English

• Heteroarene-directed transition-metal-catalyzed C—H bond activation/annulation reactions have been emerging as one of the versatile toolboxes to construct extended π-conjugated polycyclic heteroaromatic molecules (PHAs) [1]. These processes are considered atom- and step-economic compared to external directing group (EDG)-mediated C—H activation and traditional cross-coupling because the heteroarene can act as intrinsic directing group (DG) that do not require preinstalling/removing an DG or any prefunctionalization [2–5]. Notably, they offer an ample opportunity to construct cationic N-heterocyclic salts [6–9], which otherwise are always challenging via traditional methods. In this regard, Cheng [10–12], Choudhury [13–15], Wang [16–18], Huang [19], Davies & Macgregor [20], Saa [21], Li [22] and You [23] have independently made considerable efforts on the synthesis of fused isoquinolinium salts through the assist with aryl heterocycles like imidazole, pyrrole, pyridine, pyrazine, quinazolinone, and so on.

  Among the above N-heterocycles, imidazole attracts our interest more, because it is an innate DG core with all the five atoms prone to act as directing site. For instance, the C—H annulations of N1/3-aryl [13–15,24–26] and C2-aryl [27–29] in 1H-imidazoles have been well documented. In 2023, our group realized the first example of C—H activation/annulation of C4(5)-aryl in 1H-imidazoles [30]. In addition, Dong [31–35] and Cui [36] reported the seminal work on C4_aryl−H activation of C4(5)-aryl-2H-imidazoles to form neutral products, 1-acyl isoquinolines (Scheme 1a) and spiroimidazole-indenes (Scheme 1b), in which only one of the two (C4 and C5) aryls could be activated. Despite this growing interest in C−H annulation of 2H-imidazoles, the construction of cationic molecules, 2H-imidazolium salts, and their photophysical properties have not been researched. Moreover, among the massive cationic fluorophores [9], full-color emissive skeleton is rare [37]. Considering the cationic nitrogen-embedded polycyclic aromatic hydrocarbons (cNe-PAHs) have many potential applications, for example, in fuel cells, semiconductors, and liquid crystals [38–41], the synthesis of 2H-imidazole-based cNe-PAHs should be appealing.

  Scheme 1

  

  Scheme 1.  C-H activation/annulation of 2H-imidazoles.

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  Encouraging by these seminal works along with our interesting in N-heterocycles construction [42–44], we propose that the [4 + 2] annulation of 4,5-diaryl-2H-imidazoles with alkynes could stop just after the reductive elimination, forming imidazo[5,1-a]isoquinolinium (IMIQ) salts (Scheme 1c). These new cNe-PAHs have been demonstrated to exhibit continuously tunable full-color emissions (433–633 nm). Interestingly, the cNe-PAHs can further transform into neutral, non-symmetric, axial, and aggregation-induced emission (AIE) active N,N-bidentate ligands, 1,1′-biisoquinolines (1,1′-BIQs, Scheme 1d), which are always challenging via previous C−H activation/1,3-diyne strategy that giving 3,3′-, 3,4′-, and 4,4′-BIQs [45–48]. After the mechanistic insights with ESI-MS and ¹⁵N-labelling studies, we found that the ring-opening [4 + 2] annulation proceeds through two simultaneous routes: (a) NH₄OAc-promoted Hofmann elimination → hydrolysis → iminization → annulation; (b) NH₄OAc-promoted Hofmann elimination → annulation.

  Initially, 4,5-diphenyl-2H-imidazole 1a and diphenylacetylene 2a were selected as the model substrates to react under our previous conditions of Rh/Cu(OTf)₂, but only trace amount of annulative product 3a was observed (see Supporting information). To our delight, the C4_aryl−H activation/N-annulation occurred smoothly when presenting Rh/Cu(OAc)₂, giving imidazo[5,1-a]isoquinolinium tetrafluoroborate 3a in a high yield of 98% after working-up with NaBF₄ for anion-exchange (Scheme 2a). Notably, working up with other different simple non-silver salts accessed the IMIQs with various anions such as OTf, SbF₆, PF₆, and Cl conveniently (all > 90% yield). More interestingly, 3a could further react with one molecule of 2a to afford ring-opening product, 1,1′-BIQ 5a (71% yield), in the presence of Rh/Cu(OAc)₂/NH₄OAc (Scheme 2b).

