The Synthesis of Novel P,N-Ferrocenylpyrrolidine-Containing Ligands and Their Application in Pd-Catalyzed Allylic Alkylation – A Synthetic and Mechanistic Investigation

Abstract

The synthesis of a series of planar chiral P,N-ferrocenylpyrrolidine-containing ligands, with varying substituents at the phosphorus donor atom, is described. The phosphorus donor atom was introduced via a diastereoselective ortho-directed metalation of N-methylpyrrolidinyl ferrocene followed by a quench with various chlorophosphines. These P,N systems are very active in Pd-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate with dimethylmalonate (conversions of up to 100%) and demonstrated good levels of enantioselectivity (up to 85% ees). Good selectivity for the (R)-enantiomer was observed and mechanistic studies, involving X-ray crystallography and NMR spectroscopic experiments, were employed to help rationalize the observed stereochemical outcome of the reaction.

Since the serendipitous discovery of ferrocene in 1951,[1] there has been a wide range of research into the synthesis of ferrocene derivatives, now one of the most important structural motifs in materials science, organometallic chemistry, and asymmetric catalysis.[2] Planar-chiral ferrocenes with 1,2-substitution are the most studied substitution pattern in ferrocene-containing ligands for transition-metal-catalyzed asymmetric synthesis. Since the pioneering work of Ugi on the C₂ functionalization of enantiopure N,N-dimethyl-1-ferrocenylethylamine (1, Figure [1]),[3] the most common strategy for introducing planar chirality is based on the diastereoselective ortho-lithiation of monosubstituted ferrocenes containing an appropriate chiral ortho-directing group and subsequent quenching with an electrophile. The ligand PPFA (2) was the first reported example of a planar-chiral enantiopure ferrocenyl phosphinamine by Hayashi and Kumada in 1974.[4] The ease of preparing planar chiral ferrocene compounds employing diastereoselective ortho-metalating groups, including amines, sulfoxides, acetals, oxazolines, azepines, sulfoximines, and hydrazones, has contributed to the wide range of planar chiral ferrocene ligands reported to date.[5] To date, hundreds of chiral P,N-ferrocenyl ligands have been prepared, exemplified by the ferrocenyloxazoline ligands 3 developed by Richards,[6] Sammakia[7] and Uemura[8] and applied by us in Pd-catalyzed allylic amination[9] and asymmetric Heck transformations.[10] We reported the preparation of P,N ligands of type 4, possessing both planar and central chirality obtained through diastereoselective metalation of trans-(2R,5R)-2,5-dialkyl-1-(ferrocenylmethyl)pyrrolidines and their application in Pd-catalyzed allylic alkylation.[11]

Due to the successful application of ferrocene-containing ligands and wishing to extend our work on pyrrolidine-containing P,N ligands,[12] [13] [14] we investigated novel ferrocene ligands in which the α-chiral center is incorporated into a pyrrolidine unit (Figure [2]). We previously reported the enantioselective preparation of the key intermediate ferrocenepyrrolidine (R)-5 and applied it in the diastereoselective formation of a series of N,O ligands of type 6 for the diethylzinc-mediated addition to aldehydes, affording enantioselectivities of up to 95% ee.[15] In addition, we reported the synthesis of novel ferrocene-phosphinamine ligands of type 7, again obtained through diastereoselective ortho-lithiation of ferrocenepyrrolidine (R)-5, and their application in the Pd-catalyzed allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate gave enantioselectivities of up to 77% ee.[16]

The success of the P,N-ligand class 7 prompted us to test the effect of varying the substituents at the phosphorus donor atom both electronically and sterically. It has been previously demonstrated that the presence of electron-withdrawing and electron-donating groups at phosphorus can have a significant effect on the selectivity and reactivity of their metal complexes in asymmetric catalysis.[17] Herein we describe the synthesis of three novel P,N-ferrocenylpyrrolidine ligands 8a–c, in which we have varied the sterics and electronics at the phosphorus donor atom. In addition, the application of these ligands in Pd-catalyzed allylic substitution and a spectroscopic and X-ray crystallographic study of their Pd η³-allyl species will be presented.

