Organocatalytic asymmetric synthesis of atropisomeric tetraarylethenes: design, properties, and applications

Abstract

Tetraarylethenes are important π-conjugated molecules with versatile applications in OLEDs, fluorescent probes, and aggregation-induced emission (AIE) luminogens. Access to chiral TAEs is largely limited to strategies relying on preinstalled chiral elements or post-synthetic resolution, restricting structural diversity and functional exploration. Herein, we report a CPA-catalysed atroposelective transfer hydrogenation enabling the direct construction of axially chiral TAEs via desymmetrization of TAE dialdehydes with anilines. This method features broad substrate scope, excellent enantioselectivity, and remarkable AIE properties. Our work provides a practical catalytic approach to access structurally diverse chiral TAEs, offering new opportunities for the development of functional chiral luminogens and optoelectronic materials.

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Introduction

Tetraarylethenes (TAEs) represent an important class of π-conjugated organic molecules characterized by a central C=C double bond bearing four aryl substituents. Owing to their rigid conjugated frameworks, structural tunability, and distinctive electronic properties, TAEs have garnered significant attention in materials science, bio-sensors, and organic synthesis.¹ They serve as versatile building blocks for functional materials, including metal organic frameworks,² fluorescent probes,³ and aggregation-induced emission (AIE) luminogens (Scheme 1A).⁴ In addition, their unique photophysical behavior enables diverse applications in molecular switches,⁵ molecular recognition,⁶ and circularly polarized luminescence (CPL).⁷ Consequently, the development of efficient and general synthetic strategies to access structurally diverse and chiral TAEs is of great significance for advancing both fundamental studies and practical applications.

Scheme 1 Applications of chiral TAEs and our reaction design. (A) TAEs and its derivatives. (B) Traditional strategies for the preparation of chiral TAEs. (C) This work: CPA-catalysed atroposelective synthesis of axially chiral TAEs.

Traditional approaches to access optically active TAEs typically rely on the incorporation of external chiral auxiliaries or chiral side chains into the TAE framework, thereby introducing central or axial chirality (Scheme 1B).⁸ Another widely employed strategy involves exploiting helical chirality; for instance, Zheng and co-workers achieved the resolution of racemic hindered tetraphenylethylene helicates through chiral chromatography.⁹ These chiral TAEs exhibited remarkable AIE characteristics, along with notable chiral recognition and CPL properties. Despite these advances, the synthesis of chiral TAEs remains largely dependent on preinstalled chiral elements or post-synthetic resolution, which restricts structural diversity and limits further exploration of their functional potential. The lack of efficient catalytic systems capable of directly inducing chirality within the TAE core has significantly hindered the development of new chiral architecture and the in-depth study of their optoelectronic and supramolecular behaviors. Recently, Tan and co-workers reported the first CPA-catalysed enantioselective synthesis of TAEs via an electrophilic aromatic substitution-enabled desymmetrization of prochiral substrates.¹⁰ Nevertheless, catalytic asymmetric strategies that enable the direct construction of chiral TAEs remain in their infancy and are urgently needed to expand the accessible chemical space of these chiral luminogens.

Chiral phosphoric acids¹¹ have proven to be highly effective catalysts in asymmetric transfer hydrogenation (ATH) reactions of α,β-unsaturated carbonyls, imines, and heterocycles, typically employing Hantzsch esters or benzothiazolines as hydrogen donors.¹² Notably, substantial progress has been achieved in CPA- and enzyme-catalysed atroposelective transfer hydrogenations for the construction of axially chiral biaryls.¹³ In parallel, remarkable advances have also been made in the asymmetric catalytic synthesis of axially chiral aryl-alkenes over the past few years.¹⁴ Inspired by these precedents, we envisioned that axially chiral TAEs could be accessed via a desymmetrization strategy involving the condensation of TAE dialdehydes with anilines under CPA-catalysed asymmetric transfer hydrogenation. Herein, we disclose the first example of a CPA-catalysed atroposelective transfer hydrogenation enabling the direct construction of axially chiral TAEs (Scheme 1C). This method features mild reaction conditions, a broad substrate scope, and remarkable enantioselective control. Moreover, the resulting products exhibit excellent AIE characteristics.

