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. 2023 Sep 28;9(11):2036–2043. doi: 10.1021/acscentsci.3c00748

Enantioselective Rh(I)-Catalyzed C–H Arylation of Ferroceneformaldehydes

Chen-Xu Liu 1, Fangnuo Zhao 1, Qing Gu 1, Shu-Li You 1,*
PMCID: PMC10683487  PMID: 38033798

Abstract

graphic file with name oc3c00748_0007.jpg

As an important class of platform molecules, planar chiral ferrocene carbonyl compounds could be transformed into various functional groups offering facile synthesis of chiral ligands and catalysts. However, developing efficient and straightforward methods for accessing enantiopure planar chiral ferrocene carbonyl compounds, especially ferroceneformaldehydes, remains highly challenging. Herein, we report a rhodium(I)/phosphoramidite-catalyzed enantioselective C–H bond arylation of ferroceneformaldehydes. Readily available aryl halides such as aryl iodides, aryl bromides, and even aryl chlorides are suitable coupling partners in this transformation, leading to a series of planar chiral ferroceneformaldehydes in good yields and excellent enantioselectivity (up to 83% yield and >99% ee). The aldehyde group could be transformed into diverse functional groups smoothly, and enantiopure Ugi’s amine and PPFA analogues could be synthesized efficiently. The latter was found to be a highly efficient ligand in Pd-catalyzed asymmetric allylic alkylation reactions. Mechanistic experiments supported the formation of imine intermediates as the key step during the reaction.

Short abstract

Various planar chiral ferrocene carbonyl compounds, as an essential class of platform molecules, have been synthesized efficiently via Rh(I)-catalyzed asymmetric C−H arylation.

I. Introduction

Over the past decade, transition-metal-catalyzed enantioselective C–H functionalization reactions have contributed tremendously to the improvement of molecular complexity from readily available chemical feedstocks.1−9 Notably, Rh(I)-catalyzed asymmetric C–H functionalization reactions have progressed rapidly.10−13 In 2004, Bergman, Ellman, and co-workers made a breakthrough in Rh(I)-catalyzed intramolecular asymmetric C–H alkylation reaction using an imine as a directing group.14 Later, several elegant works for Rh(I)-catalyzed asymmetric C–H functionalization reactions have been demonstrated.15−26 In 2016, Glorius and co-workers achieved remarkable progress in the combination of a rhodium(I) precatalyst with a chiral N-heterocyclic carbene (NHC) or monodentate phosphonite ligand enabled asymmetric C(sp3)–H arylation in good yields and enantioselectivity.27,28 Our group recently explicated the mechanism of Rh(I)-catalyzed asymmetric C–H arylation, which first occurs by a directed C–H activation through a concerted metalation–deprotonation (CMD) pathway and the reductive elimination is the turnover-limiting step.29 In these examples, a variety of strong coordination directing groups such as pyridine, thione, thioamide, etc. have been employed to warrant high efficiency and stereoselectivity for Rh(I)-catalyzed asymmetric C–H functionalization reactions; however, these directing groups in general are difficult to remove or undergo subsequent conversions. To date, there are rare examples of rhodium-catalyzed intermolecular asymmetric ortho-C–H activation of aldehyde derivatives, likely due to the weak coordination ability of the carbonyl group and the potential competitive aldehyde C–H cleavage by rhodium catalysts.11

Ferrocene derivatives have received extensive attention in materials science, medicinal chemistry, and asymmetric catalysis because of their unique electronic and structural properties (Figure 1a). Planar chiral ferrocene carbonyl compounds are an essential class of platform molecules that can be transformed and applied in the synthesis of chiral ligands and catalysts, such as Ugi’s amine, PPFA, Josiphos, PHOX-type ligands, and so on.30−37 Traditional methods for the synthesis of planar chiral ferrocene carbonyl compounds usually relied on preinstalled chiral auxiliaries, and tedious synthetic steps were needed (Figure 1b).38 The utilization of various reactive metal reagents also resulted in poor functional group compatibility. Meanwhile, transition-metal-catalyzed enantioselective C–H functionalization is emerging as a powerful tool for synthesizing planar chiral ferrocenes.39−42 However, directing groups often need to be preloaded and are difficult to remove.43−57 Therefore, the development of an enantioselective and efficient synthesis of structurally diverse planar chiral ferroceneformaldehydes is highly desirable.