  Scheme 2

  

  Scheme 2.  Reaction evolution.

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  It is noted that 3a could transform into 1-benzoyl isoquinoline 4 just by dealing with NH₄OAc, proceeding through Hofmann elimination (giving N-alkenyl imine 3a′) and hydrolysis (Scheme 3a). Thus, both 3a and 3a′ are vital intermediates in the generation of 4 from 1a and 2a, while 3a was not mentioned in Dong's report [32]. Furthermore, compound 4 reacted with 2a smoothly to form 1,1′-BIQ 5a in quantitative yield (Scheme 3b). 5a was still obtained in 18% yield without addition of NH₄OAc, showing that the other N atom in 2H-imidazole can also act as a directing group to happen a second annulation (Scheme 3c). Then some isotope labeling experiments were conducted to gain more insights into the mechanism. Interestingly, a high but not full ¹⁵N-labelling ratio (89%) was received, identifying that the N atom of the second isoquinoline ring mainly came from NH₄OAc and minor came from initial 2H-imidazole (Scheme 3c). Deuterium incorporation of 25% was observed at the ortho position of 1a in a H/D exchange experiment, indicating the first C−H activation is reversible. Moreover, kinetic isotope effect (KIE) of k_H/k_D ≈ 2.1 was obtained from the first annulation, showing that cleavage of the C−H bond may be involved in the rate-determining step (Scheme 3d). In this regard, the mechanism of current reaction is proposed (Scheme 3e). As in the first [4 + 2] annulation, the metalation and reversible C−H activation forms five-membered rhodacycle Int-A, which undergoes alkyne insertion and reductive elimination to give cationic salt 3a. Distinctively, the second [4 + 2] annulation for the generation of 1,1′-BIQ 5a from 3a may include two pathways. First, N-alkenyl imine 3a′ is afforded through Hofmann elimination, which then drives two pathways (A and B). As in path A, 3a hydrolyze into ketone 4, followed by ketone-amine condensation to generate NH imine Int-B, which directs the second annulation with 2a to give final product 5a with a N-exchange manner (major). As in path B, N-alkenyl imine 3a′ can directly guide a C−H activation to form rhodacycle Int-C, which undergoes alkyne insertion and reductive elimination to deliver 5a with a N-retention manner (minor).

  Scheme 3

  

  Scheme 3.  Mechanistic studies.

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  On the basis of the optimal reaction conditions and mechanistic studies, a series of IMIQ salts were prepared first (Scheme 4). 3a could be also isolated in a high yield of 88% on 1 mmol scale. Spiro 2H-imidazole 1b was tolerated well to yield 3b in high yield of 95%. Non-symmetric 2,2,4-trimethyl-5-phenyl-2H-imidazole 1c reacted with 2a furnished 3c in 80% yield. The 2H-imidazoles bearing Me, OMe, and Cl substituents reacted with 2a gave 3d-f in 87%, 30%, and 75% yield, respectively. Both electron-deficient and electron-rich alkynes were compatible well and afforded the title products (3g-k) in good yields. Multiple functionalized salts 3m-o were smoothly received in 21%-60% yields. Alkyl-alkyl alkyne was intact, giving 3p in 73% yield.

  Scheme 4

  

  Scheme 4.  Synthesis of isoquinolinium salts. Reaction conditions: 1 (0.1 mmol), 2 (0.1 mmol), [Cp*RhCl₂]₂ (2 mol%) and Cu(OAc)₂·H₂O (2 euqiv.) were stirred in TFE (2 mL) at 120 ℃ for 12 h under air, then NaBF₄ (2 equiv.) in H₂O (1 mL) was added to stir at room temperature for 10 min, isolated yields.

