English

• Green chemistry plays a key role in meeting the strategic requirements of sustainable development [1]. The rational utilisation of carbon dioxide (CO₂), which is abundant, nontoxic and recyclable C1 source, in organic synthesis is a crucial part of green chemistry [2,3]. Recently, numerous promising results have been reported on applications of CO₂ in organic synthesis [4-13]. In particular, the use of CO₂ to synthesize heterocyclic compounds has attracted widespread attention [14-18].

  Quinolinofuran is a crucial heterocyclic compound that is widely used in pharmaceuticals and materials [19-21]. Therefore, its synthesis has long been a focus of attention [22-25]. Compared to traditional methods for quinolinofuran synthesis, carbonylation-rearrangement of α-aminoaryl-tethered alkylidenecyclopropanes (ACPs) has the following unique advantages: (1) ACPs are readily accessible molecules and (2) various substituents can be easily introduced into different positions along the quinolinofuran structure. Shi et al. [26] and Peng et al. [27] have successfully realized such conversion using triphosgene (BTC) or carbon monoxide (CO), respectively (Scheme 1A). However, BTC and CO are highly toxic, hampering their applications in laboratory and industry. Given our continued interest in sustainable organic synthesis and carbonylation with CO₂ [14,28-33], we wondered if CO₂ could be used in the carbonylation-rearrangement of α-aminoaryl-tethered ACPs to obtain quinolinofuran fragments. Such a scenario faces two challenges. First, the thermodynamic stability and kinetic inertness of CO₂ render it difficult to realize efficient transformations, particularly under low pressure. Second, transformations involve a series of reactions such as carbonylation with CO₂, ring opening and rearrangement, making such processes even more complex and challenging (Scheme 1B).

  Scheme 1

  

  Scheme 1.  Carbonylation-rearrangement of α-aminoaryl-tethered ACPs.

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  Considering these challenges, we began investigating the reaction using 2-(cyclopropylidene(phenyl)methyl)aniline (1a) as the substrate under 1 atm of CO₂ (Table 1). The desired product 2a was obtained in 25% yield with LiO^tBu as the base in DMF at 140 ℃ for 24 h (Table 1, entry 1). The use of other solvents, such as DMAc or o-xylene, resulted in lower yields of 2a, while the use of THF resulted in a yield comparable to that obtained using DMF (Table 1, entries 2–4). This transformation exhibited good performance in a non-polar solvent [26,27], while the carbonylation with CO₂ was typically more effective in a dipole solution [28]. Therefore, we combined the two advantages and used a mixture of a dipole solvent and non-polar solvent as the reaction medium to further screen the reaction. The yield of the desired product considerably improved under these conditions (Table 1, entries 5 and 6), especially with a combination of DMF and o-xylene, for which the yield reached 49%. From the perspective of the reaction mechanism, the reason for low yield might be the sluggish rearrangement after carbonylation and many side reactions. Based on this hypothesis, we tested various Lewis acids in the reaction to promote the opening of the ternary ring and improve the efficiency of the reaction. After a series of screenings, we found that the addition of a particular amount of FeCl₃ improved the yield substantially to 80% (Table 1, entries 7–10. Please see more information in Supporting information), thus confirming the validity of our hypothesis. Next, we evaluated various mixture proportions of solvents, reaction times and amounts of LiO^tBu (Table 1, entries 11–15). However, no yield better than 80% was achieved. When the amount of LiO^tBu was reduced, there was a noticeable decrease in the yield of the product. Combined with our previous research [28], this may because LiO^tBu does not only act as a base in the reaction but also fix CO₂ for the formation of intermediates. Therefore, the amount of base has a significant effect on the reaction (Table 1, entries 14 and 15). Once the optimal solvent proportion, reaction time and amount of LiO^tBu had been confirmed, we further screened various reaction temperatures, aiming for milder conditions. We found a slight enhancement to the product yield to 82% when reacting at 110 ℃ (Table 1, entries 16). Finally, we confirmed that FeCl₃ (Table 1, entry 18), CO₂ (Table 1, entry 19), and LiO^tBu (Table 1, entry 20) all played a crucial role in promoting the reaction.

