Manganese-Catalyzed Asymmetric Hydrosilylation of Aryl Ketones.

We disclose the synthesis of a series of manganese complexes of chiral iminopyridine oxazoline ligands and their application in the first manganese-catalyzed asymmetric ketone hydrosilylations. The most sterically hindered manganese catalyst bearing two CH(Ph)2 groups at the 2,6-ortho positions of the imino aryl ring and a tBu group on the oxazoline ring furnishes the secondary alcohols in high enantioselectivities and yields.

Introduction

The asymmetric hydrosilylation of ketones provides a n class="Chemical">convenient and efficient way to optically active secondary alcohols, which are valuable building blocks in organic synthesis.[1] As a consequence, tremendous interest has been driven to the development of efficient catalysts for this transformation.[2] Over the past several decades, a variety of noble-metal-based catalysts, such as Rh, Ir, and Pt, have been applied in the asymmetric hydrosilylation of ketones.[3] Considering the high cost and intrinsic toxicity associated with the noble metals, it is desirable to search for cheap and base-metal-based alternatives. Recently, progress has been made in the development of asymmetric ketone hydrosilylations using nonprecious metals, such as Fe,[4] Co,[5] Cu,[6] and Zn.[7]

Manganese is the third earth-abundant transition n class="Chemical">metal[8] and indispensable in many biological systems.[9] Because of the low cost and environmentally benign nature of the metal, manganese catalysis has attracted much attention during the past 5 years.[10] Among them, manganese(I) carbonyl complexes[11] and a tris(dipivaloylmethanato)Mn(III)[12] could be utilized as the catalysts for the hydrosilylation of ketones. More recently, manganese complexes with salen-,[13] carbene-,[14] or bis(imino)pyridine-based ligands[15] have proven to be highly effective for this transformation. Despite these advances, enantiomeric hydrosilylation of ketones catalyzed by manganese complex has remained unknown.

Recently, this laboratory[16] and the Lu group[17] independently designed the so-called “iminopyridine oxazoline” n class="Chemical">(IPO) ligands and applied the corresponding cobalt and iron complexes for catalytic, enantioselective reduction events. In line with our interest in developing base-metal-catalyzed organic transformations, we describe herein the synthesis of manganese complexes ligated by chiral IPO ligands and their catalytic performance in asymmetric ketone hydrosilylations.

Results and Discussion

This study started with the preparation of the chiral IPO manganese n class="Chemical">complexes (Scheme ). Treatment of the IPO ligands containing two iPr groups at the 2,6-ortho positions of the imino aryl ring with anhydrous Mn(THF)2Cl2 formed the Mn(II) dichloride complexes [(S)-IPOR]MnCl2 (R1 = Bn, (S)-3a; R1 = iPr, (S)-3b) in good yields. Following the same procedure, the sterically more demanding complexes (S)-3c and (S)-3d [(S)-(PhIPOR]MnCl2 (R2 = iPr, (S)-3c; R2 = tBu, (S)-3d) bearing the CH(Ph)2 substituents at the 2,6-aryl positions were also produced in high yields. The single-crystal X-ray diffraction analysis of (S)-3d revealed a distorted square-pyramidal geometry around the Mn center (Figure ).

Scheme 1

Synthesis of (IPO)Mn Complexes (S)-3a–d

Figure 1

Solid-state structure of (S)-3d. Selected bond distances (Å) and angles (deg): Mn1–N1, 2.304(4); Mn1–N2, 2.175(3); Mn1–N3, 2.384(4); Mn1–Cl1, 2.3148(14); Mn1–Cl2, 2.3578(15); N1–Mn1–N2, 74.07(14); N2–Mn1–N3, 69.49(13); N1–Mn1–N3, 141.16(13); N2–Mn1–Cl1, 136.66(12); N2–Mn1–Cl2, 100.90(11); and Cl1–Mn1–Cl2, 121.94(6).

Solid-state structure of (S)-3d. Selected bond distances (Å) and angles (deg): Mn1–N1, 2.304(4); n class="Gene">Mn1–N2, 2.175(3); Mn1–N3, 2.384(4); Mn1–Cl1, 2.3148(14); Mn1–Cl2, 2.3578(15); N1–Mn1–N2, 74.07(14); N2–Mn1–N3, 69.49(13); N1–Mn1–N3, 141.16(13); N2–Mn1–Cl1, 136.66(12); N2–Mn1–Cl2, 100.90(11); and Cl1–Mn1–Cl2, 121.94(6).