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  Next, the resultant IMIQ salts (3) were selected to react with another different alkynes with the aid of NH₄OAc, accessing non-symmetric 1,1′-BIQs (Scheme 5). Through the catalytic system of [Cp*RhCl₂]₂/Cu(OAc)₂/NH₄OAc, non-symmetric 1,1′-BIQs 5b-f were obtained in 44%-80% yields (Scheme 5). Like Br, t-Bu, and OMe were all intact in this novel ring-opening C−H activation and annulation. Aryl-alkyl product 3,4-diethyl-3′,4′-diphenyl-1,1′-biisoquinoline 5f was gained in 63% yield. Multiple substituted 1,1′-BIQ 5g was isolated in relatively low yield. Notably, such non-symmetric 1,1′-BIQs can also be obtained from 2H-imidazole 1 and two molecules of 2 through one-pot, two-step double C−H annulation (e.g., 5c with 50% yield). These non-symmetric 1,1′-BIQs may serve as a class of privileged bidentate ligands in organic synthesis [49].

  Scheme 5

  

  Scheme 5.  Synthesis of unsymmetric 1,1′-BIQs. Reaction conditions: 3 (0.1 mmol), 2 (0.1 mmol), NH₄OAc (3 equiv.), [Cp*RhCl₂]₂ (2.5 mol%) and Cu(OAc)₂·H₂O (2 equiv.) in TFE (2 mL) at 120 ℃ for 12 h under air.

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  With a series of novel IMIQ salts and non-symmetric 1,1′-BIQs in hand, the photophysical properties of these interesting scaffolds were studied. To clarify the fluorescence-structure relationship of IMIQ salts, the representative data are listed in Table 1 and Fig. 1. The impact of the R¹ substituent on C4 and C5 aryl of imidazole moiety is unordered. For instance, methyl (3d) shows a weak blue-shift while methoxy (3e) and chloro (3f) have red-shifts. In contrast, impact of the R² substituent is ordered and significant. For instance, an increase in the electron-withdrawing ability [50] from Cl (3g) to CF₃ (3b) enables a hypsochromic shift of emission wavelengths from 464 nm to 433 nm. Taking H as the reference R¹ substituent, the alteration of the R² substituent from H (3a) to electron-donating t-Bu (3i) and methoxy (3j) induce an obvious bathochromic shift of emission wavelengths from 462 nm to 500 and 565 nm, respectively. If locking R² as OMe and changing R¹ from H (3j) to Me (3n) and OMe (3o), the emission wavelengths only slightly change but do not enhance. The result indicates that the fluorescence of IMIQ is more sensitive on the group in C3 and C4 positions of isoquinoline moiety than that in other sites. Accordingly, a more electron-donating group (−OCH₂O−) was introduced, and the emission maximum of 4l red-shifts to 633 nm. While 3k with strong electron-donating N,N-diphenylamino (DPA) was non-emissive in the visible light region. In general, the current imidazo[5,1-a]isoquinolinium salts serve as new fluorophores with tunable full-color emissions (blue to red, 433–633 nm).

  Table 1

  

  Table 1.  Fluorescence emission maxima of selected IMIQ salts. ^a

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  Figure 1

  

  Figure 1.  (A) PL spectra of compounds 3a-o in CH₂Cl₂ (10 µmol/L). (B) The photos of 3a-o in CH₂Cl₂ (10 µmol/L) under 365 nm UV light.

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  Moreover, the photophysical properties of non-symmetric 1,1′-BIQs were tested. We previously found that this kind of compounds were AIE active [42]. As 5e and 5g were determined before, AIE properties of new compounds 5b-d and 5f were tested. As shown in Fig. 2A, the solution of 5d in THF showed faint photoluminescence (PL). Increasing the water fraction (f_w) to 95% allowed a significant emission signal peaking at 433 nm to be detected. The fluorescence intensity had an approximately 20-fold enhancement when f_w increased to 97% (Fig. 2B). Although the solution at this stage (f_w = 97%) is still clear under ambient light, the Tyndall phenomenon shows the formation of the nano-sized aggregates. 5b, 5c and 5f were also proved to be AIE-active with 7.8-, 12.2-, and 1.8-folds enhancement of intensities, respectively (Supporting information).