  Table 1

  

  Table 1.  Optimizations of reaction conditions.^a

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  With the optimal reaction conditions obtained, we began investigating the substrate scope 1 (Scheme 2). First, we examined substrates bearing substituents of the benzene ring on ACP (2b–l). As shown in Scheme 2, different functional groups, such as electron-donating groups (–Me, –^tBu, –OMe, and –OPh) and electron-withdrawing groups (–F, –Cl, –Br and –CF₃), at the para (2b–j) and meta (2k–l) positions of the phenyl ring did not affect the reaction. The electron-rich aryl groups (2b–f, 2k–l) showed higher reactivity than that of the electron-poor ones (2g–j) because the carbonylation reaction occurred more easily on substrates with electron-rich groups, similar to our previous work [28]. Furthermore, a variety of anilines with mono-substituents (2m–p) also afforded the desired products in moderate to good yields. The substrates bearing di-substituents on the aniline and phenyl ring components were also found to undergo this transformation to afford desired products with good yields (2q–u). Our protocol was also suitable for a substrate for which R^' was an alkyl group (2v), exhibiting a good yield. Furthermore, substrates bearing heterocyclic substituents, such as benzofuran group (2w) and benzothiophene group (2x), were also investigated to give the products in synthetically useful yields. Considering that carbazole groups are often used in optoelectronic materials molecules, we also investigated substrates containing carbazole groups and achieved good yield (2y).

  Scheme 2

  

  Scheme 2.  Substrate scope. 1 (0.2 mmol), CO₂ (1 atm), LiO^tBu (4.5 equiv., 0.9 mmol), FeCl₃ (10 mol%), DMF/o-Xylene (2 mL, v/v = 1/1), 110 ℃, 24 h. Isolated yields are shown.

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  Furthermore, we demonstrated its utility in organic synthesis. First, we conducted a gram-scale reaction of 1a and obtained the target product 2a in 73% yield (Scheme 3A). Next, considering the D-A structure is a common structure in organic luminescent materials [34-38], we synthesized a group of organic light-emitting materials with the D-A (electron donor-acceptor) structure by employing quinolinofuran as the electron acceptor and diphenylamine as the electron donor and tested their light-emitting properties (Scheme 3B). Based on the absorption and emission spectra, the luminescence of the two kinds of structures (i.e., oxidised (TPAQF-2) and non-oxidised (TPAQF-1)) fell within the blue-light range, with the oxidised structure being significantly red-shifted compared to the non-oxidised structure. These results provide an idea for material design (Table 2).

  Scheme 3

  

  Scheme 3.  Gram-scale reaction and derivatization reaction.

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  Table 2

  

  Table 2.  Photophysical properties of compounds.^a

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  In addition, the presence of 1D fused ring structures and heteroatoms in TPAQF-2 conforms to the structural characteristics of "hot exciton" materials [39-41]. Therefore, we conducted further theoretical calculations on the molecular structure of this material to gain a better understanding of its properties. Based on the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the distribution of electron clouds (Fig. S1 in Supporting information), the energy level distribution of S₀-S₁₀/T₁₀ (Fig. S2 in Supporting information), the distribution of natural transition orbitals (NTO) (Table S3 in Supporting information), and the spin-orbit coupling (SOC) of S₁/T_1,2,3 (Table S4 in Supporting information), it is likely that this material has the characteristics of "hot exciton" materials that undergo reverse intersystem crossing from T₃ to S₁ (further testing is required).

  To gain insight into the reaction mechanism, a series of experiments were performed. First, we tried the carbonylation-rearrangement reaction with a methylated substrate under standard conditions; no desired product was detected. This result further demonstrated that isocyanate might be one of the reaction intermediates [26-28]. As 1a could not convert into 2a under an N₂ atmosphere, CO₂ should be the key carbonyl source in the desired reaction (Scheme 4).

  Scheme 4

  

  Scheme 4.  Control experiments.