Using optimized lithiation conditions, ferrocenylpyrrolidine (R)-5 was treated with s-BuLi (1.3 equiv) in Et₂O at –78 °C for three hours and warmed to 0 °C for two hours to ensure lithiation before quenching with three chlorophosphines to afford (R,R_p )-8a–c in 80–85% de (Scheme [1]), which were subsequently purified by column chromatography in 51–61% yields.

The (R)-central chirality of the pyrrolidine and the (R)-planar chirality of the ferrocene backbone were confirmed as crystals of ligand 8b grown from pentane were suitable for X-ray crystallography analysis (Figure [3]).

Pd-catalyzed asymmetric allylic alkylation, first reported by Trost,[18] [19] [20] is one of the most important asymmetric catalytic transformations in organic chemistry for the formation of a diversity of bonds to carbon.[21–24] Significant effort has been devoted to elucidate the factors that influence the outcome of the reaction, such as metal, solvent, nucleophile, allylic substrate, and ligand, including mechanistic investigations underpinned by a combination of structural, spectroscopic, and computational studies.[25–29] The design and application of chiral P,N ligands has received much attention as, with this class of ligands, electronic desymmetrization of the intermediate Pd η³-allyl encourages preferential attack of the nucleophile at the allylic carbon trans to phosphorus, which is more electrophilic due to the larger trans effect exerted by phosphorus.[30]

Pd(η³-allyl)phosphinamine complexes 10a–c were prepared in good yields (82–90%) on reaction of the corresponding phosphinamine ligands 8a–c with palladium chloride dimer 9 and sodium tetrafluoroborate (Scheme [2] ). [31] The ³¹P NMR spectra of each of the complexes prepared contained one peak showing that only one of the two possible diastereomeric Pd η³-allyl complexes has been formed.

Prior work within the group had shown that the BSA method developed by Trost[18] proved to afford higher conversions and enantioselectivities compared with the sodium malonate procedure when Pd complexes of ligand 7a were used. Therefore, in the Pd-catalyzed allylic alkylation with complexes 10a–c, the BSA procedure was applied, using 1,3-diphenylpropenyl acetate (11) as the substrate, dimethylmalonate (12) as the nucleophile, and a catalyst loading of 2 mol% in each case (Scheme [3], Table [1]).

Variation of solvents showed that the use of both dimethylformamide (DMF) and acetonitrile resulted in excellent conversions with moderate enantioselectivities for ligands 8a (Table [1], entries 1 and 4) and 8b (Table [1], entries 2 and 5), but poor enantioselectivities for ligand 8c (Table [1], entries 3 and 6). The use of dichloromethane resulted in lower conversions, except in the case of ligand 8b (Table [1], entry 8), and moderate enantioselectivities again for ligands 8a and 8b (Table [1], entries 7 and 8), but poor for ligand 8c (Table [1], entry 9).

The size of the nucleophile’s counterion is known to have an effect on the rate and selectivity of the reaction.[33] In order to test the effect of the added base, lithium acetate and sodium acetate were also applied with DMF as the solvent. When lithium acetate was employed, optimum selectivity for all three ligands was achieved with lower conversions (Table [1], entries 13–15). Sodium acetate and potassium acetate afforded similar results (compare entries 1–3 with 10–12). The use of lithium acetate was also tested in conjunction with acetonitrile, and this resulted in slightly higher enantioselectivities than that of DMF with lithium acetate (Table [1], entries 16–18). Therefore, the combination of lithium acetate in acetonitrile was used when testing the effect of varying the temperature of the reaction.

Cooling the reaction to 0 °C did not increase the levels of enantioselectivity for any of the ligands and even after 6 days, conversions were very poor (9–44% yield, entries 19–21). Considering conversions dropped significantly when using lithium acetate at room temperature compared with any of the other bases, the idea of heating the reaction mixture seemed appropriate. In this case the enantioselectivity for Pd complexes of ligands 8a and 8c increased slightly along with the desired effect of improved conversions, with the latter affording our optimal set of results with 92% yield and 85% ee (entries 22 and 24). Although the conversion did improve for ligand 8b, the enantioselectivity did decrease to 68% ee (entry 23). The optimal result previously obtained with the analogous parent ligand 7a was quantitative conversion and 77% ee in DMF, with KOAc as base at room temperature (Table [1], entry 25). Comparing the optimum results for the three novel ligands with the original diphenylphosphine ligand 7a of 76–85% ee,[16] it can be concluded that using these substituents to change the electronic and steric environment at phosphorus does not appear to play a major role in effecting the stereochemical outcome. However, Pd complexes of the o-tolyl-derived ligand 8c, with an additional steric effect, did induce the optimal level of enantioselectivity of 85% ee.

