Catalytic asymmetric construction of remote P or other heteroatom (Si/S) stereogenic axially chiral scaffolds

Abstract

Axial chirality represents a fundamental element of stereochemistry and is a defining structural feature in many chiral compounds. Although the axial chirality of atropisomers, allenes and spiranes has been extensively studied, the chemistry of alkylidenecycloalkanes and their analogues—especially those possessing P-, S- or Si-based axial chirality—remains underdeveloped. Here we report a chiral phosphoric acid-catalyzed oxime condensation that enables the construction of remote heteroatom (P/S/Si)-stereogenic axially chiral scaffolds. The reaction accommodates a broad range of functional groups, delivering optically active axially chiral 9-heteroanthrone-based oxime ethers in good to excellent enantioselectivities (up to 98 : 2 er). We also describe a modular synthesis of dibenzoazaphosphepinones and demonstrate their application as chiral monodentate phosphine ligands in asymmetric catalysis.

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Introduction

Axially chiral compounds have garnered significant attention in synthetic^1,2 and pharmaceutical³ chemistry due to their presence in natural products and diverse applications^4–6 as bioactive drug molecules,^7–9 chiral ligands and catalysts,^10,11 and functional materials like liquid crystals.¹² Advances in catalytic asymmetric synthesis over recent decades have established robust methods for constructing axially chiral compounds, particularly (hetero)aryl-based X–Y (X, Y = C, N) systems^13–31 and allenes,^32–34 making this a vibrant subfield within asymmetric catalysis. These advances have stimulated exploration of novel axially chiral skeletons.^{22,30,35–37} In contrast to classical atropisomers, allenes and spiranes constitute a distinct class of non-atropisomeric axially chiral compounds that has recently garnered increasing attention.³⁴ The hybridization of alkene and spirane structural motifs gives rise to a distinctive class of non-atropisomeric axial stereoisomerism, termed alkylidene-cyclic molecules. However, investigation into this axial stereoisomerism is still limited^36,37 (Scheme 1A). Early synthetic approaches to such architectures typically relied on non-catalytic methods.^38–44 A landmark study by Fiaud et al. in 1988 described the enantioselective synthesis of 4-alkylcyclohexylbenzene derivatives via asymmetric palladium catalysis,⁴⁵ yet this strategy remained largely overlooked for years. Recently, more efficient routes to these novel skeletons have emerged, including: (i) photochemical deracemization of racemic alkylidenecycloalkanes;^46,47 (ii) enantioselective [2 + 2] cycloadditions between terminal alkenes and allenoates or allenyl imides;^48,49 (iii) transition metal-catalyzed desymmetrization of functionalized cycloalkanes;^50–61 and (iv) enantioselective functionalization of cyclic carbonyl compounds^62–71 (Scheme 1B).

Scheme 1 Enantioselective synthesis of axially chiral alkylidenecycloalkanes.

Notably, the construction of axially chiral tricyclic aromatic scaffolds bearing remote heteroatom (P/S/Si)-stereogenic centers remains rare. This may be attributed to the challenge of exercising stereocontrol at distal heteroatom sites: while desymmetrization of monocyclic substrates enables effective control of carbon stereocenters, the bulky tricyclic aromatic framework complicates stereochemical manipulation at remote P/S/Si centres.⁶⁶ Moreover, substitution of distal carbon atoms with phosphorus, sulfur or silicon elongates the C(sp²)–P(sp³), C(sp²)–S(sp³), C(sp²)–Si(sp³) bonds relative to C(sp²)–C(sp³), increasing ring strain and conformational constraint. The inherent structural distortion in 9-heteroanthrones may further complicate precise stereochemical control⁷² (Scheme 1C).

