical–radical cross-coupling strategy for guanylation of benzoyl-thiourea derivatives with diverse secondary amines to afford bioactive benzoyl-guanidine hybrids under blue LED irradiation in a greener reaction medium. Moreover, this effective, high-atom-economy (90%) protocol involves molecular cleavage (via Mumm rearrangement), formation of revised fragments, and reassembly of selective radical fragments. In addition, the work demonstrates the post-modification of benzoyl guanidine to afford bioactive isoquinolinone scaffolds via ruthenium-catalyzed oxidative annulation with diphenylacetylene.

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The commanding influence of amine functionality on small molecules can significantly tune the pharmacokinetics and bioactivity of functional molecules.^1a Hence, the inclusion of such functionalities has foremost importance in synthetic organic chemistry and chemical biology. In this regard, guanidine, a N-enriched bioactive hybrid, has received considerable attention in the pharmaceutical and functional heterocycles due to its well-known bioactivity,^1b,c such as anti-bacterial,^2a anti-fungal,^2a anti-protozoal,^2b anti-tumor,^2c and anti-leishmanial activity.^2d Guanidine-based drug moieties, including pinacidil (treatment of hypertension),^3a NC-147 (a synthetic sweetener),^3b HBD-F (an anti-bacterial agent),^3c and SM-111^3d (an anti-HIV agent), have brought guanidine to the forefront of research (Scheme 1a). These compounds are not only biologically significant but also important intermediates in synthetic chemistry.⁴

Scheme 1 (a) Importance of guanidine frameworks, (b) synthetic exploration of benzoyl-guanidine, (c) photoredox catalysed synthesis of benzoyl-guanidine.

Moreover, the strong basicity of guanidine makes it an efficient organic superbase, enabling its application in both stoichiometric and catalytic organic transformations.⁵ Additionally, guanidines are also used as chiral auxiliaries and chiral Bronsted base organo-catalysts in numerous asymmetric synthesis reactions, including Mannich-, Michael-, and Henry-type reactions.⁶ However, the synthetic routes for guanylation are limited and suffer from several disadvantages, such as the use of stoichiometric oxidants, metals, and desulfurizing agents (Scheme 1b).⁷ To promote guanylation, guanylating precursors like carbodiimides,^8a isothioureas,^8b triflylguanidines,^8c cyanamides,^8d amidines,^8e pyrazole-1-carboximidamides,^8f and benzoyl-thioureas^8g are mainly required. In the previously documented methodologies, Wacharasindhu et al. (2020) reported a Ru(bpy)₃Cl₂ photocatalytic approach to access guanidine.^9a However, the synthesis of benzoyl-guanidine has several challenges, yet seminal reports by Badshah et al. (2012)^9b and Saeed et al. (2025)^9c notably utilized highly toxic HgCl₂ in their respective preparation. Besides, the Brito group (2015) and Tamminana et al. (2021) reported the synthesis of benzoyl-guanidine from benzoyl-thiourea and amines using TBHP and iodine (50 mol%), respectively, as oxidants (Scheme 1b).^9d,e

Driven by the growing demand for sustainable¹⁰ and energy-efficient processes,¹¹ photoredox chemistry has emerged as a powerful platform for rapid organic transformations under mild conditions.¹² Although rearrangement reactions involving bond cleavage and reconstruction offer atom-economical synthetic routes,^13a–c many reported methods suffer from poor efficiency, are not selective, and result in the loss of components during fragmentation.^13d To address these limitations and the longstanding desulfurization challenges in efficient guanylation, we have introduced a mild metal-free photoredox electron transfer protocol in a green solvent under blue LED irradiation, which offers high atom economy (90%).

The exploration commenced with a systematic optimization of the reaction parameters (Tables S1–S5). Initially, we employed benzoyl-thiourea 1a and piperidine 2a as model substrates. The reaction was carried out in acetonitrile as a solvent using K₂CO₃ as a base and rose bengal (RB) as a photocatalyst under blue LED irradiation for 8 hours. Under these circumstances, the desired benzoyl-guanidine 3a (CCDC no. 2524079) was observed with a commendable 65% yield. Furthermore, screening of photocatalysts revealed that eosin-Y, rhodamine-6G, and g-C₃N₄ remained inferior to RB and produced 20–60% yields, respectively (Table S1). Furthermore, the systematic screening of solvents like polar, non-polar, and a mixture of solvents revealed that EtOH/H₂O (9 : 1) was found to be the optimal solvent, producing 3a in 80% yield (Table S2). Subsequently, after screening various bases (Table S3), we found that the K₂CO₃/EtOH : H₂O (9 : 1) combination was the best condition by considering its low toxicity and cost-effectiveness. Additionally, the effect of light and the photocatalyst was studied (Tables S4 and S5). In the absence of blue LED irradiation and a photocatalyst, the absence of guanylation emphasizes the importance of photonic activation during the course of excitation and the importance of the photocatalyst in guanylation via the electron transfer process, respectively. Moreover, the role of oxygen in the reaction was investigated by carrying out the reaction under an argon atmosphere. The suppressed yield of the desired product indicates the importance of oxygen in the photocatalytic cycle.