Considerable effort has been made to understand the critical enantiodifferentiating carbon–carbon bond-forming step from the initial investigations of Bosnich[31] employing diphosphine ligands to the more recent work of Brown, Pfaltz, Togni, and Helmchen employing phosphinamine ligands.[30] ^, [34] [35] [36] [37] [38] Their key findings propose that the transition state of Pd η³-allyl complexes contains an allyl group which has reoriented itself into a η²-alkene product-like geometry, rather than the η³-allyl starting complex, thus facilitating nucleophilic attack on the allyl terminal carbon trans to phosphorus. The lability of the diastereomeric 1,3-diphenylallyl complexes is influenced by such electronic factors and by intracomplex steric clashes between the ligand and the allyl. In the case that the 1,3-diphenylallyl Pd complexes only adopt a syn-syn geometry, two isomeric complexes can be envisaged, namely one having the central allyl proton pointing above the P–N plane (endo) and one having the central allyl proton pointing below the P–N plane (exo). Upon nucleophilic attack trans to phosphorus the endo diastereomer will lead to the enantiomeric product to the one obtained through reaction of the exo diastereomer (Scheme [4]).

Thus, the η³-1,3-diphenylallylpalladium tetrafluoroborate salts 14a–c were prepared in excellent yields (94–99%) from the reaction between (R,R_p )-8a–c with di-μ-chloro-bis(1,3-diphenylallyl)dipalladium and sodium tetrafluoroborate in dry degassed dichloromethane (Scheme [5]).

Only one complex (14b) gave spectra that were amenable to unambiguous interpretation. The ³¹P NMR spectrum of 13b confirmed the presence of both diastereomeric allyl complexes, as two peaks at δ = 17.9 and 20.2 ppm in a 1.8:1 ratio were observed.

With the aid of a TOCSY experiment, the allyl protons of both diastereomeric intermediates were identified. For the major isomer, the allyl proton trans to nitrogen appeared as a doublet at δ = 5.46 ppm and the central allyl proton appeared as a triplet at δ = 6.22 ppm. An apparent triplet at δ = 5.68 ppm corresponds to the allyl proton trans to phosphorus on both the major and minor isomers. The allyl proton trans to nitrogen and the central allyl proton of the minor isomer appeared as a doublet at δ = 3.78 ppm and a multiplet at δ = 6.73–6.77 ppm, respectively.

A 2D NOESY confirmed the syn,syn configuration of both the major and minor π-allyl diastereomers. A 1D NOE experiment was carried out by irradiating the apparent triplet at δ = 5.68 ppm, which represents both allyl protons trans to phosphorus. A NOE enhancement of each of the signals for the allyl protons trans to nitrogen was observed.

In addition, a NOE was observed for the singlet at δ = 2.50 ppm (the N–Me protons of the minor diastereomer). This concludes that the minor diastereomeric intermediate is endo-syn-syn 14b, hence the major diastereomer is exo-syn-syn 14b (Figure [4]).

The application of this catalyst in Pd-catalyzed allylic alkylation afforded the (R)-enantiomer in preference and this is predicted to arise from nucleophilic attack trans to phosphorus on the endo-η³-allyl intermediate. Therefore, in this case the preferred diastereomeric species in solution does not give rise to the major (R)-enantiomer obtained in catalysis. This indicates that the major diastereomer interconverts in solution via the π–σ–π mechanism to afford the endo diastereomer, which is alkylated at a faster rate to afford the (R)-enantiomer.

We failed to grow crystals of 14b suitable for analysis by X-ray crystallography. Similar efforts to obtain good quality crystals of both the η³-1,3-diphenylallyl and the free ligand 8c, the one that afforded the best results in Pd-catalyzed allylic alkylation, also proved futile. However, a palladium dichloride complex 16c was prepared and X-ray quality crystals were grown (Figure [5]).

It was not possible to study the η³-1,3-diphenylallyl of the dicyclohexyl ligand 14a by ¹H NMR spectroscopy. As with the di(o-tolyl)phosphine-containing ligand 8c, a Pd dichloride complex 16a was analyzed by X-ray crystallography (Figure [6]).