The catalytic performance of the manganese n class="Chemical">complex 3a was initially investigated in the asymmetric hydrosilylation of 4-chloroacetophenone (4a) using several commercially available hydrosilanes (Table , entries 1–4). Upon activation with NaBHEt3 (2 mol %),[18]3a (1 mol %) exhibits high activity with phenylsilane (PhSiH3) as the reductant at room temperature, furnishing the desired alcohol in high yield and moderate enantioselectivity after hydrolysis (entry 1). A secondary silane, Ph2SiH2, gave a slightly lower selectivity than PhSiH3 (entry 2). The tertiary silane (EtO)3SiH is active for hydrosilylation but gives low enantioselectivity, whereas methylmethoxysilane (MD’M) did not deliver the hydrosilylation product (entries 3 and 4). The enantioselectivity is affected by using different silane sources, which indicates that silane might be involved in the enantio-determining transition state.[4k] Hence, PhSiH3 was chosen as the silane source for further studies.

Table 1

Optimization of Reaction Conditionsa

entry	cat.	silane	yield (%)	e.r.
1	(S)-3a	PhSiH3	94	76.5:23.5
2	(S)-3a	Ph2SiH2	91	71.5:28.5
3	(S)-3a	(EtO)3SiH	82	59.5:40.5
4	(S)-3a	MD’M	N/A	N/A
5b	(S)-3a	PhSiH3	89	75:25
6b	(S)-3b	PhSiH3	89	77.5:22.5
7b	(S)-3c	PhSiH3	92	80.5:19.5
8b	(S)-3d	PhSiH3	98	88:12
9b,c	(S)-3d	PhSiH3	98	95.5:4.5
10b,c,d	(S)-3d	PhSiH3	86	90:10

Reaction conditions: 4a (0.5 mmol), silane (1 equiv), IPO manganese complex (1 mol %), and NaBEt3H (2 mol %) in toluene (1 mL) at 25 °C. Yields were determined by 1H NMR spectroscopy using mesitylene as an internal standard. The e.r. values were determined by HPLC analysis.

4b was used as the substrate.

Toluene (5 mL) was used.

NaOtBu (2 mol %) was used instead of NaBHEt3.

Reaction conditions: 4a (0.5 mmol), n class="Chemical">silane (1 equiv), IPO manganese complex (1 mol %), and NaBEt3H (2 mol %) in toluene (1 mL) at 25 °C. Yields were determined by 1H NMR spectroscopy using mesitylene as an internal standard. The e.r. values were determined by HPLC analysis.

NaOtBu (2 mol %) was used instead of n class="Chemical">NaBHEt3.

Next, manganese complexes (S)-3a–3d with difn class="Chemical">ferent IPO ligands were evaluated in the hydrosilylation of ketone 4b (Table , entries 5–8). The reaction with (S)-3a or (S)-3b bearing iPr substituents at the 2,6-ortho position of the imino aryl ring provided high reactivity but moderate enantioselectivity (entries 5 and 6). Increasing the steric hindrance of the ligand by replacing the iPr groups in (S)-3b with two CH(Ph)2 units, [(S)-3c], led to a significantly improved enantioselectivity (entry 7, 80.5:19.5 e.r.). The substitution of tBu for iPr on the oxazoline moiety resulted in a further enhancement of the enantioselectivity (entry 8, 88:12 e.r.).

Screening of the solvents indicated that toluene was the optimal medium, while the reactions with other n class="Chemical">common solvents resulted in either decreased reaction rate or lower enantioselectivity. In addition, the concentration of the substrates had an important impact on the enantioselectivity, and the best result was obtained when the concentration of ketone was 0.1 M (for detailed optimization studies, see Table S1 in the Supporting Information). Using NaOtBu instead of NaBHEt3 could afford the desired product in a relatively lower yield and enantioselectivity (Table , entry 10). Finally, under optimized conditions, hydrosilylation using the sterically most demanding complex (S)-3d afforded high enantioselectivity (99.5:4.5 e.r.) and excellent isolated yield (98%) (Table , entry 9).

