Cobalt-catalyzed asymmetric phospha-Michael reaction of diarylphosphine oxides for the synthesis of chiral organophosphorus compounds

Abstract

The asymmetric Michael addition of phosphorus nucleophiles to electron-deficient alkenes is one of the most direct and atom-economical methods to provide chiral organophosphorus compounds with high efficiency in recent years. Herein, we report a cobalt-catalyzed imidazolyl-directed asymmetric phospha-Michael-type reaction of diarylphosphine oxides with electron-deficient alkenes for synthesizing chiral organophosphorus compounds in moderate to good yields and good to excellent enantioselectivities (25 examples, up to 99% yield, and 99% ee). This protocol features broad substrate scope, good functional group tolerance, and mild conditions as well as avoids the release of massive metal wastes and the use of noble transition metal catalysts. The excellent enantioselectivity of the phospha-Michael reaction can be due to the adoption of a novel chiral N4-ligand. Furthermore, the DFT calculation indicates that the bulky 2,4,6-(i-Pr)₃C₆H₂ group of the ligand induces large steric hindrance which blocks the nucleophilic attack from the Si-face.

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Chiral organophosphorus compounds are not only used as versatile ligands, or organocatalysts in asymmetric catalytic transformation,¹ but also frequently found in natural products, pharmaceuticals, agrochemicals or functional materials due to their unique properties.² Therefore, numerous efforts have been made toward the catalytic asymmetric synthesis of organophosphorus compounds. Among them, the asymmetric phospha-Michael addition³ is one of the most direct and atom-economical methods to provide chiral organophosphorus compounds with high efficiency. In the past few decades, transition metal-catalyzed (including Pd, Cu, etc.),⁴ organocatalyzed (i.e., cinchona alkaloids, proline derivatives, N-heterocyclic carbenes, chiral phosphoric acids, etc.)⁵ and Lewis acid-catalyzed (i.e., Zn, Mg, etc.)⁶ asymmetric phospha-Michael addition of secondary phosphine oxides to electron deficient alkenes have been well developed. Recently, Liu and coworkers⁷ reported the first Cu(I)-catalyzed enantioselective phosphinocyanation of styrenes through a tandem radical relay strategy; in 2020, Fu et al.⁸ developed a nickel-catalyzed enantioconvergent reductive hydroalkylation of olefins with an α-phosphorus alkyl electrophile to construct chiral organophosphorus compounds; recently, Wang's group⁹ reported an enantioselective palladium-catalyzed hydrophosphinylation of allenes and a copper-catalyzed hydroamination for the synthesis of chiral organophosphorus compounds, respectively. Shortly afterwards, Xu and coworkers¹⁰ presented a Ni/photoredox-catalyzed enantioconvergent reductive cross-coupling between α-bromoalkylphosphates and aryl iodide for the synthesis of chiral α-aryl phosphorus compounds. Despite these remarkable advances, exploring a novel and environmentally friendly phospha-Michael-type reaction for the preparation of enantioenriched organophosphorous compounds under mild conditions is still highly desired.

Cobalt, a cheap and low toxic element in comparison with other transition metals, was found to exhibit excellent activity in asymmetric catalysis and has received growing attention from the synthetic community in the last two decades.¹¹ Recently, Duan's group¹² reported a cobalt-catalysed asymmetric phospha-Michael addition for the synthesis of P-stereogenic compounds, in which a pyridyl moiety was adopted as the coordinating group in secondary phosphine oxides in order to obtain good enantioselectivity (Scheme 1a). In 2019, we reported a visible-light-induced cobalt-catalyzed enantioselective radical conjugate addition when the novel chiral Co-centered octahedral complex was used as the catalyst and an imidazolyl moiety as the coordinating group in the Michael acceptors (Scheme 1b).¹³ Based on our continuing interest in the synthesis of biologically active organophosphorus compounds,¹⁴ herein, we report a cobalt-catalyzed asymmetric phospha-Michael-type reaction of diarylphosphine oxides with electron-deficient alkenes by introducing an imidazolyl moiety to coordinate with the cobalt complex catalyst in a bidentate chelating form in order to obtain a high enantioselectivity control (Scheme 1c).