In addition, and the first occasion on which we have been able to analyze a 1,3-(diphenylallyl)palladium complex of our P,N-ligands by X-ray crystallography, we were gratified to grow suitable crystals of 14a and analyze accordingly (Figure [7]). From this the configuration of the allyl group was assigned as endo-syn-syn. This ligand induced an excess of the (R)-13 product, which we propose arises from attack on the allyl carbon (C-36) trans to phosphorus.[16]

Coordination about the Pd has a pseudo square planar geometry, with the characteristic Pd–C bond trans to the phosphorus donor atom displaying the longer distance of 2.38 Å, compared to the Pd–C bond trans to nitrogen which is 2.13 Å.

In conclusion, three novel P,N-ferrocenylpyrrolidine ligands 8a–c were synthesized[39] and their Pd(η³-allyl)-complexes were applied in the Pd-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate with dimethylmalonate as the nucleophile. Good selectivity for the (R)-enantiomer was observed with an optimal ee of 85% for the di(o-tolyl) ligand (R,R_p )-8c being observed. The electronic and steric properties of the ligands did significantly affect the rate and selectivity of the reaction. The importance of the choice of reaction conditions was highlighted, as depending on the electronic or steric environment present on the ligands, a particular combination of base and solvent was required for optimum selectivity and reactivity. These P,N-systems are very active in Pd-catalyzed allylic alkylation (conversions of up to 100%) and have a good enantiodiscriminating ability (76–85% ees). X-ray crystal structures were obtained of the free di(p-fluorophenyl) ligand (R,R_p )-8b, the di(cyclohexyl)phosphine ligand (R,R_p )-8a complexed to palladium dichloride, and the di(o-tolyl)phosphine ligand (R,R_p )-8c complexed to palladium dichloride. An X-ray crystal structure of the Pd η³-(diphenylallyl) complex of ligand (R,R_p )-8a was also acquired. This, along with 2D NMR experiments, helped to rationalize the stereochemical outcome of the reaction, with the favored (R)-enantiomer being obtained in all cases from nucleophilic attack trans to phosphorus on the minor endo-syn-syn diastereomeric intermediates 14a–c.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

The authors wish to thank Dr. Yannick Ortin of the UCD NMR Centre in the School of Chemistry/CSCB for help with NMR spectroscopic studies and Dr. Jimmy Muldoon and Dr. Dilip Rai for the acquisition of mass spectra.