Having the optimized conditions established, we evaluated the sn class="Chemical">cope of the manganese-catalyzed asymmetric ketone hydrosilylation (Table ). With the most sterically hindered IPO manganese complexes (S)-3d as the precatalyst, hydrosilylations of a variety of aryl alkyl ketones occurred smoothly at the ambient temperature with low catalyst loadings (1 mol %). In most cases, high isolated yields and good enantioselectivities were achieved. This method is effective with acetophenones bearing both electron-donating and electron-withdrawing groups. The substituent pattern on the aryl rings of acetophenones can have a significant influence on the activity as well as the enantiomeric selectivity. Substrates with chloro group and methoxy group at the ortho position barely formed the reduction products at 25 °C, but they could be transformed to the corresponding alcohols smoothly at 60 °C with good enantioselectivity (5i, 92:8 e.r.; 5l, 91.5:8.5 e.r.). In addition, although high yield and e.r. value were obtained for the meta-methoxy-substituted substrate (5k), the para-methoxy-substituted substrate afforded the product in relatively low enantioselectivity (5j). However, for the chloro-substituted acetophenones, the position of the chloro group did not affect the enantioselectivity significantly. The substrates with chloro groups at the ortho, meta, or para position were reduced in comparable enantioselectivities (5a, 5h, and 5i). Remarkably, the bromo (5m), iodo (5n), and cyano (5r) group could also be tolerated, affording the desired products in moderate to high yields and enantioselectivities. The sterically demanding 1-mesitylethanone underwent hydrosilylation successfully to form the product in 96% yield and 95:5 e.r. (5g). The reaction using 2-acetylnaphthalene as the substrate afforded the desired product in a high yield and moderate enantioselectivity (5s).

Table 2

Mn-Catalyzed Asymmetric Hydrosilylation of Various Aryl Ketonesa

Reaction conditions: 4 (0.5 mmol), PhSiH3 (0.5 mmol), (S)-3d (1 mol %), NaBEt3H (2 mol %), and toluene (2.0 mL) at 25 °C. Isolated yields. The e.r. values were determined by HPLC analysis. The absolute configurations were assigned by comparison with the reported optical rotations.

Reaction was conducted at 60 °C.

(S)-3c was used as the precatalyst.

Reaction conditions: 4 (0.5 mmol), n class="Chemical">PhSiH3 (0.5 mmol), (S)-3d (1 mol %), NaBEt3H (2 mol %), and toluene (2.0 mL) at 25 °C. Isolated yields. The e.r. values were determined by HPLC analysis. The absolute configurations were assigned by comparison with the reported optical rotations.

Aryl alkyl ketones in which the alkyl group is larger than the methyl group were also explored in the asymmetric hydrosilylation (Table , 5t–y). Good enantioselectivity n class="Chemical">could be obtained when propyl phenyl ketone was employed as the substrate (5t, 94:6 e.r.). However, a further increase of the steric hindrance of the substituent at the benzylic position resulted in reduced enantioselectivity (5u, 69.5:30.5 e.r.). The reaction of the substrate bearing a cyclopropyl group was sluggish under standard conditions. Thus, a less sterically hindered IPO manganese complex (S)-3c was used as the precatalyst, which gave a satisfied yield and moderate enantioselectivity (5v, 89% yield, 71.5:28.5 e.r.). Cyclic aryl ketones resulted in moderate stereoselectivity (5w and 5x). The hydrosilylation of 1,1-diaryl ketones gave a low e.r. value (5y).

Conclusions

In summary, we have developed the first manganese-catalyzed asymmetric hydrosilylation of n class="Chemical">ketones using a chiral IPO manganese pincer complex as the precatalyst. This protocol provides an efficient way for the synthesis of optically active secondary alcohols under mild conditions with low catalyst loadings. Further studies on the development of new asymmetric transformations using the IPO manganese catalysts are currently underway in our laboratory.

Experimental Section

General Description

Unless otherwise noted, all reagents were purchased from commercial suppliers and used without further purification. All manipulations were carried out using standard Schlenk, high-van class="Chemical">cuum, and glovebox techniques. Tetrahydrofuran (THF) and toluene were distilled from sodium benzophenone ketyl prior to use.