Inspired by these precedents and the potential applications of 9-heteroanthrones,^73–76 we envisioned a chiral phosphoric acid-catalyzed enantioselective condensation between 9-heteroanthrones and hydroxylamines to construct axially chiral 9-heteroanthrone-based oxime ethers (Scheme 1D). We proposed that stereocontrol in this condensation would arise during the dehydration step, wherein the CPA catalyst engages the NH group via hydrogen bonding and activates the hydroxyl group through protonation to facilitate water elimination⁶⁵ (Scheme 1C). Moreover, in this work, we also developed a modular synthesis of dibenzoazaphosphepinones via axial-to-point chirality conversion from 9-phosphaanthrone-derived oxime ethers and explored their applications as chiral monodentate phosphine ligands in asymmetric reactions (Scheme 1D).

Results and discussion

We began our investigation by examining the model reaction between 5-phenyl-10H-acridophosphin-10-one 5-oxide (1a) and O-(4-nitrophenyl)hydroxylamine (2a) using a series of chiral phosphoric acid (CPA) catalysts with varied scaffold structures in DCE at 60 °C (Table 1, entries 1–7). The less sterically encumbered CPA C1 afforded only trace product (entry 1). Pleasingly, C2 delivered the target product 3a in 65% yield with 93.5 : 6.5 enantiomeric ratio (er) (entry 2). Both the 2,4,6-tricyclohexyl-substituted C3 and the 9-phenanthrenyl-based C4 afforded moderate enantioselectivity (85 : 15–87.5 : 12.5 er) but with diminished yields (entries 3–4). The 8H-binaphthyl-derived CPA C5 provided acceptable yield and er (entry 5). The 2,4,6-triisopropyl-substituted spirocyclic CPA C6 yielded only trace product (entry 6), whereas the 9-phenanthrenyl-substituted spirocyclic CPA C7 afforded good enantioselectivity (93 : 7 er) with 79% yield (entry 7). Subsequent solvent screening identified CCl₄ as the optimal choice, delivering improved enantioselectivity (97 : 3 er, entries 8–15). Reducing the temperature to 50 °C maintained high stereocontrol but lowered the yield (75%, entry 16). Omitting molecular sieves or reducing the catalyst loading adversely affected reaction efficiency (55–67%, entries 17 and 18). Further optimization (see SI) established the standard conditions: C2 (10 mol%) and 5 Å molecular sieves (50 mg) in CCl₄ (0.3 mL) at 60 °C for 72 hours.

Table 1 Optimization of the reaction conditions^a

a Reaction conditions: 1a (0.10 mmol), 2a (0.12 mmol), CPA (10 mol%) and 5 Å molecular sieves (50 mg) in solvent (0.3 mL) at 60 °C for 72 h unless otherwise noted.b Isolated yield.c Enantiomeric ratio (er) determined by chiral HPLC analysis.d Reaction performed at 50 °C.e Without 5 Å molecular sieves.f With 5 mol% catalyst loading.
Entry	CPA	Solvent	Yield^b (%)	er^c (%)
1	C1	DCE	Trace	—
2	C2	DCE	65	93.5 : 6.5
3	C3	DCE	49	87.5 : 12.5
4	C4	DCE	12	85 : 15
5	C5	DCE	57	92.5 : 7.5
6	C6	DCE	Trace	—
7	C7	DCE	79	93 : 7
8	C2	PhCl	63	89 : 11
9	C2	PhCF3	83	87.5 : 12.5
10	C2	Toluene	65	91 : 9
11	C2	DCM	57	93.5 : 6.5
12	C2	CHCl3	47	82 : 18
13	C2	CCl4	99	97 : 3
14	C2	THF	Trace	—
15	C2	Decaline	35	95 : 5
16^d	C2	CCl4	75	96.5 : 3.5
17^e	C2	CCl4	55	96 : 4
18^f	C2	CCl4	67	96.5 : 3.5