After the optimization of reaction conditions, we expanded the scope of the reaction by testing diverse amines with 1a under the optimized conditions. Secondary aliphatic amines such as 4-benzylpiperidine, morpholine, thiomorpholine, pyrrolidine, and allylamine were smoothly reacted with 1a under optimized conditions, producing the respective benzoyl-guanidines 3b–3f in high yields (70–76%). Furthermore, the scope was extended with diverse piperazines, and the reaction yielded the desired products in good yields. When N-Boc-piperazine, N-phenylpiperazine, N-(4-methoxyphenyl)piperazine, N-benzylpiperazine, and N-benzhydrylpiperazine were reacted with 1a, the corresponding benzoyl-guanidines 3g–3k were formed in moderate to good yields (65–71%). Next, we examined the effects of various substituents on 1a. For instance, when a benzoyl-thiourea containing halide (R² = 4-Br) was reacted with piperidine, the desired product 3l was obtained in 60% yield. The decrease in yield may be due to the electron-withdrawing (EWG) inductive effect of Br, which reduces the nucleophilicity of the thiourea nitrogen.^14a Similarly, benzoyl-thiourea substituted with an electron-donating group (EDG) like n-butyl (R² = 4-ⁿBu) was reacted with piperidine, providing benzoyl-guanidine 3m in good yield (76%). Unfortunately, a strong electron-withdrawing group (R² = 4-NO₂) substituted benzoyl-thiourea failed to produce 3n, likely because the strong electron-withdrawing nature of the nitro group significantly decreases the electron density on the thiourea nitrogen and adjacent sulfur atom (Scheme 2).^14b

Scheme 2 Substrate scope. ^aReaction conditions: 1a (0.5 mmol), amine (1.5 equiv.), K₂CO₃ (2 equiv.), and rose bengal (1 mol%) in EtOH : H₂O (9 : 1, 4 mL) under blue LED irradiation at room temperature for 8 h. ^bGram-scale synthesis (4 mmol).

Moreover, the effect of substituents on the benzoyl side was also investigated, where R¹ = 4-Me, 4-Cl, and 4-NO₂ substituted benzoyl-thioureas were smoothly reacted with piperidine, producing the compounds (3o–3q) in yields of 64–68%. After successful guanylation of benzoyl thiourea using secondary amines and piperazine derivatives, we further expanded the scope of the reaction with heterocyclic thiourea. When thiophenoyl-thiourea 1g and furanoyl-thiourea 1h were reacted with piperidine, only thiophenoyl-thiourea was able to produce the desired product in 60% yield (3aa and 3ba). This difference likely arises from the higher oxidative sensitivity and low stability of the furan ring compared to thiophene under photoredox conditions, thereby making it more susceptible to oxidative decomposition.^14c,d Furthermore, the substrate scope was extended by reacting 1g with various secondary amines such as 4-benzylpiperidine, morpholine, thiomorpholine, pyrrolidine, and allylamine, producing the respective benzoyl-guanidines 3ab–3af in moderate yields (60–65%). Moreover, the heterocyclic scope was extended with diverse piperazine derivatives. N-Boc-piperazine, N-phenylpiperazine, N-(4-methoxyphenyl)piperazine, N-benzylpiperazine, and N-benzhydrylpiperazine were reacted with 1g, affording the desired heterocyclic-guanidines 3ag–3ak in moderate to good yields (61–69%). Furthermore, we assessed the viability of our protocol for gram-scale reactions using 4 mmol of 1a under standard conditions. The targeted product 3a was obtained in 70% yield, highlighting that the protocol was readily scalable. Moreover, the post-modifications of the obtained product were demonstrated through ruthenium-catalyzed oxidative annulation of compounds 3a and 3c with diphenylacetylene to afford bioactive isoquinolinone scaffolds (4a and 4b) in 64–69% yields (Scheme 3a).¹⁵

Scheme 3 (a) Post-synthetic modification of benzoyl-guanidine, (b–d) control experiments, (e and f) radical scavenger studies, (g) byproduct trapping study.

