Simple, one-step syntheses of tri- and tetracyclic B,N,S-heterocycles from reactions of a diboracumulene with thiols

Abstract

Boron-containing fused heterocycles have received considerable attention due to their unique structures, bonding, and intriguing properties. However, progress in this area has been hampered by a lack of efficient synthetic methodologies. In this study, we demonstrate the reactivity of a diboracumulene, LBBL (L = cyclic (alkyl)(amino)carbene, CAAC), with various nitrogen-containing heterocyclic thiols. These straightforward reactions result in the partial or complete cleavage of the B–B multiple bonds in the diboracumulene, along with S–H bond activation in the thiol units, leading to the formation of a range of tricyclic and tetracyclic B,N,S-heterocycles. Notably, the tricyclic B,N,S-heterocycle exhibits π aromaticity in all three of its rings.

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Introduction

Fused heterocyclic compounds are common structural motifs found in natural products, pharmaceuticals, and functional materials, and they have garnered significant attention in recent years.^1–7 In particular, nitrogen-containing fused heterocycles play a crucial role in the development of various drugs, pesticides, and functional materials.^2,3,5–7 Notably, the incorporation of electron-deficient boron atoms into these fused heterocycles has led to the discovery of a wide range of unique compounds (Fig. 1A), owing to boron's ability to modulate the energies and distributions of frontier molecular orbitals and their ability to interact with biologically-relevant molecules.^8–19 For example, boron-containing heterocyclic drugs such as tavaborole (I) and crisaborole have been approved by the United States FDA, while many others are currently undergoing clinical trials.⁸ Boron-doped heterocycles II–IV display distinctive photophysical and optoelectronic properties (Fig. 1A).^9–11 Despite these promising applications, existing synthetic methods for constructing boron-based fused heterocycles remain limited, primarily relying on boron-doping via salts or small-molecule elimination, or metallacycle transfer.^20–26 Also, these methodologies are mostly restricted to the use of B₁-containing reagents (Fig. 1B). However, Liu and coworkers recently demonstrated that the cycloaddition reaction between bis-diazidoborane and isonitriles can yield boron-containing tricyclic heterocycles.²⁷ Further, we showed that the insertion of azides into the B–B single bond of cyclic diboranes results in the formation of novel B,N-heterocycles.²⁸ In light of the increasing interest in boron-containing heterocycles, efforts have intensified to identify unique diboron-based synthons and to develop innovative strategies for their incorporation into the synthesis of fused heterocycles.

Fig. 1 (A) Examples of fused B,N/O-heterocycles. (B) General synthetic route to fused B-containing heterocycles. (C) Synthesis of fused B,N,S-heterocycles via B–B multiple bond activation (this work). (L = CAAC).

In this context, the highly reactive diboracumulene [B₂(CAAC)₂] (1),²⁹ featuring electron-rich B–B and B–C(CAAC) multiple bonding, exhibits a broad and versatile reactivity profile.^30–41 Notably, B₂L₂ (L = CAAC or NHC)^29,42 species featuring B–B multiple bonds exhibit a wide range of intermolecular bond activation reactions via 1,2-addition mechanisms, including the activation of H–H,³³ B–H,³⁶ C–H,^39,40 B–B,³⁷ C–O,^34,35 S–S,³⁸ and Se–Se³⁸ bonds. Thus, diboracumulene 1 was seen as a promising diboron source for the synthesis of boron-based fused heterocycles. Recently, we have shown that the reaction of diboracumulene 1 with pyridine yielded a tricyclic diazadiborinine derivative.⁴³ On the other hand, the S–H bond has proven to be highly reactive. N-containing heterocyclic thiols, which possess such reactive S–H units, are well-suited for 1,2-addition reactions involving both nitrogen and sulfur centres following S–H activation.⁴⁴ Thus, the combination of diboracumulene 1 with various N-containing heterocyclic thiols offers a potential strategy for constructing fused B,N,S-heterocycles. Herein, we report the synthesis of various tricyclic and tetracyclic B,N,S-heterocycles via S–H bond activation, accompanied by partial or complete cleavage of the B–B multiple bonds.