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With optimized conditions in hand, we first explored the substrate scope with respect to the hydroxylamine component (Table 2, A). Using O-(4-tert-butylphenyl)hydroxylamine (2b) afforded oxime ether 3b in 93% yield and 97 : 3 er. A phenyl substituent at the para-position gave 3c in 85% yield with 95 : 5 er. Hydroxylamines bearing an OCF₃ group provided 3d in 92% yield and 95.5 : 4.5 er. Other electron-withdrawing para-substituents—halogen (2e), ester (2f) and cyano (2g)—participated effectively, yielding products (3e–3g) with high stereoselectivity (90–93% yield, 94.5 : 5.5–96.5 : 3.5 er). meta-Bromo (2i) and ester (2j) substituents also afforded excellent stereocontrol (94.5 : 5.5 and 96.5 : 3.5 er, respectively), while a meta-methoxy group (2h) led to lower er (89.5 : 10.5) and yield (75%), likely due to its electron-donating character. ortho-Substitution (2k) compromised both enantioselectivity (87 : 13 er) and yield (76%), presumably due to steric hindrance. Benzyl-substituted hydroxylamine 2p (95% yield, 90.5 : 9.5 er) and aliphatic allylic hydroxylamine 2q (85% yield, 88.5 : 11.5 er) also underwent efficient condensation with the selected 9-phosphaanthrone (1a). Absolute configurations were assigned based on X-ray crystallographic analysis of 3a (CCDC 2478635).

Table 2 Substrate scope with respect to 9-heteroanthrone derivatives and hydroxylamines

a Reaction conditions: (1) (0.10 mmol), (2) (0.12 mmol), (C2) (10 mol%) and 5 Å molecular sieves (50 mg) in solvent (0.3 mL) at 60 °C. Yields are isolated. The er determined by chiral HPLC analysis.b Na₂SO₄ used instead of 5 Å molecular sieves.c Decaline instead of CCl₄.

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Given its superior reactivity and ease of product isolation, O-(4-tert-butylphenyl)hydroxylamine (2b) was selected to evaluate the substrate scope of 9-heteroanthrones (Table 2, B). para-Substituted electron-donating methyl or methoxy groups on phenyl group afforded 3r and 3s in excellent yields (90–92%) and enantioselectivities (94 : 6–96 : 4 er). The condensation tolerated electron-withdrawing para-substituents, including halogens and trifluoromethyl groups, yielding products 3t–3v with high enantioselectivity (95 : 5–96 : 4 er). Oxime ethers 3w and 3x were obtained in good yields (84–91%) from meta-substituted methyl and chlorophenyl groups. ortho-Methyl group afforded 3y in 89% yield and 94.5 : 5.5 er. 3,5-Dimethyl-phenyl 9-phosphaanthrone reacted smoothly to give 3z (86% yield, 96 : 4 er). Reaction of cyclohexyl 9-phosphaanthracene or phosphoramidate proceeded in 80–85% yield to give 3aa or 3ab with excellent stereoselectivity (97 : 3–98 : 2 er). By contrast, 3,6- or 2,7-dimethyl-substituted 9-phosphaanthrone provided 3ac or 3ad in moderate yields (55–62%) but good enantioselectivities (91 : 9–94.5 : 5.5 er). To further demonstrate generality, we investigated the desymmetrization of 9-Si-anthrones (1ba, 1bb) and a 9-S-anthrone (1ca) for constructing remote S/Si-stereogenic axially chiral scaffolds (see SI for condition screening). Axially chiral 9-Si-anthrone-based oxime ethers (3ba, 3bb) were obtained in excellent yields with moderate to good er (83.5 : 16.5–86.5 : 13.5). The 9-S-anthrone (1ca) reacted smoothly, delivering the product (3ca) in moderate yield (69%) with moderate enantioselectivity (79 : 21 er) (Table 2, B). The consistently lower enantioselectivities observed for these S- and Si-containing scaffolds compared to their phosphorus analogues prompt us to consider factors beyond bond length considerations, such as the steric hindrance of substituents and other secondary effects.

