Synthesis and Application of Planar Chiral Cyclic (Amino)(ferrocenyl

2 days ago - New cyclic (amino)(ferrocenyl)carbene (CAFeC) ligands containing the Cp* group have been developed as a modification of their prototype C...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Synthesis and Application of Planar Chiral Cyclic (Amino)(ferrocenyl)carbene Ligands Bearing FeCp* Group Risa Yasue† and Kazuhiro Yoshida*,†,‡ †

Department of Chemistry, Graduate School of Science and ‡Molecular Chirality Research Center, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

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S Supporting Information *

ABSTRACT: New cyclic (amino)(ferrocenyl)carbene (CAFeC) ligands containing the Cp* group have been developed as a modification of their prototype Cp version. The generation of the new carbenes was indirectly confirmed by trapping experiments in which a carbene precursor was reacted with sulfur or [IrCl(cod)]2 in the presence of a base. The electronic properties of the new CAFeCs were evaluated by determining the Tolman electronic parameter (TEP) of the Ir dicarbonyl complex [IrCl(CO)2(CAFeC)] that was synthesized via [IrCl(cod)(CAFeC)]. It was revealed that the donor strengths of the new CAFeCs were very high and comparable to those of cyclic (amino)(alkyl)carbenes (CAACs). The influence of the steric effect on the enantioselectivity by changing Cp to the Cp* group of CAFeCs was confirmed to be significant in an Ir-catalyzed asymmetric transfer hydrogenation of cyclic N-sulfonylimine where CAFeCs were used as chiral ligands.

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(CAACs),8 which also possess only one nitrogen atom.2c The purpose of this study was to develop Cp* version 1b in order to enhance the coordination ability of CAFeCs. It was also anticipated that new carbenes 1b would create a more attractive chiral environment than 1a as chiral ligands because of the bulkiness of the Cp* group (Figure 1, right).

INTRODUCTION The pentamethylcyclopentadienyl ligand (Cp*) is one of the persubstituted versions of cyclopentadienyl ligands that coordinate to metals in the pentahapto (η5-) bonding mode.1 In comparison to the common cyclopentadienyl ligand (Cp), Cp* is bulkier and more electron-rich; thus, its complexes are more soluble in nonpolar solvents. Over the past few years, we have developed planar chiral cyclic (amino)(ferrocenyl)carbenes (CAFeCs) (Rp)-1a as new ferrocene-based N-heterocyclic carbene (NHC) ligands (Figure 1, left).2,3 CAFeCs possess only one nitrogen atom

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RESULTS AND DISCUSSION The synthetic route to carbene precursor (Rp)-[1bmH]OTf is shown in Scheme 1. Here, the same strategy as that established by us for prototype ligand (Rp)-1a was employed.,2a,9 First, known chiral iodoferrocene (S,S,Sp)-2b,10 which was obtained by a diastereoselective ortho-functionalization strategy,11 was subjected to the Mizoroki−Heck reaction with acrylamide to form 3b having an enamide side chain.12 Then, resulting 3b was treated with LiAlH4 to give primary amine 4b. Without purification, 4b was treated with hydrochloric acid for acetal hydrolysis, and this was followed by a spontaneous intramolecular cyclization reaction to yield aldimine 5b. Desired iminium salt (Rp)-[1bmH]OTf was obtained by reacting 5b with isopropyl iodide, followed by an anion exchange with AgOTf.13 One of the merits of this synthetic strategy is the simple introduction of many substituents on the nitrogen atom of 5b by only changing the electrophiles. Because it was confirmed that CAFeC (Rp)-1am was not isolable and decomposed into a mixture even at low temperature, the generation of (Rp)-1am was indirectly confirmed by carbene trapping experiments with sulfur in our previous study.2a The same trapping experiment was also conducted to confirm the generation of (Rp)-1bm. As a result,

Figure 1. Improvement of planar chiral cyclic (amino)(ferrocenyl)carbene (Rp)-1a to (Rp)-1b.