Spectra were recorded on an Agilent 400 MHz or a Varian Mern class="Chemical">cury 400 MHz instrument. 1H NMR chemical shifts were referenced to residual protio solvent peaks or tetramethylsilane signal (0 ppm), and 13C NMR chemical shifts were referenced to the solvent resonance. Data for 1H NMR were recorded as follows: chemical shift (δ, ppm), multiplicity (s = singlet, d = doublet, t = triplet, quint = quintuplet, sext = sextuplet, m = multiplet or unresolved, coupling constant(s) in Hz, integration). Data for 13C NMR were reported in terms of chemical shift (δ, ppm). Enantiomeric excesses were determined by high-performance liquid chromatography (HPLC) with a Dionex or Agilent chromatography [Phenomenex Lux 5u Cellulose-1 (0.46 × 25 cm), Phenomenex Lux 5u Cellulose-3 (0.46 × 25 cm), Phenomenex Lux 5u Cellulose-4 (0.46 × 25 cm), Daicel CHIRALPAK AD-H (0.46 × 25 cm), Daicel CHIRALPAK OD-H (0.46 × 25 cm), CHIRALPAK IC (0.46 × 25 cm), and Lux 5u Amylose-2 (0.46 × 25 cm)] in comparison with authentic racemic materials. Optical rotations were measured on a Rudolph Research Analytical Autopol I Polarimeter. Elemental analyses and high-resolution mass spectrometry were carried out by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS).

Synthesis of Complex [(S)-Bn-IPO]MnCl2 [(S)-3a]

To a yellow solution of (S)-1a (260 mg, 0.66 mmol) in approximately 30 mL of THF, 179 mg (0.66 mmol) of n class="Chemical">MnCl2(THF)2 was added. The resulting mixture was stirred at 60 °C for 24 h. After that, it was filtered through a plug of Celite, and the volatiles were removed in vacuo. The residue was washed with toluene (3 mL*5) to afford product (S)-3a as an orange solid (390 mg, 96%). Anal. Calcd for C29H33Cl2MnN3O: C, 61.60; H, 5.88; N, 7.43. Found: C, 61.72; H, 5.75; N, 7.33.

Synthesis of Complex [(S)-iPr-IPO]MnCl2 [(S)-3b]

This compound was prepared in a manner similar to (S)-3a with 178 mg (0.66 mmol) of (S)-1b, 179 mg of n class="Chemical">MnCl2(THF)2 (0.66 mmol), and approximately 30 mL of THF. This procedure yielded 330 mg (96%) of an orange solid identified as (S)-3b. Anal. Calcd for C46H43Cl2MnN3O: C, 70.86; H, 5.56; N, 5.39. Found: C, 70.82; H, 5.55; N, 5.64.

Synthesis of Complex [Dibenzhydryl-(S)-iPr-IPO]MnCl2 [(S)-3c]

This compound was prepared in a manner similar to (S)-3a with 430 mg (0.66 mmol) of (S)-2a, 178 mg of n class="Chemical">MnCl2(THF)2 (0.66 mmol), and approximately 20 mL of THF. This procedure yielded 460 mg (90%) of an orange solid identified as (S)-3c. Anal. Calcd for C47H45Cl2MnN3O: C, 71.12; H, 5.71; N, 5.29. Found: C, 70.71; H, 5.56; N, 5.19.

Synthesis of Complex [Dibenzhydryl-(S)-tBu-IPO]MnCl2 [(S)-3d]

This compound was prepared in a manner similar to (S)-3a with 162 mg (0.24 mmol) of n class="Chemical">(S)-2b, 65 mg of MnCl2(THF)2 (0.24 mmol), and approximately 20 mL of THF. This procedure yielded 178 mg (90%) of an orange solid identified as (S)-3d. Anal. Calcd for C47H45Cl2MnN3O: C, 71.12; H, 5.71; N, 5.29. Found: C, 70.71; H, 5.56; N, 5.19.

General Procedure for the Hydrosilylation of Ketones with the Manganese Complex

In a nitrogen-filled glovebox, to a solution of (S)-3d (0.005 mmol, 4.0 mg) in 2 mL of n class="Chemical">toluene, a solution (1.0 M in THF) of NaBHEt3 (10 μL, 0.01 mmol) was slowly added at 25 °C. After stirring for 1 min, PhSiH3 (54.1 mg, 0.5 mmol, 1 equiv) and the ketone substrate 4 (0.5 mmol) were sequentially added. The reaction mixture was stirred at 25 °C for 3 h and then was quenched by exposing the solution to air. Then, MeOH (1.5 mL) and 10% NaOH (2 mL) were added with vigorous stirring for 10 h. The resulting solution was extracted with EtOAc, washed with brine (15 mL), and dried over anhydrous Na2SO4. After filtration and evaporation of the solvent, the residue was purified by flash column chromatography with EtOAc/petroleum ether to give the desired product.