Results and discussion

Reaction condition optimization

To gain initial insights into the reaction parameters, we selected TAE dialdehyde 1a and 4-methoxyaniline 2a as model substrates to investigate the reaction under atroposelective transfer hydrogenation conditions, using the Hantzsch ester as the hydrogen source and a chiral phosphoric acid (CPA) as the catalyst (Table 1). Under the preliminary conditions (C1, 10 mol% in toluene at room temperature for 12 h), the desired product 3a was obtained in 63% yield with a modest enantioselectivity 11% ee (Table 1, entry 1). Next, a series of chiral BINOL-derived CPAs C2–C6 were evaluated (entries 2–6), among which catalyst C4 bearing bulky aryl substituents afforded the best enantioselectivity of 59% ee (entry 4). To further enhance the asymmetric induction, various axially chiral backbones of CPAs were then examined (entries 7–11). Notably, the 1,1′-spirobiindane-diol-derived C10 exhibited superior catalytic performance, affording the desired product 3a in 70% yield with 80% ee (entry 10). Subsequent solvent screening showed that DCE was the most effective medium for enantiocontrol among CCl₄, DCM, and DCE (entries 12–14), affording 3a in 70% yield and an excellent 91% ee (entry 14). Lowering the temperature to 0 °C further improved the enantioselectivity to 93% (entry 15). Finally, extending the reaction time to 24 h led to a slightly higher isolated yield (72%) while maintaining the same excellent enantioselectivity (entry 16). Notably, the bis-reductive amination byproduct arising from a second reductive amination of 3a was consistently detected during reaction optimization. When the loading of catalyst C10 was reduced to 5 mol%, product 3a was obtained in only 53% yield with a decreased ee of 91% (entry 17).

Table 1 Optimization of the reaction conditions^a

Entry	CPA (10 mol%)	Solvent	T (°C)	Time	Yield of 3a^a (%)	ee of 3a^b (%)
a Unless otherwise specified, the reaction conditions were as follows: 1a (0.10 mmol), 2a (0.10 mmol), 10 mol% CPA, and 1.2 equiv. of HE in 1.0 mL of solvent at 0–25 °C for 12–24 h. ^a Determined by ^1H-NMR analysis. ^b Determined by chiral HPLC analysis. ^c Isolated yield in the parentheses. ^d 5 mol% C10.
1	C1	Toluene	25	12 h	63	11
2	C2	Toluene	25	12 h	55	27
3	C3	Toluene	25	12 h	40	7
4	C4	Toluene	25	12 h	65	59
5	C5	Toluene	25	12 h	63	29
6	C6	Toluene	25	12 h	65	57
7	C7	Toluene	25	12 h	50	53
8	C8	Toluene	25	12 h	46	3
9	C9	Toluene	25	12 h	72	75
10	C10	Toluene	25	12 h	70	80
11	C10	Toluene	25	12 h	55	63
12	C10	CCl4	25	12 h	75	85
13	C10	DCM	25	12 h	62	89
14	C10	DCE	25	12 h	70	91
15	C10	DCE	0	12 h	58	93
16	C10	DCE	0	24 h	78(72)^c	93
17^d	C10	DCE	0	24 h	53	91

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Substrate scope

With the optimized reaction conditions in hand, we next explored the substrate scope of the CPA-catalysed atroposelective transfer hydrogenation using TAE dialdehyde 1a as the standard substrate (Table 2). A diverse array of aromatic amines 2 bearing various electronic and steric features were examined to evaluate the generality of this transformation. Anilines containing electron-donating substituents such as alkoxy (2a–2c), methyl (2d), or aryl (2e–2f) groups underwent smooth conversion to the corresponding axially chiral products 3a–3f in good to excellent yields (65–81%) and high enantioselectivities (89–93% ee). Similarly, para-halogenated anilines (F, Cl, Br) were well tolerated, delivering the desired products 3g–3i with excellent enantioselectivities. The absolute configuration of this class of products was unambiguously determined to be S by single-crystal X-ray diffraction analysis of compound 3i (CCDC 2494773), thereby establishing the stereochemical correlation for this series. Anilines bearing polar or functionalized substituents such as phenolic hydroxyl, benzoyl, or pyrrolyl groups were also compatible, affording the corresponding products 3j–3l in moderate yields with good enantioselective control. Furthermore, disubstituted anilines with 3,4- or 3,5-methyl/methoxy patterns smoothly furnished the products 3m–3p in up to 80% yield and 93% ee, demonstrating that sterically encumbered substrates are also well accommodated. Notably, ortho-substituted 3-amino-4-methylphenol gave the product 3q in slightly reduced yield but maintained excellent enantioselectivity, indicating that steric hindrance near the reaction site primarily affects the reactivity rather than the asymmetric induction. Moreover, anilines containing 3,4,5-trisubstitution patterns were also suitable, affording 3r with good efficiency. The reaction proved to be highly general, extending to aromatic amines bearing naphthyl, anthracenyl, and heteroaromatic moieties, which furnished the corresponding products 3s–3ab in moderate yields and high enantioselectivities (83–93% ee). We experimentally determined the rotational barrier of 3ab to be 33.2 kcal mol⁻¹ at 140 °C in mesitylene. Finally, to demonstrate the synthetic versatility of this methodology, we extended the reaction to structurally complex anilines bearing secondary alkylamine or lactam substituents at the para-position. Both substrates underwent smooth transformation to afford the corresponding axially chiral TAE derivatives 3ac–3ae in good yields with excellent enantioselectivities or diastereoselectivity (>20 : 1 dr), underscoring the robustness and broad applicability of this asymmetric transformation for the late-stage modification of complex molecules.