  Figure 2

  

  Figure 2.  (A) PL spectrum of 5d in THF and THF/water mixtures (10 µmol/L) with different water fractions (f_w). (B) Plots of relative maximum emission intensity (I/I₀) of 5d versus the solvent composition of THF/water mixture. Inset: (left) photos of 5d at f_w = 0 and 97% under 365 nm UV light; (right) the Tyndall phenomenon.

  DownLoad: Full-Size Img PowerPoint

  In summary, the rhodium-catalyzed C4(5)_aryl−H activation and ring-retentive annulation of 2H-imidazoles with internal alkynes to build imidazo[5,1-a]isoquinolinium (IMIQ) salts with high yields and broad scope has been disclosed. These novel IMIQ salts serve as new full-color emissive fluorophores with emission wavelengths raging of 433–633 nm, just by simply modifying the substituents on C3 and C4 positions of isoquinoline ring. Furthermore, the cationic IMIQ salts can react with another different alkynes and NH₄OAc, through ring-opening annulation, to give non-symmetric and AIE-active 1,1′-biisoquinolines (1,1′-BIQs), in which NH₄OAc accounts for Hofmann elimination and imine formation, thus leading to an unprecedented imine dance: cyclic imine → N-alkenyl imine → NH imine. This transformation includes two pathways of N-exchange (major) and N-retention (minor), offering a convenient route to ¹⁵N labelled 1,1′-BIQs. Further applications of the process developed in this effort are currently under study.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We are grateful for the financial support from the National Natural Science Foundation of China (Nos. 22261013 and 22001049), Guangxi Natural Science Foundation (No. 2020GXNSFBA297003), and Magneto-Chemical Functional Materials (No. EMFM20221102). The authors also thank Prof. Ming-Hua Zeng (GXNU) for helpful ESI-MS analysis.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109533.

  
  
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• 

  Scheme 1  C-H activation/annulation of 2H-imidazoles.

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  Scheme 2  Reaction evolution.

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  Scheme 3  Mechanistic studies.

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  Scheme 4  Synthesis of isoquinolinium salts. Reaction conditions: 1 (0.1 mmol), 2 (0.1 mmol), [Cp*RhCl₂]₂ (2 mol%) and Cu(OAc)₂·H₂O (2 euqiv.) were stirred in TFE (2 mL) at 120 ℃ for 12 h under air, then NaBF₄ (2 equiv.) in H₂O (1 mL) was added to stir at room temperature for 10 min, isolated yields.

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  Scheme 5  Synthesis of unsymmetric 1,1′-BIQs. Reaction conditions: 3 (0.1 mmol), 2 (0.1 mmol), NH₄OAc (3 equiv.), [Cp*RhCl₂]₂ (2.5 mol%) and Cu(OAc)₂·H₂O (2 equiv.) in TFE (2 mL) at 120 ℃ for 12 h under air.

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  Figure 1  (A) PL spectra of compounds 3a-o in CH₂Cl₂ (10 µmol/L). (B) The photos of 3a-o in CH₂Cl₂ (10 µmol/L) under 365 nm UV light.

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  Figure 2  (A) PL spectrum of 5d in THF and THF/water mixtures (10 µmol/L) with different water fractions (f_w). (B) Plots of relative maximum emission intensity (I/I₀) of 5d versus the solvent composition of THF/water mixture. Inset: (left) photos of 5d at f_w = 0 and 97% under 365 nm UV light; (right) the Tyndall phenomenon.

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  Table 1.  Fluorescence emission maxima of selected IMIQ salts. ^a

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•