We next investigated the conversion of remote phosphorus axial chirality to phosphorus-centered chirality via a Beckmann rearrangement. Pd-catalyzed deallylation of 3q (88 : 12 er) afforded hydroxylamine 4, which underwent a Beckmann rearrangement to give intermediate 5 (82% over two steps, 83 : 17 er). The slight erosion in enantioselectivity may be attributed to facile isomerization of the hydroxyl group in 4. Standing at room temperature for 24 h converted compound 5 to 7b, and subsequent trichlorosilane reduction afforded 6a in 60% yield over two steps (82.5 : 17.5 er). Recrystallization yielded enantiopure R-6a (>99.5 : 0.5 er), the structure of which was confirmed by X-ray crystallography (CCDC 2478637). Reduction of R-6a with a borane-dimethyl sulfide complex gave 7a in 65% yield with retained er (>99.5 : 0.5). The P(III) center in R-6a was oxidatively functionalized using H₂O₂, S₈ and Se to furnish 7b, 7c and 7d, respectively, in excellent yields (94–99%) with complete chirality retention (>99.5 : 0.5 er). Treatment of 7b with Lawesson's reagent afforded thioamide 8a in 90% yield and 99 : 1 er. Methylation of 8a gave 9a quantitatively (99%) with 99.5 : 0.5 er. These transformations highlight the versatility of the method for accessing diverse functionalized products with high efficiency and stereochemical fidelity (Scheme 2).

Scheme 2 Synthetic transformations of 3q.

Furthermore, we present R-6a—a novel cyclic triarylphosphine scaffold that was previously inaccessible in enantiopure form through conventional synthetic routes. In contrast to conventional triarylphosphines, this structure exhibits a narrower ∠C1P1C2 bond angle (98.7° vs. 102.0°) and enhanced rigidity, features that are anticipated to enhance its performance in asymmetric catalysis (Scheme 3A). Racemization studies revealed a high inversion barrier of 30.3 kcal mol⁻¹ for R-6a in toluene at 87 °C (Scheme 3B). Notably, R-6a (>99.5 : 0.5 er) functioned effectively as a chiral monodentate phosphine ligand in Pd-catalyzed asymmetric allylic alkylation (Scheme 3C). For instance, the reaction of indole with (E)-1,2-diphenylallyl acetate 10 delivered the C3-phenylallyl-substituted indole 12 in 97% yield and 95.5 : 4.5 er. Similarly, using benzylamine 13 as the nucleophile afforded product 14 in 93% yield and 92.5 : 7.5 er. The absolute configurations of 12 and 14 were assigned based on literature reports.^77,78 These results underscore the promising potential of cyclic chiral triarylphosphines in the field of asymmetric catalysis.

Scheme 3 The bond angle analysis, racemization studies, and synthetic application of cyclic triarylphosphine 6a.

Conclusions

In summary, we have established an efficient and enantioselective construction of remote P/S/Si-stereogenic axially chiral frameworks from 9-heteroanthrones and hydroxylamines via chiral phosphoric acid-catalyzed asymmetric condensation. This work overcomes longstanding challenges in stereocontrol at distal heteroatom centers and provides access to a novel class of axially chiral oxime ethers in high enantiopurity (up to 98 : 2 er). Furthermore, these products serve as versatile platforms for diversity-oriented synthesis: through a Beckmann rearrangement, axial chirality is efficiently converted into phosphorus-centered point chirality, yielding enantioenriched dibenzoazaphosphepinones. The resulting cyclic triarylphosphine architecture not only exhibits enhanced rigidity and configurational stability but also functions as an effective chiral monodentate ligand, demonstrating promising utility in catalytic asymmetric allylation reaction. This work opens avenues for the design and application of heteroatom axially chiral systems in synthesis and catalysis.

Author contributions

W. L. performed most of the experiments, with support from Z. L. in the catalyst and substrate preparation. W. L., Y.-Z. L. and W.-P. D. conceived of the idea and designed the experiments. W. L., Y.-Z. L. and W.-P. D. wrote the manuscript. Y.-Z. L. and W.-P. D. directed the project. All authors contributed to discussions.

Conflicts of interest

There are no conficts to declare.

Data availability

CCDC 2478635 (3a) and 2478637 (6a) contain the supplementary crystallographic data for this paper.^79a,b

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental procedures, characterization data, NMR spectra, HPLC traces, and additional figures supporting the results presented in the main text. See DOI: https://doi.org/10.1039/d5sc08143b.

Acknowledgements

We are grateful for the financial support from the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (2022R01007), the NSFC (22201253), and the Start-up Research Grant from Zhejiang Normal University.

Notes and references

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