• References and Notes

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• 39 Preparation of 2-[(2R)-N-Methylpyrrolidin-2′yl]-(1R)-ferrocenyldi(aryl/alkyl)-phosphines 8a–c To a solution of (R)-1-ferrocenyl-2-methylpyrrolidine (7, 0.63 g, 2.12 mmol) in dry diethyl ether (7 mL) at –78 °C, sec-butyllithium (1.97 mL, 1.4 M, 2.76 mmol) was added dropwise. The orange suspension was stirred for 3 h at this temperature and at 0 °C for 90 min to ensure complete lithiation. The relevant chlorophosphine (2.76 mmol) was added as a solution in diethyl ether (3 mL) at 0 °C, and the reaction was stirred at room temperature for a further hour. The reaction was quenched with 10 % ammonium chloride (5 mL) and extracted with diethyl ether (3 × 8 mL). The combined organic layers were washed with water (10 mL), brine (10 mL), dried (Na₂SO₄), and concentrated to give the crude product as an orange oil. The crude product contained a mixture of diastereomers (80–85% de), which were separated by column chromatography. Analytical Data of Compound (R,R _p)-8a Isolated as an orange sticky solid in 51% yield; R_f = 0.64 (Al, 5:1 pentane/ethyl acetate); [α]_D +184.0 (c = 0.5, CHCl₃). IR (KBr disc): ν_max = 3020, 2926, 2400, 1216 cm^–1. ¹H NMR (600 MHz, CDCl₃): δ = 1.15–1.50 (m, 9 H, Cy), 1.55–1.75 (m, 8 H, Cy, H_4′b), 1.85–1.90 (m, 3 H, Cy, H_4′a), 1.95–2.10 (m, 3 H, Cy), 2.18–2.25 (m, 2 H, H_3′b, H_5′b), 2.27 (s, 3 H, N–Me), 2.32–2.38 (m, 1 H, Cy), 2.40–2.48 (m, 1 H, H_3′a), 2.97 (app t, J = 8.0 Hz, 1 H, H_2′), 3.07 (app t, J = 8.0 Hz, 1 H, H_5′a), 4.10 (br s, 1 H, CpH₅), 4.13 (s, 5 H, unsub. Cp), 4.21 (br s, 1 H, CpH₃), 4.23 (app t, J = 2.1 Hz, 1 H, CpH₄). ¹³C NMR (151 MHz, CDCl₃): δ = 23.0 (d, ⁵ J _P,_C = 1.0 Hz, C_4′), 26.5 (d, ⁴ J _P,_C = 1.0 Hz, Cy), 26.6 (d, ⁴ J _P,_C = 1.0 Hz, Cy), 27.3 (d, ³ J _P,_C = 7.5 Hz, Cy), 27.6 (d, ³ J _P,_C = 7.4 Hz, Cy), 27.7 (d, ² J _P,_C = 13.2 Hz, Cy), 28.2 (d, ² J _P,_C = 12.3 Hz, Cy), 29.1 (d, ³ J _P,_C = 7.5 Hz, Cy), 30.1 (d, ³ J _P._C = 8.8 Hz, Cy), 32.1 (d, ² J _P,_C = 16.4 Hz, Cy), 32.7 (d, ² J _P,_C = 20.2 Hz, Cy), 33.6 (d, ⁴ J _P,_C = 12.1 Hz, C_3’), 35.0 (d, ¹ J _P,_C = 12.5 Hz, Cy), 36.3 (d, ¹ J _P,_C = 13.0 Hz, Cy), 40.9 (N–Me), 58.1 (C_5′), 66.0 (d, ³ J _P,_C = 1.7 Hz, C_2′), 67.5 (CpC₄), 69.5 (Cp), 70.8 (d, ³ J _P,_C = 3.0 Hz, CpC₃), 71.0 (d, ² J _P,_C = 2.5 Hz, CpC₅), 77.7 (d, ¹ J _P,_C = 22.7 Hz, CpC₁), 94.2 (d, ² J _P,_C = 16.7 Hz, CpC₂). ³¹P NMR (243 MHz, CDCl₃): δ = –11.0. Anal. Calcd for C₂₇H₄₀FeNP: C, 69.67; H, 8.66; N, 3.01. Found: C, 70.07; H, 8.75; N, 2.74. ESI-HRMS: m/z calcd for C₂₇H₄₀FeNP [M + H]: 466.2326; found: 466.2343.