Figure 1.

Figure 1

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Enantioselective synthesis of planar chiral ferrocenes. (a) Molecules containing a ferrocene scaffold. (b) Previous synthesis of planar chiral ferroceneformaldehydes. (c) Asymmetric C–H arylation of ferroceneformaldehydes.

Inspired by these above pioneering results, we recently found that Rh(I)-catalyzed asymmetric C–H arylation of ferroceneformaldehydes was realized by a strategy of the in situ formation of imines (Figure 1c). In the presence of a Rh(I) catalyst derived from chiral phosphoramidite, direct arylation of ferroceneformaldehydes with readily available aryl halides proceeded in excellent enantioselectivity. Herein, we report the results of this study.

II. Results and Discussion

II.1. Reaction Development

Imines are known as efficient directing groups for Rh(I)-catalyzed C–H functionalization reactions.58−60 To overcome the weak coordinating ability of aldehyde, in situ generated imine from ferroceneformaldehyde 1a and benzylamine was examined in the presence of 5 mol % [Rh(C2H4)2Cl]2, 20 mol % L1, and 2.0 equiv of LiOtBu. To our delight, the arylation product 3aa was obtained in 66% NMR yield and 99% ee (Table 1, entry 1). Other aryl-substituted (TADDOL)-derived phosphoramidite ligands such as 3,5-Me2-C6H3 and 2-naphthyl reduced the yield of the reaction (entries 2 and 3, 22–24% NMR yields, 98% ee). Meanwhile, the substituents attached to the nitrogen atom in the ligand were also investigated, but the yield and enantioselectivity decreased when more bulky substituents were introduced (entries 4 and 5, 21% NMR yield, 50–71% ee). Chiral phosphonite ligands L6 and L7 also enabled the reaction to proceed smoothly (entries 6 and 7, 53–60% NMR yields, 99% ee). The utilization of diastereoisomers of the Feringa ligand (L8 or L9) resulted in a marked decrease in both yield and enantioselectivity (entries 8 and 9, 12–24% NMR yields, 42–73% ee). Unfortunately, SPINOL- or BINOL-derived chiral phosphoramidite ligands L10 or L11 failed to give 3aa (entries 10 and 11). In addition, NaOtBu as the base also led to excellent enantioselectivity but a moderate yield (entry 12). Then, screening a variety of solvents disclosed that dioxane could provide better results, leading to 3aa in 80% NMR yield and >99% ee (entry 13). When (R)-phenylethylamine was used instead of benzylamine, together with either configuration of L1 or PPh3 as ligand, the target product was not formed, likely due to the steric hindrance of the chiral imine. Overall, the optimized reaction conditions were obtained as the following: [Rh(C2H4)2Cl]2 (5 mol %), (R, R)-L1 (20 mol %), and LiOtBu (2.0 equiv) in dioxane at 80 °C.

Table 1. Optimization of the Reaction Conditionsa.

II.1.

entry ligand base solvent NMR yield (%)b ee (%)c
1 L1 LiOtBu THF 66 99
2 L2 LiOtBu THF 24 98
3 L3 LiOtBu THF 22 98
4 L4 LiOtBu THF 21 71
5 L5 LiOtBu THF 21 50
6 L6 LiOtBu THF 60 99
7 L7 LiOtBu THF 53 99
8 L8 LiOtBu THF 12 73
9 L9 LiOtBu THF 24 –42
10 L10 LiOtBu THF <5  
11 L11 LiOtBu THF <5  
12 L1 NaOtBu THF 20 98
13 L1 LiOtBu dioxane 80 (76)d >99
14 L1 LiOtBu toluene 20 99
15 L1 LiOtBu DCE 8  
16 L1 LiOtBu MeCN <5  
17 L1 LiOtBu MeOH <5  
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a

Reaction conditions: 1a (0.1 mmol), BnNH2 (0.11 mmol) in DCE (0.5 mL) at 80 °C for 4 h, then 2a (0.2 mmol), [Rh(C2H4)2Cl]2 (0.005 mmol), ligand (0.02 mmol), and base (0.2 mmol) in solvent (1.0 mL) at 80 °C.

b

Determined using 1,3,5-trimethoxybenzene as an internal standard.

c

Determined by HPLC analysis with a chiral stationary phase.

d

Isolated yield.