Table 2 Reaction scope of aromatic amines^a

a Reaction conditions: 1a (0.10 mmol), 2 (0.10 mmol), 10 mol% CPA, and 1.2 equiv. of HE in 1.0 mL of DCE at 0 °C for 24 h; isolated yield by silica gel chromatography; ee values were determined by chiral HPLC. ^a 72 h. ^b −10 °C, 72 h.

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The substrate scope was next explored using a series of TAE dialdehydes 1 and 4-methoxyaniline 2a as the model substrate (Table 3). We first examined TAEs bearing para-substituents on the aryl rings at the distal position of the benzaldehyde moiety. Methyl, and halogen groups were all well tolerated, affording the corresponding products 4a–4c in 52–68% yields with 81–96% ee. Notably, the fluorene-substituted TAE also performed well under the standard conditions, delivering 4d in 63% yield and 91% ee. Subsequently, TAEs bearing para- or meta-substituents on the aryl ring directly attached to the same carbon center as the benzaldehyde were also evaluated, furnishing the desired axially chiral TAEs 4e–4h in 62–88% yields with good enantioselectivities (90–91%). Moreover, excellent enantioselectivity was achieved in the reaction of an unsymmetrical TAE (Z/E = 1/1) with 2a, affording 4i (Z/E = 1/1) in 71% yield.

Table 3 Reaction scope of TAE dialdehydes^a

a Reaction conditions: 1 (0.10 mmol), 2a (0.10 mmol), 10 mol% CPA, and 1.2 equiv. of HE in 1.0 mL of DCE at 0 °C for 24 h; isolated yield by silica gel chromatography; ee values were determined by chiral HPLC.

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Synthetic applications and mechanistic studies

To further demonstrate the practicality and scalability of this atroposelective strategy for constructing axially chiral TAEs, a large-scale reaction was carried out (Scheme 2A). The reaction between TAE dialdehyde 1a and p-toluidine 2d on a 2.0 mmol scale proceeded smoothly under the standard conditions, affording product 3d in 75% yield while maintaining its high enantioselectivity. Subsequently, a series of derivatization transformations of 3d was conducted to showcase the synthetic versatility of this method (Scheme 2B). Reduction of the aldehyde moiety with NaBH₄ furnished benzyl alcohol 5a in 61% yield without any loss of enantioselectivity. Protection of the N–H group in 3d with TsCl afforded sulfonamide 5b in moderate yield, with only a slight decrease in enantiopurity. Condensation of the aldehyde with hydroxylamine hydrochloride and R-tert-butylsulfinamide produced oxime 5c and sulfinamide 5d in high yields (56–72%) with excellent enantioselectivity (91% ee) or diastereoselectivity (>20 : 1 dr), respectively. Moreover, 3d could undergo further reductive amination to afford 5e in 46% yield, albeit with a slight decrease in enantiomeric excess. To evaluate the configurational stability of these axially chiral TAEs, racemization experiments were performed on 3p, revealing a rotational barrier of 33.8 kcal mol⁻¹ at 120 °C in toluene (Scheme 2C), indicating that this class of compounds is configurationally stable at room temperature and does not readily undergo racemization under the reaction conditions. Subsequently, non-linear effect studies exhibited a linear correlation between the ee of product 3p and that of catalyst C10, suggesting that a monomeric catalyst species is involved in the enantio-determining transition state (Scheme 2D). Finally, control experiments were conducted to probe the reaction mechanism (Scheme 2E). Under the standard conditions but in the absence of the Hantzsch ester, the reaction between 1a and 2a afforded the corresponding imine 3a′ in 63% yield as a racemate. In addition, treatment of rac-3d with 2p under the standard conditions delivered enantioenriched 3d in 27% yield with 87% ee, together with ent-5e in 60% yield and 60% ee (Scheme 2F). These results suggest that stereocontrol is established during the CPA-catalyzed ATH reduction of the imine intermediate, while a subsequent kinetic resolution process further enhances the enantioselectivity.