Scheme 1 Cobalt-catalyzed asymmetric conjugate addition reactions.

We commenced our investigation on the cobalt-catalyzed asymmetric phospha-Michael-type reaction of diarylphosphine oxides with electron-deficient alkenes for synthesizing chiral organophosphorus compounds using enone (1a) containing a phenylimidazolyl directing group and diphenylphosphine oxide (2a) as the model substrates, CoCl₂·6H₂O as the catalyst in the presence of various chiral N4 ligands in dichloromethane (DCM) at room temperature. To our delight, when cyclohexyldiamine-derived benzoimidazolyl N4 ligand (L1, 8.6 mol%) was used, the asymmetric phospha-Michael reaction indeed occurred, giving the product 3a in excellent yield with moderate enantioselectivity (Table 1, entry 1: 99% yield, 49% ee). Screening of N4 ligands showed that increasing the steric hindrance of the benzene ring of N4 ligands could improve the enantioselectivity of the phospha-Michael reaction (Table 1, entries 4 and 5, 86% ee and 91% ee, respectively); replacement of the chiral cyclohexane-1,2-diamine unit with the 1,2-diphenylethane-1,2-diamine skeleton could further increase the enantioselectivity of the phospha-Michael reaction slightly (Table 1, entry 6, 93% ee); however, when N4 ligands containing a benzoxazolyl or benzothiazolyl moiety were explored, both the reactivity and enantioselectivity of the phospha-Michael reaction decreased remarkably (Table 1, entries 7–10). In order to investigate how the steric environments of the cobalt complex catalyst affect the reaction activity and enantioselectivity of the phospha-Michael reaction, we obtained the chiral cobalt octahedral complexes ΛCo9 and ΛCo10 by stirring ligand L9 or L10 with CoCl₂·6H₂O in acetonitrile at room temperature, their structures were confirmed by X-ray crystallography analysis (Scheme 2).¹⁵ As shown in Scheme 2, the coordination of two nitrogen atoms of the chiral diamine moiety with the Co center, though preferably induces the chirality of the cobalt center, makes the sterically hindered tert-butyl groups move far away from the plane comprising Co and two Cl atoms or a Cl atom and an O atom of H₂O (distance: SΛCo9 = 4.95 Å, SΛCo10 = 4.69 Å vs. SΛCo2 = 5.02 Å). Furthermore, by analyzing the bond angle of Cl–Co–Cl (or O) in different octahedral complexes (∠2 = 95.9° for ΛCo2, ∠2′ = 100.3° for ΛCo9, ∠2′′ = 99.9° for ΛCo10, vs. ∠2′′′ = 87.8° for Meggers’ catalyst ΛRh1¹⁶), the potential coordination environments of ΛCo9, ΛCo10 and ΛCo2 were found to provide relatively unrestricted access to nucleophiles compared to Meggers’ catalyst ΛRh1. Therefore, these steric characteristics would be unfavorable for the stereoinduction capacity of the cobalt catalysts ΛCo2, ΛCo9 and ΛCo10 (Fig. 1) in contrast to Meggers’ catalyst ΛRh1. So, we concluded that these cobalt complexes (ΛCo2, ΛCo9 and ΛCo10) were not good candidate catalysts for the phospha-Michael reaction with good stereochemical outcomes. In addition, DFT calculation indicates that the bulky 2,4,6-(i-Pr)₃C₆H₂ group of the ligand L6 induces a large steric hindrance which blocks the nucleophilic attack from the Si-face, so the cobalt complex ΛCo6 was a suitable catalyst for guaranteeing the phospha-Michael reaction with good enantioselectivities.

Scheme 2 Synthesis and crystal structures of the cobalt-complex catalysts ΛCo9 and ΛCo10.