II.2. Scope and Synthetic Applications

With the optimized conditions in hand, the substrate scope of this reaction was investigated (Figure 2). First, a series of aryl halides was examined. Interestingly, the utilization of 4-methoxychlorobenzene could afford a moderate yield and excellent enantioselectivity (65% yield, 99% ee), and 4-methoxyiodobenzene was also tolerated (62% yield, 84% ee). Unfortunately, 4-methoxyphenyl 4-methylbenzenesulfonate led to no reaction. The chiral catalytic system readily promoted the reactions of ferroceneformaldehydes with various para-substituted phenyl bromides, and the desired planar chiral ferroceneformaldehydes 3ab–an were obtained in reasonable yields (45–80%) with high enantioselectivity (96–>99% ee). For meta-substituted phenyl bromides, the reaction proceeded smoothly to generate the arylated products 3ao,ap (75–82% yields, 99 to >99% ee). Good yields and excellent enantioselectivity were obtained for multisubstituted phenyl bromides (3aq–at, 78–83% yields, >99% ee). When 5-benzofuryl-, 5-benzothienyl-, 6-benzothienyl-, 5-(N-methyl)indolyl- or 3-thienyl-bromides were applied, the corresponding products 3au–ay were delivered in 68–73% yields with 98–>99% ee. Meanwhile, the ruthenoceneformaldehydes participated in this reaction to give the products with excellent enantioselective control (3ba,ca, 60–72% yields, 98–>99% ee). Deuterated aldehyde substrate 1d could lead to comparable results (3da, 75% yield, >99% ee). However, acetylferrocene was not reactive under the standard conditions likely due to the difficulty in the formation of ketimine.

Figure 2.

Figure 2

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Scope of Rh(I)-catalyzed C–H arylation of ferroceneformaldehydes. Reaction conditions: 1 (0.2 mmol), BnNH2 (0.22 mmol) in DCE (1.0 mL) at 80 °C for 4 h, then 2 (0.4 mmol), [Rh(C2H4)2Cl]2 (0.01 mmol), (R,R)-L1 (0.04 mmol), and LiOtBu (0.4 mmol) in dioxane (2.0 mL) at 80 °C. Yields of isolated products are reported. ee values were determined by HPLC analysis on a stationary chiral stationary phase. Note: (a) [Rh(C2H4)2Cl]2 (0.02 mmol) and (R, R)-L1 (0.08 mmol) were used.

Next, the reactions of ferroceneformaldehydes bearing diverse substituents, including alkyl (methyl, ethyl, and n-butyl), alkenyl (vinyl and 2-propenyl), aryl (phenyl, 4-methoxyphenyl, 1-naphthyl, 9-anthracenyl), and trimethylsilyl on the other Cp ring, were explored under the optimized conditions. Generally, the asymmetric induction of these reactions was very high, and excellent enantioselectivity was obtained for the corresponding ferroceneformaldehyde products 3fa–oa (98–>99% ee). However, the increased bulkiness of the other Cp ring reduced the reactivity of these ferroceneformaldehyde derivatives. For example, the reaction of 1′-(9-anthracene)ferroceneformaldehyde 1n could give acceptable results (3na, 51% yield, >99% ee) only with increased catalyst loading (10 mol % [Rh(C2H4)2Cl]2 and 40 mol % (R,R)-L1).

As a further demonstration of the utility of this method, a gram-scale reaction of 1a (10 mmol) and 2a was carried out. Pleasingly, the corresponding product 3aa was obtained in 70% yield and >99% ee with only 2.0 mol % of [Rh(C2H4)2Cl]2. The absolute configuration of 3aa was assigned as Sp by the X-ray diffraction analysis of an enantiopure sample. To further demonstrate the potential utility of this reaction, various transformations of product (Sp)-3aa (>99% ee) were carried out (Figure 3). The aldehyde group could be transformed into diverse functional groups smoothly, such as methyl (4a), vinyl (4b), acetenyl (4c), hydroxyl (4d), primary amine (4e), secondary amine (4f), and tertiary amine (4g) without the loss of enantioselectivity. Meanwhile, the aldehyde group on 3aa could be protected by thiols via simple protocols (4h, 72% yield, 99% ee). In addition, the Perkin reaction could be conducted to generate carboxylic acid 4i (70% yield, >99% ee).

Figure 3.