that stabilizes the adjacent carbene carbon and are characterized by their ability to coordinate more strongly to metals than common NHC ligands4,5 having two nitrogen atoms. In fact, from the Tolman electronic parameter (TEP) estimated from the CO stretching frequencies6 of the [IrCl(CO)2(1am)] complex, the donor strength of CAFeC 1am was found to be similar to those of cyclic (amino)(aryl)carbenes (CAArCs)7 and cyclic (amino)(alkyl)carbenes © XXXX American Chemical Society

Received: March 14, 2019

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DOI: 10.1021/acs.organomet.9b00171 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Then, we turned our attention to the synthesis of Ir complexes bearing (Rp)-1bm. Because we found that the carbene transfer procedure via copper complex was suitable for the synthesis of Ir complex in a previous study of (Rp)-1am,2c the same strategy was employed this time as well. As a result, treatment of iminium salt (Rp)-[1bmH]OTf with LiHMDS in the presence of [IrCl(cod)]2 and CuCl gave desired product (Rp)-7bm (Scheme 3). Because of its stability in air and

Scheme 1. Synthesis of CAFeC Precursor (Rp)-[1bmH]OTf

Scheme 3. Synthesis of Dicarbonyl Complex (Rp)-8bm

the addition of LiHMDS as a base to a mixture of iminium salt (Rp)-[1bmH]OTf and sulfur in THF gave desired sulfur adduct(Rp)-6bm in 53% yield (Scheme 2).

moisture, the purification of 7bm was performed by silica gel chromatography. We subsequently synthesized [IrCl(CO)2(1bm)] complex 8bm from 7bm by replacing the COD ligand with CO. The resulting dicarbonyl complex was also stable in air and moisture. However, in contrast to 7bm, 8bm was a mixture of diastereomeric atropisomers ((Sa,Rp)8bm/(Ra,Rp)-8bm = 0.42/1 at 20 °C) in solution due to the restricted rotation around the Ir−Ccarbene bond owing to steric bulkiness.14 This phenomenon was not observed in 7bm that contained a COD coligand that is bulkier than CO. The atropisomers in the dicarbonyl complex were also observed in the case of [IrCl(CO)2(1am)] 8am,2c but the ratio of (Sa,Rp)8am/(Ra,Rp)-8am was 0.88/1 at 20 °C. The bias in the ratio of the atropisomers in 8bm relative to that in 8am could be attributed to the steric influence of the Cp* group, which is bulkier than the Cp group. The result of the X-ray diffraction analysis of a crystal obtained by vapor diffusion of n-pentane into a saturated CH2Cl2 solution of 8bm is shown in Figure 3 (CCDC 1902307). The structure corresponds to one of the two atropisomers, (Ra,Rp)-8bm and is very similar to that of (Ra,Rp)-8am bearing the Cp group.2c Some characteristics of the structure of (Ra,Rp)-8bm are as follows: (1) The Ir-carbene bond length of 2.094(5) Å is comparable to the values observed in related [IrCl(CO)2(NHC)] complexes.6a,15 (2) The carbene ligand coordinates nearly orthogonally to the metal−ligand plane (dihedral angle (Cl−Ir−C1−N) = −82.7(4)°).6a,15 (3) The two Ir−Ccarbonyl bond lengths are clearly different (Ir−Ccarbonyl bond trans to 1bm: 1.891(6) vs cis to 1bm: 1.989(7) Å), indicating a strong trans influence of the CAFeC ligand. (4) An anagostic interaction16 exists between Ir···H15 (definition:17 M−H distance ≈ 2.3−2.9 Å, M−H−C 110−170°), and a weak electrostatic interaction is noted between Ir···H7. In order to classify the donor ability of new CAFeC 1bm, analysis of the CO stretching frequencies of 8bm was carried out.18 In our previous study,2c we found that the TEP of prototype CAFeC 1am (2044 cm−1) is positioned between