Fig. 1 Comparison of the stereochemical parameters of ΛCo9 and ΛCo10 with ΛCo2 (CCDC 1870638).¹³

Table 1 Optimization of the reaction conditions^a

Entry	Ligand	Solvent	Yield^b (%)	ee ratio^c (%)
a Unless noted otherwise, reactions were performed with 1a (0.10 mmol), 2a (0.15 mmol, 1.5 equiv.), CoCl2·6H2O (8 mol%), L (8.6 mol%), AgOTf (16 mol%) in DCM (1.0 mL) at room temperature for 48 h. b Determined by ^1H NMR using triphenylmethane as an internal standard. c Determined by chiral HPLC analysis. d 2.0 mL of DCM was used. e 1.0 equiv. of 2a was used. f 2.0 equiv. of 2a was used. g Replaced AgOTf with AgClO4 (16 mol%). h Isolated yield in parenthesis.
1	L1	DCM	99	49
2	L2	DCM	99	53
3	L3	DCM	99	48
4	L4	DCM	99	86
5	L5	DCM	88	91
6	L6	DCM	98	93
7	L7	DCM	42	3
8	L8	DCM	59	43
9	L9	DCM	36	24
10	L10	DCM	40	47
11	L6	Acetonitrile	75	48
12	L6	Acetyl acetate	93	6
13	L6	CHCl3	92	84
14	L6	THF	99	18
15	L6	Et2O	80	2
16^d	L6	DCM	96	86
17^e	L6	DCM	94	93
18^f	L6	DCM	95	85
19	L6	DCM	99 (92)	96

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Encouraged by this result, we continued to evaluate the effect of solvents and found that, highly polar solvents such as acetonitrile or medium polar ethyl acetate and ether solvents such as THF or ethyl ether are not effective at improving the yields of products and enantioselectivity; however, halogenated solvents such as DCM or chloroform are suitable one, and DCM is the best solvent for improving the outcome of the phospha-Michael reaction (Table 1, entries 6 and 11–15). When the amount of DCM used was increased to 2 mL, the enantioselectivity of the phospha-Michael reaction decreased slightly (Table 1, entry 16). Subsequently, increasing the molecular ratio of substrates 1a and 2a to 1 : 2 or decreasing the ratio to 1 : 1, could not improve the reaction efficiency and stereoselectivity (Table 1, entries 17 and 18). In addition, the addition of bases is not beneficial for the phospha-Michael reaction (please see the ESI†). Furthermore, when the additive AgClO₄ was used instead of AgOTf, the ratio of excess enantioselectivity was further increased to 96% (Table 1, entry 19). Finally, the results of the control experiments showed that the cobalt source, N4 ligand and the additive AgClO₄ were crucial for the asymmetric phospha-Michael addition (please see the ESI†).

After the optimal conditions were established, the scope of β-aryl substituted α,β-unsaturated enones containing phenylimidazolyl directing group 1 was then investigated. As shown in Table 2, substrates 1 bearing either an electron-donating group (i.e., methyl-, methoxy-, tert-butyl, N,N-dimethylamino) or an electron-withdrawing one (i.e., fluoro-, chloro-, bromo-, iodo-, trifluoromethyl, methoxycarbonyl) on the benzene ring, in either ortho-, meta- or para-substituted pattern, were found to be compatible for the cobalt-catalyzed asymmetric phospha-Michael reactions, generating the corresponding products in moderate to excellent yields and good to excellent enantioselectivities (3a–3p, 68%–99% yield, 87%–99% ee). It's worth mentioning that furyl and thienyl substituted enones were tolerated well in this reaction and furyl substituted enone gave the target compound (3q) with the highest yield and stereochemistry. Furthermore, β-alkyl substituted α,β-unsaturated enone was also found to be suitable for the reaction, and the corresponding product 3s was obtained with excellent enantioselectivity (>99% ee), albeit in moderate yield (50% yield). Next, we turned to investigate whether other diaryl phosphine oxides or dialkyl phosphites could act as effective phosphorus nucleophiles when 1a was used (Table 2). To our delight, a series of diaryl phosphine oxides were all good candidates in this reaction. As highlighted in Table 2, diaryl phosphine oxides 2 bearing either diphenyl, naphthyl substituted or different halogen (fluoro or chloro) substituted or the halogen atom in different positions on the benzene ring all worked well and the corresponding products (3u–3y) were obtained in excellent yields and enantioselectivities (92–98% yields, 93–99% ee). However, dialkyl phosphites (for example, dimethyl or diethyl phosphite) were not active in the cobalt-catalyzed asymmetric phospha-Michael-type reaction, which might probably be due to the poor nucleophilicity of the dialkyl phosphites. In order to determine the absolute configuration of products 3, a single crystal of 3a was cultured and unambiguously confirmed by X-ray single-crystal diffraction analysis.¹⁵ The result showed that 3a was presented in S-configuration.