Corresponding Author

Publication History

Received: 23 August 2022

Accepted after revision: 06 September 2022

Article published online:
12 October 2022

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• References and Notes

• 1 Kealy TJ, Pauson PL. Nature 1951; 168: 1039
  CrossrefSearch in Google Scholar
• 2 Togni A, Hayashi T. In Ferrocenes: Homogenous Catalysis-Organic Synthesis-Materials Science. VCH; Weinheim: 1995
  Search in Google Scholar
• 3 Marquarding D, Klusacek H, Gokel G, Hoffmann P, Ugi I. J. Am. Chem. Soc. 1970; 92: 5389
  CrossrefSearch in Google Scholar
• 4 Hayashi T, Yamamoto M, Kumada M. Tetrahedron Lett. 1974; 4405
  Crossref
• 5 Cunningham L, Benson A, Guiry PJ. Org. Biomol. Chem. 2020; 18: 9329
  CrossrefPubMedSearch in Google Scholar
• 6 Richards CJ, Mulvaney AW. Tetrahedron: Asymmetry 1996; 7: 1419
  CrossrefSearch in Google Scholar
• 7 Sammakia T, Latham HA, Schaad DR. J. Org. Chem. 1995; 60: 10
  CrossrefSearch in Google Scholar
• 8 Nishibayashi Y, Uemura S. Synlett 1995; 79
  Thieme Connect
• 9 Malone YM, Guiry PJ. J. Organomet. Chem. 2000; 603: 110
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• 39 Preparation of 2-[(2R)-N-Methylpyrrolidin-2′yl]-(1R)-ferrocenyldi(aryl/alkyl)-phosphines 8a–c To a solution of (R)-1-ferrocenyl-2-methylpyrrolidine (7, 0.63 g, 2.12 mmol) in dry diethyl ether (7 mL) at –78 °C, sec-butyllithium (1.97 mL, 1.4 M, 2.76 mmol) was added dropwise. The orange suspension was stirred for 3 h at this temperature and at 0 °C for 90 min to ensure complete lithiation. The relevant chlorophosphine (2.76 mmol) was added as a solution in diethyl ether (3 mL) at 0 °C, and the reaction was stirred at room temperature for a further hour. The reaction was quenched with 10 % ammonium chloride (5 mL) and extracted with diethyl ether (3 × 8 mL). The combined organic layers were washed with water (10 mL), brine (10 mL), dried (Na₂SO₄), and concentrated to give the crude product as an orange oil. The crude product contained a mixture of diastereomers (80–85% de), which were separated by column chromatography. Analytical Data of Compound (R,R _p)-8a Isolated as an orange sticky solid in 51% yield; R_f = 0.64 (Al, 5:1 pentane/ethyl acetate); [α]_D +184.0 (c = 0.5, CHCl₃). IR (KBr disc): ν_max = 3020, 2926, 2400, 1216 cm^–1. ¹H NMR (600 MHz, CDCl₃): δ = 1.15–1.50 (m, 9 H, Cy), 1.55–1.75 (m, 8 H, Cy, H_4′b), 1.85–1.90 (m, 3 H, Cy, H_4′a), 1.95–2.10 (m, 3 H, Cy), 2.18–2.25 (m, 2 H, H_3′b, H_5′b), 2.27 (s, 3 H, N–Me), 2.32–2.38 (m, 1 H, Cy), 2.40–2.48 (m, 1 H, H_3′a), 2.97 (app t, J = 8.0 Hz, 1 H, H_2′), 3.07 (app t, J = 8.0 Hz, 1 H, H_5′a), 4.10 (br s, 1 H, CpH₅), 4.13 (s, 5 H, unsub. Cp), 4.21 (br s, 1 H, CpH₃), 4.23 (app t, J = 2.1 Hz, 1 H, CpH₄). ¹³C NMR (151 MHz, CDCl₃): δ = 23.0 (d, ⁵ J _P,_C = 1.0 Hz, C_4′), 26.5 (d, ⁴ J _P,_C = 1.0 Hz, Cy), 26.6 (d, ⁴ J _P,_C = 1.0 Hz, Cy), 27.3 (d, ³ J _P,_C = 7.5 Hz, Cy), 27.6 (d, ³ J _P,_C = 7.4 Hz, Cy), 27.7 (d, ² J _P,_C = 13.2 Hz, Cy), 28.2 (d, ² J _P,_C = 12.3 Hz, Cy), 29.1 (d, ³ J _P,_C = 7.5 Hz, Cy), 30.1 (d, ³ J _P._C = 8.8 Hz, Cy), 32.1 (d, ² J _P,_C = 16.4 Hz, Cy), 32.7 (d, ² J _P,_C = 20.2 Hz, Cy), 33.6 (d, ⁴ J _P,_C = 12.1 Hz, C_3’), 35.0 (d, ¹ J _P,_C = 12.5 Hz, Cy), 36.3 (d, ¹ J _P,_C = 13.0 Hz, Cy), 40.9 (N–Me), 58.1 (C_5′), 66.0 (d, ³ J _P,_C = 1.7 Hz, C_2′), 67.5 (CpC₄), 69.5 (Cp), 70.8 (d, ³ J _P,_C = 3.0 Hz, CpC₃), 71.0 (d, ² J _P,_C = 2.5 Hz, CpC₅), 77.7 (d, ¹ J _P,_C = 22.7 Hz, CpC₁), 94.2 (d, ² J _P,_C = 16.7 Hz, CpC₂). ³¹P NMR (243 MHz, CDCl₃): δ = –11.0. Anal. Calcd for C₂₇H₄₀FeNP: C, 69.67; H, 8.66; N, 3.01. Found: C, 70.07; H, 8.75; N, 2.74. ESI-HRMS: m/z calcd for C₂₇H₄₀FeNP [M + H]: 466.2326; found: 466.2343.

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