Scheme 2 Synthetic applications and mechanistic studies. Reaction conditions: (a) NaBH₄ (1.2 equiv.), THF/H₂O (4/1), 0 °C, 2 h; (b) TsCl (1.5 equiv.), Et₃N (3.0 equiv.), DMAP (10 mol%), DCM, 0 °C to rt, 12 h; (c) NH₂OH·HCl (1.2 equiv.), NaOAc (1.2 equiv.), MeOH/H₂O (9/1), rt, 5 min; (d) (R)-^tBuSA (3.0 equiv.), Ti(OEt)₄ (3.0 equiv.), DCM, 50 °C, 12 h; (e) 2p (1.0 equiv.), (PhO)₂PO₂H (10 mol%), HE (1.0 equiv.), Et₂O, rt, 1 h. (A) 2.0 mmol-scale reactions. (B) Transformations of axially chiral 3d. (C) Racemization experiments. (D) Non-linear effect of the C10. (E) Control experiment. (F) Kinetic resolution study.

Optical property investigation

A comprehensive investigation of the photophysical characteristics of selected axially chiral TAEs was carried out (Fig. 1). In the UV-vis spectra, an intense absorption feature appeared near 240 nm, accompanied by weaker transitions around 305 nm (Fig. 1A). Upon photoexcitation at 340 nm, the compounds emitted fluorescence centered at approximately 510 nm. Moreover, the AIE properties of the newly developed axially chiral TAE framework were evaluated (Fig. 1B and C). Compound 3a exhibited only weak fluorescence in THF/H₂O mixtures when the water fraction (f_h) was below 80%. As f_h increased, the emission intensity gradually intensified. Notably, when f_h > 90%, a distinct fluorescence band appeared at 530 nm, clearly confirming the pronounced AIE behavior of these chiral TAE derivatives. The CD spectra of the two enantiomers, S-3p and R-3p, exhibited mirror-image Cotton effects (Fig. 1D), demonstrating that the chiral integrity of the molecules was well preserved; in all cases, the S-isomers consistently showed positive Cotton responses. CPL measurements revealed distinct emission activity for 3k, 3z, S-3p, and R-3p (Fig. 1E), with their corresponding luminescence dissymmetry factors (|g_lum|) determined from the emission profiles (Fig. 1F). Among these, compound 3z displayed the highest |g_lum| value of 0.00222, underscoring its promise as a potential CPL-active material for optoelectronic applications.

Fig. 1 Photophysical and optical property investigations. (A) Absorption spectra and emission spectra. (B) The PL intensity of 3a in THF/H₂O mixed solvents. (C) Relative PL intensity of 3a. (D) CD spectra. (E) CPL spectra. (F) g_lum values–wavelength curve.

Conclusions

In summary, we have developed a highly efficient CPA-catalysed atroposelective transfer hydrogenation for the direct construction of axially chiral TAEs. This method features broad substrate scope, excellent enantioselective control, and scalability. The resulting chiral TAEs exhibit remarkable optical and potential AIE properties, highlighting their potential in optoelectronic and chiroptical applications. Further exploration of the AIE mechanism and structure–property relationship is currently ongoing in our laboratory.

Author contributions

M. A., C.-J. L., F. X., Y.-J. S., and Z. S. performed the experiments. All authors contributed to analysing the experimental results. R.-R. L. and L. X. conceived the study, supervised the project, and wrote the paper.

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, NMR spectra, high resolution mass spectrometry data, HPLC data and crystallographic data of 3i. See DOI: https://doi.org/10.1039/d6sc03274e.

CCDC 2494773 contains the supplementary crystallographic data for this paper.¹⁵

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22371152, 22501151), and National Natural Science Foundation of Shandong (ZR2023JQ006, ZR2024QB091).

Notes and references

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Footnote

† These authors contributed equally.

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