Results and discussion

The room-temperature reaction of diboracumulene 1 with two equivalents of 1-phenyl-1H-tetrazole-5-thiol in benzene led to a colour change from purple to colourless (Scheme 1). The ¹¹B NMR spectrum of the resulting colourless reaction mixture showed a new chemical shift at δ = 10.0 ppm, significantly upfield from that of diboracumulene 1 (δ = 80.0 ppm). After workup, a colourless solid (2) was isolated in 75% yield. A single-crystal X-ray diffraction (SCXRD) analysis revealed 2 to comprise a tetracyclic B,N,S-heterocycle, resulting from the S–H activation of two thiol groups and their symmetrical coordination to the B₂ unit from both sides through nitrogen and sulfur centres in a μ₂,η² binding mode (Fig. 2 (left)). The tetracyclic 2 is formed by the fusion of two planar five-membered CN₄ rings and two envelope-shaped five-membered CB₂NS rings. The hydrogen atoms from the S–H units migrate to the CAAC ligands upon activation (H1 and H2, Fig. 2 (left)), which is also evident from a singlet ¹H NMR signal at δ = 4.40 ppm, consistent with reported chemical shifts for protonated CAAC units.^43,45,46 Notably, the B–B bond (1.846(6) Å) in 2 is significantly longer than the B–B multiple bond (1.489(2) Å) in 1 (ref. 29) and falls in the expected range of B–B single bond of diboranes.^30,31,47,48 The B1–C3 (1.639(7) Å) and B2–C4 (1.649 (5) Å) bond distances in 2 also indicate single covalent character. The fusion of two N-containing heterocyclic thiol units across the B–B bond in 2 leads to a nearly eclipsed conformation with a cis arrangement of (H–CAAC)–B–B–(CAAC–H) moiety. The small S1–B1–B2–N5 (22.9(1)°) and S2–B2–B1–N1 (22.6(1)°) dihedral angles reflect the rigidity of the fused tetracyclic system. The N–B distances (N1–B1: 1.594(6) Å; N5–B2: 1.590(5) Å) fall within the range typical for dative N→B bonds,^44,49 whereas the S–B distances (S1–B1: 2.032(5) Å; S2–B2: 2.017(8) Å) indicate single covalent bonds. Thus, the fused tetracyclic heterocycle 2 can also be interpreted as a doubly base-stabilised, symmetrical cyclic diborane(4).

Scheme 1 Synthesis of tetracyclic B,N,S-heterocycles 2–4.

Fig. 2 Crystallographically-determined molecular structures of 2 (left) and 4 (right). Ellipsoids are shown at the 50% probability level. All hydrogen atoms and ellipsoids of the ligand periphery are omitted for clarity. Selected bond lengths of 2 (in Å): B1–B2 1.846(6), B1–S1 2.032(5), B2–S2 2.017(8), B1–N1 1.594(6), B2–N5 1.590(5), B1–C3 1.639(7), and B2–C4 1.649 (5). Selected bond lengths of 4 (in Å): B1–B2 1.733(2), B1–S1 1.954(1), B2–S2 1.980(1), B1–N2 1.599(2), B2–N1 1.622(1), and B2–C3 1.637(2).

Further, we have carried out the room-temperature reaction of diboracumulene 1 with another five-membered heterocyclic thiol, namely 1-methyl-1H-imidazole-2-thiol (2 equivalents) in benzene, which also led to the isolation of a colourless solid 3 in 77% yield (Scheme 1). The ¹¹B NMR spectrum of 3 showed a signal at δ = 15.1 ppm, slightly downfield of that of tetracyclic 2 (δ = 10.0 ppm). Also, the ¹H NMR spectrum of 3 displayed a pattern similar to that of tetracyclic 2. For example, a singlet ¹H NMR signal at δ = 4.19 ppm was observed for 3, which is slightly upfield relative to the peak at δ = 4.40 ppm corresponding to the migrated hydrogens from S–H to the CAAC ligands in tetracyclic 2. These findings suggest the formation of an analogue of 2. Indeed, a SCXRD analysis confirmed 3 as a tetracyclic B,N,S-heterocycle, in which four five-membered rings are fused (Fig. S21). However, due to disorder, the structural parameters of 3 cannot be discussed in detail.