Scheme 2. Synthesis of Sulfur Adduct (Rp)-6bm

The crystal structure of (Rp)-6bm is shown in Figure 2 (CCDC 1902306). This structure is almost identical to the sulfur adduct of prototype 1am, in which an intramolecular hydrogen bond exists between the sulfur atom and a hydrogen atom of the isopropyl group.2a,b

Figure 2. Crystal structure of (Rp)-6bm. Selected bond lengths (Å) and angles (deg): C1−S 1.682(2), C1−N 1.357(2), C1−C9 1.462(3), C2−N 1.473(3), C9−C10 1.432(3), H2−S 2.6169, C2− H2−S 106.0, S−C1−N 122.6(2), N−C1−C9 115.7(2), C9−C1−S 121.7(2), S−C1−C9−C10 −35.6(3). B

DOI: 10.1021/acs.organomet.9b00171 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Figure 5. %Vbur and steric map representation of ligand (Rp)-1bm in (Ra,Rp)-8bm, calculated using the SambVca web application (with default setting: bondi radii scaled by 1.17, sphere radius 3.5 Å, mesh spacing 0.10 Å, hydrogen atoms omitted).

Figure 3. Crystal structure of (Ra,Rp)-8bm. Selected bond lengths (Å) and angles (deg): Ir−C1 2.094(5), Ir− Cl 2.338(2), Ir−C18 1.891(6), Ir−C19 1.989(7), O1−C18 1.131(8), O2−C19 0.83(1), C1−N 1.319(6), C1−C6 1.443(8), Ir−H7 2.9362, Ir−H15 2.4737, Ir−H7−C7 102.9, Ir−H15−C15 127.1, Cl− Ir−C1−N −82.7(4).

Finally, we examined the application of the new ligand to an asymmetric reaction. The test reaction chosen for this purpose is the asymmetric transfer hydrogenation of cyclic Nsulfonylimine, which was reported to be promoted by NHC/ Ir complexes.22 Motivated by the work of Crabtree et al.23 and Metallinos et al.,24 we chose [Ir(cod)(NHC)(PAr3)]PF6 (9) as the basic catalyst structure that shows high catalytic activity in the asymmetric hydrogenation of quinolines. As shown in Scheme 4, the preparation of 9 containing our CAFeC was accomplished by simply adding various phosphines and KPF6 to [IrCl(cod)(CAFeC)] (7).

those of representative classical diaminocarbene A (2051 cm−1)6a and representative CAAC B,8a which is known to have strong donor ability (2041 cm−1) (Figure 4). The IR spectrum

Scheme 4. Conversion of (Rp)-7 into (Rp)-9 and Structures of Ir Complexes for Asymmetric Transfer Hydrogenation

Figure 4. Comparison of donor abilities of selected NHC and phosphine ligands.

The results of the asymmetric transfer hydrogenation are shown in Table 1. Although we tested phosphine-free Ir complex 7bm as the catalyst first, the result was unsatisfactory in terms of both yield and enantioselectivity (entry 1). In contrast, phosphine-containing catalyst 9bmu showed much better performance and gave product (S)-11 in 57% yield with 50% ee (entry 2).25 To investigate the influence of substituents on the nitrogen atom of CAFeC, the isopropyl group (m) was replaced by the less bulky ethyl group (n) or the bulkier diphenylmethyl group (o). With these catalysts, 9bnu and 9bou, catalytic activity was retained, but enantioselectivities were decreased (entries 3 and 4). Then, we used 1bm as CAFeC ligand and conducted a screening for phosphines at the Ir center. Although the replacement of PPh3(u) by P(2MeC6H4)3 (v) or P(3-MeC6H4)3 (w) proved to be detrimental to both catalytic activity and selectivity (entries 5 and 6), the replacement by P(4-MeC6H4)3 gave the best enantioselectivity (65% ee) in this study (entry 7). We assume that the excessive bulkiness of the two former ligands (v and w) gave rise to the dissociation of the phosphine from the metal center during the reaction, so the results obtained with the catalysts bearing