Table 2 Substrate scope of the cobalt-catalyzed asymmetric conjugate addition reactions^a,b,c

a Unless noted otherwise, all reactions were performed with 1 (0.20 mmol), 2 (0.30 mmol, 1.5 equiv.), CoCl₂·6H₂O (0.016 mmol, 8 mol%), L6 (0.018 mmol, 8.6 mol%), AgClO₄ (0.032 mmol, 16 mol%) in dichloromethane (2.0 mL) at room temperature for 48 h. b Isolated yield. c Determined by chiral HPLC analysis using Chiral OD-H or AS-H column, and hexane/i-PrOH as the eluent. d Reaction time of 7 h. e Reaction time of 72 h.

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In order to evaluate the synthetic utility of the cobalt-catalyzed asymmetric phospha-Michael-type reaction, a gram-scale reaction of 1a (3.5 mmol) and 2a (5.25 mmol, 1.5 equiv.) was performed under standard conditions, the target product 3a (1.60 g) was obtained in 91% isolated yield with 96% ee (Scheme 3).

Scheme 3 Scale-up preparation and synthetic transformations of 3a.

Furthermore, the target compounds could be conveniently transformed into functionalized chiral organophosphorus compounds. 3a could easily transform into chiral organophosphorus compound bearing an ester group 4a in 93% yield with 81% ee in a simple one-step procedure; additional, chiral ketone compound containing phosphorus 5a was obtained in moderate yield with excellent stereoselectivity (54% yield, 90% ee) by the addition of 3a with Grignard reagent (Scheme 3).

On the basis of some related literature reports^12,13 and our control experiment results, a plausible mechanism is suggested in Fig. 2. Initially, CoCl₂ combined with N4 ligand L6 to form a cobalt complex, which acted as a Lewis acid to activate enones 1 by coordinating with the N atom of phenylimidazole and O atom of enone 1 in a bidentate chelating form (Int-1). Subsequently, diphenylphosphine oxide 2a could tautomerize to 2a′ and undergo conjugate addition with activated enone (Int-1) to form Int-2. Product 3 was generated through a proton transfer process and subsequently the liberation of the cobalt complex catalyst.

Fig. 2 A plausible reaction mechanism and the suggested stereoinduction model.

In addition, in order to gain insight into the origin of stereocontrol of the cobalt complex ΛCo6 that catalyzed the phospha-Michael addition reactions, the DFT calculations were carried out and a stereoinduction model was also proposed to rationalize the stereochemical outcome. As shown in Fig. 2, the Re-face addition of the phosphorus nucleophile to the β-position of enones, is preferential because of the reduced steric impulsion from the N4 ligand.

In conclusion, we developed a cobalt-catalyzed imidazolyl-directed asymmetric phospha-Michael-type reaction of diarylphosphine oxides with electron-deficient alkenes for synthesizing chiral organophosphorus compounds. This protocol provides an efficient access to chiral organophosphorus compounds in moderate to excellent yields with good to excellent enantioselectivities under mild reaction conditions, meanwhile avoiding the release of massive metal wastes and the use of noble transition metal catalysts. The excellent enantioselectivity of the phospha-Michael reaction can be due to the adoption of a novel chiral N4-ligand. In addition, the phospha-Michael adducts 3 can be conveniently transformed into functionalized chiral organophosphorus compounds.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (No. 51573066) and the Region Joint Funds of the National Natural Science Foundation of China (No. U21A20384) for support of this research.

References

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Footnote

† Electronic supplementary information (ESI) available. CCDC: 2131088, 2131089 and 2131091. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qo01483a

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