To investigate the effect of heterocyclic thiol ring size on the formation of fused heterocycles, diboracumulene 1 was treated with a six-membered heterocyclic thiol, namely pyridine-2-thiol (2 equivalents). After 16 hours, the reaction led to the isolation of a colourless solid 4 in 92% yield (Scheme 1). Unlike the symmetrical tetracyclic B,N,S-heterocycles 2 and 3, the ¹¹B NMR spectrum of 4 exhibited two distinct signals at δ = 10.8 and 0.8 ppm. In line with the ¹¹B NMR spectrum, the SCXRD study of 4 showed an unsymmetrical tetracyclic B,N,S-heterocyclic species (Fig. 2 (right)). Although the core geometry of 4 is composed of four fused rings similar to tetracyclic 2 and 3, it differs by consisting of two six-membered and two five-membered rings. The dissymmetry in tetracyclic 4 arises from the presence of one B–H unit and one B–(CAAC–H) unit, rather than two B–(CAAC–H) units as found in 2 and 3. This also suggests that one of the CAAC ligands of diboracumulene 1 is substituted by a hydrogen atom transferred from the S–H group of pyridine-2-thiol to form 4. The presence of a terminal B–H unit in fused 4 is further confirmed by the ¹H{¹¹B} NMR signal at δ = 4.45 ppm and the IR stretching band observed at 2465 cm⁻¹. Meanwhile, the other hydrogen from a second S–H group migrates to the carbene carbon of the second CAAC ligand, facilitating the formation of compound 4. The B–B bond length in 4 (1.733(2) Å) is significantly shorter than that in fused 2, though it still falls within the typical range for B–B single bonds in diboranes.^30,31,47,48 The S–B distances in tetracyclic 4 (S1–B1: 1.954(1) Å; S2–B2: 1.980(1) Å) are also slightly shorter than those observed in tetracyclic 2. In contrast, the B2–C3 (1.637(2) Å) and N–B (N2–B1: 1.599(2) Å; N1–B2: 1.622(1) Å) distances of tetracyclic 4 are comparable to those in 2, indicating covalent B–C and dative N→B single bonds, respectively. Similar to tetracyclic 2, the fusion of two N-containing six-membered heterocyclic thiol moieties across the B–B bond in 4 results in a nearly eclipsed structure, featuring a cis arrangement of (H–CAAC)–B–B–(H). Notably, the dihedral angles S1–B1–B2–N1 (10.4(1)°) and S2–B2–B1–N2 (5.9(1)°) in tetracyclic 4 are considerably smaller than those in 2, suggesting a more rigid fused tetracyclic framework in 4.

Although no intermediates were detected for tetracyclic 2 and 3, the reactions likely proceed via initial adduct formation between the diboracumulene and two thiol units at the nitrogen center, followed by rapid 1,2 S–H addition to both B–C_CAAC units (Scheme S1). In the case of tetracyclic 4, initial adduct formation is also expected; however, the use of a six-membered thiol introduces steric repulsion between the CAAC and the pyridine derivative. This rapidly results in the simultaneous dissociation of one CAAC ligand, 1,1 S–H addition at the exposed boron center, and 1,2 S–H addition at the other B–C_CAAC unit, leading to the formation of 4 (Scheme S1).