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  In the results of experiments, when the reaction was conducted in the absence of FeCl₃, a sharp decrease in the yield of product 2a was observed (48%). To verify the role of the iron salt, we performed DFT calculations by setting R1 as the starting material [42-44]. Due to the excess amount of LiO^tBu, we assumed Fe(O^tBu)₃ could be formed. Once it formed, coordination of Fe(O^tBu)₃ with R1 leads to the generation of R2 exergonic by 9.5 kcal/mol. The subsequent rearrangement occurs via transition state TS1 with a free activation energy of 27.6 kcal/mol, resulting in the generation of intermediate P1 by the energy release of 31.1 kcal/mol. After the dissociation of iron salt, the final product P2 could be released. The computational results show that the rearrangement process in the absence of iron salt via transition state TS2 requires an energy barrier of 29.8 kcal/mol. The relative free energy of TS2 is 11.7 kcal/mol higher than that of TS1, which demonstrates the iron salt promoting this reaction by lowering energy barriers of ring opening and rearrangement process (Scheme 5).

  Scheme 5

  

  Scheme 5.  DFT calculations. Free energy profile and structural information on the reaction pathway to explore the role of the FeCl₃ in the reaction. Energy levels are given in kcal/mol.

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  Based on the abovementioned results and previous work, we propose the following plausible mechanism (Scheme 6). First, ACP 1 reacts with CO₂ in the presence of LiO^tBu to generate isocyanate-tethered ACP A [28], followed by a thermally induced 6π-electrocyclisation to produce the intermediate B (based on our previous research and relevant reports, the 6p-electrocyclization is relatively facile. There are studies specifically addressing the theoretical aspects of this process, which also presented a structure very similar to that of B) [26,28,42-44]. Activated by the Lewis acid FeCl₃, the intermediate B subsequently undergoes a rearrangement to produce the corresponding furoquinoline 2.

  Scheme 6

  

  Scheme 6.  Proposed mechanism of iron-promoted carbonylation–rearrangement of α-aminoaryl-tethered ACPs with CO₂.

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  In conclusion, we realize a carbonylation–rearrangement of α-aminoaryl-tethered ACPs to synthesize the key compound furoquinoline in moderate to excellent yields by using CO₂ as a user-friendly carbonyl source. The Lewis acid FeCl₃ effectively promoted the ring opening and rearrangement of cyclopropane. Notably, this process is efficient and eco-friendly, making it an attractive method for the industry. Finally, these reactions feature a broad substrate scope, satisfactory functional group tolerance, facile scalability and easy derivatisation of the products.

  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.

  CRediT authorship contribution statement

  Zhen Zhang: Writing – review & editing, Writing – original draft, Validation, Supervision, Investigation, Funding acquisition, Formal analysis, Conceptualization. Xue-ling Chen: Methodology, Investigation, Formal analysis, Data curation. Xiu-Mei Xie: . Tian-Yu Gao: Methodology, Investigation, Formal analysis, Data curation. Jing Qin: Methodology, Investigation, Formal analysis, Data curation. Jun-Jie Li: Methodology, Formal analysis, Data curation. Chao Feng: Methodology, Investigation, Formal analysis. Da-Gang Yu: Writing – review & editing, Validation, Supervision, Funding acquisition, Conceptualization.

  Acknowledgments

  We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21801025, 22225106), Natural Science Foundation of Sichuan Province (No. 2022NSFSC0200), Sichuan Science and Technology Program (No. MZGC20230100). We also acknowledge Dr. Li-Li Liao, Dr. Linghui Gu, and Prof. Wenbo Ma for their help and support in this work.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110056.

  
  
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• 

  Scheme 1  Carbonylation-rearrangement of α-aminoaryl-tethered ACPs.

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  Scheme 2  Substrate scope. 1 (0.2 mmol), CO₂ (1 atm), LiO^tBu (4.5 equiv., 0.9 mmol), FeCl₃ (10 mol%), DMF/o-Xylene (2 mL, v/v = 1/1), 110 ℃, 24 h. Isolated yields are shown.

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  Scheme 3  Gram-scale reaction and derivatization reaction.

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  Scheme 4  Control experiments.

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  Scheme 5  DFT calculations. Free energy profile and structural information on the reaction pathway to explore the role of the FeCl₃ in the reaction. Energy levels are given in kcal/mol.

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  Scheme 6  Proposed mechanism of iron-promoted carbonylation–rearrangement of α-aminoaryl-tethered ACPs with CO₂.

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  Table 1.  Optimizations of reaction conditions.^a

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  Table 2.  Photophysical properties of compounds.^a

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•