of 8bm showed two strong CO signals at 2048 and 1966 cm−1 (CH2Cl2: νav(CO) = 2007 cm−1), which were converted into the TEP of 2036 cm−1 using a well-accepted equation.19 The lowest TEP of 1bm indicates that the donor ability of 1bm is the strongest among these compounds (Figure 4). It is clear that the well-known electron-rich character1c of the Cp* group enhances the donor strength of CAFeC, an innately strong donor. To confirm the steric properties of CAFeC (Rp)-1bm, the percent buried volume (%Vbur)20 and the steric map were calculated from the solid-state structure of dicarbonyl complex (Ra,Rp)-8bm. As a result, %Vbur of 1bm was found to be 29.7 (Figure 5). Because the value of prototype ligand 1am was calculated to be 30.0 in our previous study,2c it means that there is no significant difference between the values of 1am and 1bm. This is, however, a natural consequence because the Cp and Cp* groups in the complexes are located outside the sphere of the default setting of %Vbur (3.5 Å for the radius).21 C

DOI: 10.1021/acs.organomet.9b00171 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Chemical shifts are reported in δ ppm referenced to an internal SiMe4 standard for 1H NMR and chloroform-d (δ 77.0) for 13C NMR. Highresolution mass spectra were recorded on Orbitrap mass spectrometers. Single crystal X-ray diffraction data were collected at 173 K on a CCD diffractometer with Mo Kα (λ = 0.71073 Å) radiation and graphite monochromator. The purity of all products was routinely established by 1H and 13C NMR spectra of bulk samples. The spectra have been provided in the Supporting Information. Materials. THF and Et 2 O were distilled from sodium benzophenone-ketyl under argon prior to use. CH2Cl2 was distilled from CaH2 under nitrogen and stored in a glass flask with a Teflon stopcock under nitrogen. Anhydrous CH3CN, anhydrous N,Ndimethyformamide, diphenylmethyl bromide, isopropyl iodide, ethyl iodide, tert-butyllithium, acrylamide, sodium hydrogen carbonate, tetrabutylammonium chloride (TBAC), LiAlH4, 1 M HCl, silver triflate, lithium hexamethyldisilazide, sulfur, copper(I) chloride, triphenylphosphine, tri(p-tolyl)phosphine, potassium hexafluorophosphate, ammonium formate, and dichlorobis(triphenylphosphine)palladium were used as received. [IrCl(cod)]2,27 (Sp)-1-iodo-2[(2S,4S)-4-(methoxymethyl)-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-pentamethylferrocene ((S,S,Sp)-2b),10 and 3-methylbenzo[d]isothiazole 1,1dioxide (10)28 were prepared according to the reported procedures. Procedures for the Preparation of Iminium Salt ((Rp)-[1bmH]OTf). (Rp)-(−)-1-((E)-(3-Amino-3-oxoprop-1-en-1-yl))-2-[(2S,4S)-4(methoxymethyl)-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-pentamethylferrocene ((S,S,Rp)-(−)-3b). A mixture of (Sp)-1-iodo-2-[(2S,4S)-4(methoxymethyl)-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-pentamethylferrocene ((S,S,Sp)-2b) (36.9 mg, 72.0 μmol), PdCl2(PPh3)2 (10.1 mg, 14.4 μmol), acrylamide (15.4 mg, 216 μmol), NaHCO3(20.6 mg, 245 μmol), and TBAC (16.0 mg, 58 μmol) in DMF (0.25 mL) was stirred at 120 °C for 24 h. Then, the mixture was concentrated under reduced pressure. The crude mixture was purified by silica gel column chromatography (CH2Cl2/MeOH = 30/1) to afford the title compound (28.0 mg, 61.5 μmol) (85% yield) as red oil. 1H NMR (CDCl3): δ 1.44−1.50 (m, 1H), 1.78 (s, 15H), 1.74−1.85 (m, 1H), 3.35 (s, 3H), 3.45 (dd, J = 10.4, 3.6 Hz, 1H), 3.47 (dd, J = 10.4, 7.2 Hz, 1H), 3.89 (t, J = 2.4 Hz, 1H), 3.91−3.93 (m, 1H), 3.96 (td, J = 12.0, 2.4 Hz, 1H), 4.03−4.10 (m, 1H), 4.16−4.18 (m, 1H), 4.30 (ddd, J = 11.6, 4.9, 0.7 Hz, 1H), 5.35 (s, 1H), 5.45 (br s, 1H), 5.85 (br s, 1H), 6.25 (d, J = 16.0 Hz, 1H), 7.40 (d, J = 15.6 Hz, 1H). 13C NMR (CDCl3): δ 10.6, 27.3, 59.2, 66.7, 72.9, 74.3, 75.6, 76.4, 77.5, 81.6, 82.9, 99.4, 117.8, 141.1, 168.7. HRMS (ESI) calcd for C24H33FeNO4(M−e−) 455.1754, found 455.1748. [α]18 D − 408 (c 0.01, CHCl3). (Rp)-1-(3-Aminopropyl)-2-[(2S,4S)-4-(methoxymethyl)-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-pentamethylferrocene ((S,S,Rp)-4b). To a suspension of LiAlH4 (184 mg, 4.83 mmol) in THF (3.5 mL) was slowly added a solution of (S,S,Rp)-(−)-3b (315 mg, 691 μmol) in THF (8.6 mL) via cannula at 0 °C. The reaction mixture was then warmed to refluxing temperature and stirred for 6 h. The mixture was then cooled to 0 °C and carefully quenched with Na2SO4·10H2O. After adding excess Na 2SO 4, the mixture was filtered and concentrated on a rotary evaporator to afford the title compound as yellow oil that was used to the next step without purification. (R p)-(−)-4,5-Dihydro-3H-1′,2′,3′,4′,5′-pentamethylferroco[c]azepine ((Rp)-(−)-5b). To a solution of (S,S,Rp)-4b (85.3 mg, 192 μmol) in acetone (3.7 mL) was slowly added 1 M HCl (0.29 mL) at 0 °C. The reaction mixture was stirred at the same temperature for 15 min. Then the mixture was quenched with saturated NaHCO3 aqueous and extracted with CH2Cl2 four times. The organic layers were combined, dried over Na2SO4, filtered, and concentrated on a rotary evaporator. The crude mixture was purified by PTLC on silica gel (CH2Cl2/MeOH/Et3N = 10/1/0.01) to afford the title compound (22.1 mg, 68.4 μmol) (35% yield, 2 steps) as orange oil. 1H NMR (CDCl3): δ 1.78−7.83 (m, 1H), 1.82 (s, 15H), 1.92−2.01 (m, 1H), 2.50 (ddd, J = 16.4, 12.0, 4.4 Hz, 1H), 2.62 (dt, J = 8.4, 3.6 Hz, 1H), 3.18−3.26 (m, 1H), 3.82−3.86 (m, 3H), 4.05 (dd, J = 14.4, 7.2 Hz, 1H), 7.73 (s, 1H). 13C NMR (CDCl3): δ 10.7, 26.0, 27.7, 53.6, 74.7, 75.3, 80.9, 87.4, 161.8. HRMS (ESI) calcd for C19H26FeN (M+H+) 324.1409, found 324.1395. [α]18 D − 210 (c 0.01, CHCl3).

Table 1. Asymmetric Transfer Hydrogenation of Cyclic NSulfonylimine 10 with (Rp)-7 or (Rp)-9a

entry

Ir complex

yield (%)b

1 2 3 4 5 6 7 8 9 10

(Rp)-7bm (Rp)-9bmu (Rp)-9bnu (Rp)-9bou (Rp)-9bmv (Rp)-9bmw (Rp)-9bmx (Rp)-9bmy (Rp)-9bmz (Rp)-9amu

9 57 58 75 35