Having prepared a series of tetracyclic B,N,S-heterocycles 2–4, we carried out density functional theory (DFT) calculations at the ωB97X-D/Def2-SVP level of theory to gain insight into their structure and bonding. The B–B bond in optimised 4 (1.737 Å) is shorter than those of fused 2 (1.848 Å) and 3 (1.854 Å), possibly due to reduced steric crowding resulting from the smaller substituent at B1 in the former. Nonetheless, all three B–B bond lengths fall within the typical range for B–B single bonds, which is also evident from the Wiberg bond indices (WBIs) of the B–B bonds in 2–4 (Table 1). On the other hand, the natural population analysis of 4 showed a lower natural charge on the boron atom bearing a hydrogen substituent (q_B1 = 0.01) than the other boron atom (q_B2 = 0.39), suggesting comparatively higher electron density at B1. This polarisation in the B–B bond in 4 is also reflected in the experimental (δ = 0.8 and 10.8 ppm) and calculated (δ = 2.2 (B1) and 11.5 (B2) ppm) ¹¹B NMR spectra. Also, the calculated ¹¹B NMR chemical shifts for fused 2 (δ = 11.7 ppm) and 3 (δ = 11.9 ppm) are in good agreement with the experimental values (δ = 10.0 and 15.1 ppm, respectively). Further, the highest occupied molecular orbitals (HOMOs) of fused 2 and 3 mainly correspond to the σ-bonding interactions through the H–C_CAAC–B–B–C_CAAC–H unit (Fig. 3), while the HOMO–4 of fused 4 depicts σ-bonding interactions in the S–B–B–S and B–H units (Fig. 3).

Table 1 Experimental and calculated bond lengths (d), WBIs, natural charges at boron (q_B) and ¹¹B NMR chemical shifts of fused 2–4^a

	B–B			B–N			B–S			qB	Exp. ^11B NMR [ppm]	Cal. ^11B NMR [ppm]
	Exp. d [Å]	Calc. d [Å]	WBI	Exp. d [Å]	Calc. d [Å]	WBI	Exp. d [Å]	Calc. d [Å]	WBI
a Due to disorder, the structural parameters of 3 cannot be discussed.
2	1.846 (6)	1.848	0.91	1.594 (6)	1.594	0.87	2.032 (5)	2.020	0.94	0.41	10.0	11.7
				1.590 (5)	1.594	0.87	2.017 (8)	2.020	0.94
3	a	1.854	0.91	a	1.590	0.87	a	2.021	0.94	0.42	15.1	11.9
4	1.733 (2)	1.737	0.96	1.622 (1)	1.622	0.82	1.980 (1)	1.981	1.01	0.39 (B2)	10.8 (B2)	11.5 (B2)
				1.599 (2)	1.597	0.85	1.954 (1)	1.956	1.05	0.01 (B1)	0.8 (B1)	2.2 (B1)

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Fig. 3 Selected frontier molecular orbitals of fused tetracyclic compounds 2–4 (isovalue 0.043 a.u.).

We subsequently turned to bicyclic thiol systems to investigate whether steric effects influence the formation of fused rings. The room-temperature reaction of diboracumulene 1 with two equivalents of benzo[d]thiazole-2-thiol in benzene afforded a colourless solid 5 in 75% yield (Fig. 4). The ¹¹B NMR spectrum of 5 exhibited two distinct chemical shifts at δ = 50.3 and −10.8 ppm, indicating a significant difference of 61 ppm. This large disparity suggests that the two boron atoms reside in markedly different electronic environments.

Fig. 4 Synthesis of tetracyclic B,N,S-heterocycle 5. Inset: crystallographically-determined molecular structure of 5 (ellipsoids shown at the 50% probability level). All hydrogen atoms and ellipsoids of ligand periphery are omitted for clarity. Selected bond lengths of 5 (in Å): B1–B2 1.715(2), B1–N1 1.609(2), B1–N3 1.585(2), C1–S1 1.823(2), B2–N2 1.414(2), B2–C1 1.599(2), and C5–S3 1.669(1).

A SCXRD analysis revealed that compound 5 adopts a tetracyclic B,N,S-heterocyclic core structure, but one that completely differs from those of tetracyclic B,N,S-heterocycles 2–4. The core tetracyclic geometry of 5 is composed of one five-membered and three six-membered rings (Fig. 4 (inset)). Following S–H activation, one of the thiol heterocycles participates in fused-ring formation with the B₂ unit and the ring-expanded CAAC moiety, while the other functions as a substituent at one of the boron centres (B1). The B–B bond length in 5 (1.715(2) Å) is shorter than those observed in fused 2 and 4, yet still falls within the expected range for B–B single bonds.^30,31,47,48 Furthermore, the WBI value of 0.96 for the B–B bond in 5 confirms the presence of a single bond. One of these boron atoms (B1) is tetra-coordinate, while the other boron atom (B2) is tricoordinate—consistent with the upfield (δ = −10.8 ppm) and downfield (δ = 50.3 ppm) ¹¹B NMR signals, respectively. These assignments are further supported by DFT-calculated ¹¹B NMR chemical shifts at δ = −12.4 (B1) and 49.9 ppm (B2), which closely match the experimental values. The distinction between the two boron centres is also reflected in their natural charges (q_B1 = 0.19 and q_B2 = 0.78).