Figure 3

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Gram-scale reaction and transformations of (Sp)-3aa.

Subsequently, starting from (Sp)-3aa, chiral benzyl alcohol (R,Sp)-5 could be synthesized efficiently in 80% yield and 19:1 dr via Grignard addition (Figure 4). (R,Sp)-5 could be transformed into Ugi’s amine derivative (R,Sp)-6 by a one-pot procedure, and its absolute configuration was confirmed by comparing with the literature61 (see the Supporting Information for details). Meanwhile, the chiral PPFA ligand derivative (R,Sp)-7 could be synthesized in 75% yield and >19:1 dr and was found to be an efficient ligand in a Pd-catalyzed asymmetric allylic alkylation reaction (95% yield, 93% ee). In addition, chiral thioether (R,Sp)-8 and monophosphine (R,Sp)-9 were synthesized in good yields and diastereoselectivity (70–80% yields, >19:1 dr). These transformations further enhance the synthetic utility of the current method.

Figure 4.

Figure 4

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Product synthetic utility.

II.3. Mechanistic Studies

Preliminary experiments were carried out to gain insight into the reaction mechanism. Product 3aa was not observed when the reaction of 1a was carried out in the absence of benzylamine, and only product 13 was obtained in 10% yield (Figure 5a). This shows that the formation of an imine is necessary. In situ NMR and HRMS experiments further confirmed the formation of imine intermediate I (Figure 5b). Notably, a parallel KIE experiment in which the kinetic isotope effect of 1a is only 1.10 (kH/kD) suggests that C–H cleavage is not the rate-determining step (Figure 5c). The competitive experiment between 4-bromoanisole 2a and 4-bromobenzonitrile 2n revealed that substrates bearing an electron-deficient group are more reactive than those with an electron-rich group (Figure 5d). Based on the above studies and a previous report,27 a plausible catalytic cycle was proposed for this enantioselective Rh(I)-catalyzed C–H arylation of ferroceneformaldehydes. As shown in Figure 5e, ferroceneformaldehyde 1 is dehydrated with benzylamine to form imine intermediate I. The coordination of I with the Rh precursor generates the intermediate II. Then, the intermediate II first is generated by directed C–H activation through a concerted metalation–deprotonation (CMD) pathway, delivering the intermediate III. Next, the oxidative addition of aryl halide 2 and reductive elimination proceed to give intermediate V. Finally, product 3 is obtained via hydrolysis of intermediate V.

Figure 5.

Figure 5

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Mechanistic studies.

III. Summary and Conclusions

In conclusion, we have developed a highly efficient synthesis of planar chiral ferroceneformaldehyde derivatives by enantioselective Rh(I)-catalyzed C–H arylation with readily available aryl halides under mild reaction conditions. These processes occur with excellent levels of monoarylation selectivity, enantioselectivity, and efficiency. Diverse functional groups were tolerated, and chiral Ugi’s amine and PPFA ligand analogues could be synthesized efficiently through simple protocols. The obtained chiral ferrocene ligand was found to be efficient in a Pd-catalyzed asymmetric allylic alkylation reaction. In situ NMR and HRMS experiments confirmed the formation of the imine intermediate. Further studies on enantioselective Rh(I)-catalyzed C–H functionalization reactions toward more diverse chiral molecules are ongoing in this laboratory.

Acknowledgments

We thank the National Key R&D Program of China (2021YFA1500100), NSFC (21821002, 92256302, 22071260) and the Science and Technology Commission of Shanghai Municipality (21520780100 and 22JC1401103) for generous financial support. S.-L.Y. acknowledges support from the Tencent Foundation through the XPLORER PRIZE and the New Cornerstone Science Foundation.

Supporting Information Available

(PDF) The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.3c00748.

  • Experimental procedures, complete characterization data, and copies of 1H, 13C, and 19F NMR spectra (PDF)

  • Crystallographic data for 3aa (CIF)

  • NMR fid files (ZIP)

Author Contributions

† C.-X.L. and F.Z. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

oc3c00748_si_001.pdf (11.7MB, pdf)
oc3c00748_si_002.cif (264.3KB, cif)
oc3c00748_si_003.zip (68MB, zip)

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oc3c00748_si_001.pdf (11.7MB, pdf)
oc3c00748_si_002.cif (264.3KB, cif)
oc3c00748_si_003.zip (68MB, zip)

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