To gain insight into the mechanism, a series of experiments were performed (Schemes 3b–g). 1a was subjected to irradiation with a blue LED under standard conditions for 8 hours in the absence of an amine source. Surprisingly, the starting material 1a was consumed, and two different fragments, Int-I¹⁶ and Int-II, were observed, which were isolated and characterized by NMR spectroscopy and HRMS. Additionally, the crystal structure of Int-II was obtained from SC-XRD (CCDC no. 2524076). Moreover, when the isolated fragments Int-I and Int-II were reacted with piperidine under standard conditions, product 3a was obtained in 75% yield. These findings suggested that Int-I and Int-II were the first key intermediates in this photoredox pathway. Next, Int-I was reacted with thiourea Int-III under standard conditions, resulting in the formation of guanidine 3a with 70% yield. Moreover, the radical scavenging experiment was also performed. When the reaction of 1a and piperidine was carried out under standard conditions in the presence of a radical scavenger (TEMPO), the yield of 3a was suppressed. This confirms that the reaction proceeds through a radical pathway. Moreover, we succeeded in trapping the three crucial radical intermediates 5, 6, and 7 in the radical scavenging experiment, as observed by HRMS (Scheme 3e). Similarly, when Int-I was irradiated with a blue LED in the presence of TEMPO (2 equiv.) in EtOH : H₂O, the radical intermediate 8 was observed in HRMS. Furthermore, we succeeded in trapping the precipitate of the sulfur byproduct after the addition of Pb(OAc)₂ into the reaction mixture (Scheme 3g). The trapping experiment was conducted in MeCN as a solvent using DBU as a base to preclude interference from solid K₂CO₃.¹⁷ The resulting yellow precipitate was filtered and characterized by powder XRD (Fig. S5) and the resulting data corroborate that the byproduct was lead sulfate.

Moreover, UV-visible absorption spectra of all compounds (1a, piperidine, K₂CO₃, Int-I, Int-II, and RB) in EtOH : H₂O (9 : 1) were recorded (see the SI). The observations reveal that electron transfer prevails rather than the energy transfer process, which is because the absorption spectra of the individual molecules do not overlap with the emission spectrum of RB (Fig. S8).¹⁸ Furthermore, we conducted Stern–Volmer fluorescence quenching experiments¹⁸ in the presence of RB (Fig. S11–S16). The results demonstrate that the excited state of RB* was majorly quenched by Int-III, which facilitated the first reductive quenching cycle of RB*. Moreover, we measured the redox potential of Int-III and RB by cyclic voltammetry experiments¹⁹ (see the SI, Fig. S17 and S18). From the experimental results, it was found that the oxidation potential of Int-III (E_ox) was 0.68 V vs. Ag/AgCl and the reduction potential of excited state rose bengal (RB*) (E_red*) was 0.91 V vs. Ag/AgCl. These findings suggest that Int-III can participate in single electron transfer (SET) with RB*. Accordingly, it is reasonable to propose that RB* can oxidize Int-III through a reductive quenching pathway.

Based on the results obtained from the above investigations, we proposed a mechanistic pathway. The reaction proceeded via a Mumm-type rearrangement^20a–d of 1a to 1a′ intermediate, which was then fragmented into two separate intermediates (Int-I and Int-II) in the presence of a base and ethanol. Furthermore, the thioamidation^20e,f of Int-II occurred to produce intermediate Int-III (E_ox = 0.68 V vs. Ag/AgCl), which was later deprotonated by a base, followed by SET with RB* (E_RB*/RB = 0.91 V vs. Ag/AgCl)^21a,b to generate radical intermediate Int-IV. Subsequently, RB was regenerated by a back single-electron-transfer (BET) from reduced state RB to molecular oxygen O₂ (in air).^21c,d This resulted in the formation of superoxide (O₂), which further participated in oxidative coupling with radical intermediate Int-IV to produce peroxysulfur intermediate Int-V, followed by elimination of sulfate to produce the radical intermediate²² Int-VI. Meanwhile, intermediate Int-I under irradiation with a blue LED produced radical intermediate Int-VII, which further underwent radical–radical cross-coupling with radical intermediate Int-VI to generate intermediate Int-VIII. Ultimately, hydrolysis of intermediate Int-VIII yielded the target benzoyl-guanidine 3a (Scheme 4).

Scheme 4 Proposed mechanism.

We have developed an efficient and sustainable approach to access benzoyl-guanidine hybrids via a radical–radical cross-coupling mechanism. A library of benzoyl-guanidines were synthesised with high atom economy (up to 90%). The reaction proceeds via molecular cleavage (via Mumm-type rearrangement), fragment modification, and selective reassembly of radical fragments.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental procedures, detailed information regarding optimization of the reaction conditions, detailed mechanistic investigations, characterization data of products, NMR (¹H and ¹³C) spectral data, HRMS spectral data of new products, and X-ray crystallographic data of products. See DOI: https://doi.org/10.1039/d6cc01721e.

CCDC 2524076 and 2524079 contain the supplementary crystallographic data for this paper.^23a,b

Acknowledgements

Pratheepkumar Annamalai gratefully acknowledges DST-SERB, India (Project File No. SRG/2023/002161) for financial support and Vellore Institute of Technology, Vellore, for providing the Seed Grant Fund (SG20220089). Krishnarao Desai acknowledges Vellore Institute of Technology, Vellore, for providing a research fellowship. We thank VIT-SIF for the instrumentation facilities.

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