In contrast, the reaction of diboracumulene 1 with a different bicyclic thiol, benzo[d]oxazole-2-thiol, yielded the yellow solid 6 in 88% yield. The ¹¹B NMR spectrum of 6 showed two chemical shifts at δ = 41.8 and 6.4 ppm, like tetracyclic 5, indicating two distinct chemical environments for the boron atoms. However, the complex ¹H NMR spectrum of 6 revealed the presence of two rotational isomers 6a and 6b in a 2 : 1 ratio. An SCXRD analysis revealed a tricyclic core structure comprising one six-membered ring and two five-membered rings (Fig. 5). The formation of the fused tricyclic core involves complete cleavage of the B=B double bond in 1, followed by insertion of a sulfur atom, originating from the thiol unit, between the two boron atoms. The remaining fragment of the bicyclic thiol bonded to the B₂ fragment from the opposite side, resulting in the observed fused tricyclic architecture. One of the boron centres (B1) is covalently single-bonded to the CAACH unit (B1–C4: 1.584(2) Å), while the other boron atom (B2) is partially double-bonded to the CAAC unit (B2–C5: 1.506(2) Å), which is also evident from WBI, MO, and NLMO analyses (Fig. 6, S29 and S30). Note that the molecular structure shown in Fig. 5 (inset) corresponds to isomer 6a, which is identified as the major isomer by ¹H NMR spectroscopy. Rotation of the CAAC ligand by 180° yields isomer 6b (Fig. 5 and S31), which is only 1.48 kcal mol⁻¹ higher in energy than the major isomer 6a (Gibbs free energies calculated at 298 K).

Fig. 5 Synthesis of tricyclic B,N,S-heterocycle 6 and representation and ratio of the two rotamers of 6 observed by NMR spectroscopy. Inset: crystallographically-determined molecular structure of 6a; front view (left), top view (right). Ellipsoids shown at the 50% probability level. All hydrogen atoms and ellipsoids of ligand periphery are omitted for clarity. Selected bond lengths of 6a (in Å): B1–N1 1.461(1), B2–C1 1.485(2), B1–S1 1.791(1), B2–S1 1.880(1), C1–N1 1.395(2), C3–N1 1.410(2) Å, C1–O1 1.384(1), and C2–O1 1.379(2).

Fig. 6 (A) Optimised geometry of 6a. Interatomic distances (in Å), WBIs, NICS(1)_zz and NICS(–1)_zz are given in black, red, green and sky blue, respectively. (B) Selected π bonding interactions of 6a, obtained from AdNDP analysis. The occupancy threshold of 1.8 is used for the selection of candidate localized orbitals. Hydrogens omitted for clarity (isovalue 0.043 a.u.).

Interestingly, all three rings of the tricyclic framework in 6a are coplanar, as shown in Fig. 5 (top view). Moreover, the B–N (1.461(1) Å), B–C (1.485(2) Å), B–S (1.791(1) and 1.880(1) Å), C–N (1.395(2) and 1.410(2) Å), C–O (1.379(2) and 1.384(1) Å), and C–C (1.379(2)–1.397(2) Å) bond distances within the tricyclic framework of 6a are comparable to those reported for corresponding partial double bonds, indicating extended delocalization.^43,50,51 Also, the calculated bond distances and WBIs support the partial double bonding in the tricyclic core of 6a (Fig. 6A). We performed adaptive natural density partitioning (AdNDP)^52,53 analysis, which revealed 144 localized orbitals containing 288 valence electrons. These were distributed into three lone pairs, 254 σ-bonding electrons (2c–2e), and 28 π-bonding electrons. Of the π-bonding electrons, 16 participate in delocalized multicenter–two-electron (mc–2e) π-bonding interactions spanning all three rings of the tricyclic framework (Fig. 6B). Furthermore, the natural localized molecular orbital (NLMO) analysis of fused 6a supported the presence of these delocalized π-bonding interactions (Fig. S30). All these bond parameters and bonding interactions suggested the aromaticity of the coplanar tricyclic rings in 6a, which prompted us to perform nucleus-independent chemical shift (NICS) analysis.⁵⁴ The NICS_zz values 1 Å above (NICS(1)_zz) and below (NICS(–1)_zz) the six-membered ring are more negative than those of both five-membered rings in fused 6a (Fig. 6A), indicating greater π aromaticity in the six-membered ring. Nevertheless, the significantly negative NICS(1)_zz and NICS(–1)_zz values for all three rings support the classification of the entire tricyclic core of 6a as π aromatic.

To identify the absorption responsible for the yellow color of tricyclic 6, we recorded its UV-vis spectrum, which showed a band at λ = 460 nm (Fig. S17). Time-dependent DFT calculations (Table S2 and Fig. S32) attribute this absorption to a HOMO→LUMO transition, primarily involving excitation of π_{(C1–B2–C5)} electrons to the corresponding π* orbitals.

Despite differing only by the heteroatom (S or O), the two thiol substrates exhibited markedly different reactivities, yielding tetracyclic 5 and tricyclic 6. Benzo[d]oxazole-2-thiol is more prone to losing its terminal sulfur atom (SH group) than benzo[d]thiazole-2-thiol. This difference primarily arises from the differing electronegativities of the heteroatoms (S vs. O) in their respective ring systems. Oxygen, being more electronegative than sulfur, withdraws electron density from the terminal SH group, thereby weakening the adjacent C–S bond in benzo[d]oxazole-2-thiol and making it more susceptible to cleavage. As a result, terminal C–S bond cleavage was observed in benzo[d]oxazole-2-thiol, leading to the formation of tricyclic 6. In contrast, benzo[d]thiazole-2-thiol underwent only S–H or N–H bond activation (associated with thione–thiol tautomerism), resulting in the formation of tetracyclic 5. Notably, the reaction of the diboracumulene with one equivalent of benzo[d]thiazole-2-thiol resulted in only partial formation of tetracyclic 5; full conversion required two equivalents of benzo[d]thiazole-2-thiol. Another likely factor is the higher aromaticity of benzo[d]thiazole-2-thiol compared to benzo[d]oxazole-2-thiol, due to greater π-electron delocalization in the former. In pursuit of increased aromatic stabilization, benzo[d]oxazole-2-thiol favours the formation of an aromatic tricyclic framework 6.

Conclusions

In summary, a series of fused B,N,S-heterocycles, comprising either four- or three-ring systems, was synthesised. These complex and novel fused heterocycles were synthesised from simple one-pot reactions between a diboracumlene and various heterocyclic thiols via B–B multiple bond and S–H bond activations. Monocyclic thiols produced symmetrical, butterfly-shaped tetracyclic cores, while bicyclic thiols yielded an unsymmetrical tetracyclic core and an aromatic tricyclic framework. Given the growing interest in boron-based polyheterocycles, this synthetic approach represents a significant advancement and is expected to accelerate the discovery of novel fused heterocycles.

Author contributions

Supervision, funding acquisition, conceptualization: H. B.; conceptualization, formal analysis, investigation: C. M.; investigation, formal analysis, visualization, writing – original draft: S. K.; formal analysis: L. L.; investigation: K. H.; writing – review & editing: R. D. D.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2467669, 2467670, 2467671, 2467672 and 2467673 contain the supplementary crystallographic data for this paper.^55a–e

The data supporting this article have been included as part of the SI. Supplementary information: Synthetic, spectroscopic, X-ray crystallographic and computational details. See DOI: https://doi.org/10.1039/d5sc04751j.

Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged (466954611). S. K. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship.

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