Abstract
Asymmetric Mannich-type reaction of hydrazones with difluoroenoxysilanes using chiral zinc(II)–imidazoline–phosphine complexes as catalysts have been established, giving the corresponding adducts in good to excellent enantioselectivity and chemical yields under mild conditions.
Organofluorine chemistry has attracted a considerable interest in the past several decades because, the introduction of fluorine atoms onto target molecules can cause significant enhancement in their chemical, physical, and pharmacological properties.1,2 In fact, the development of more efficient approaches to optically active fluorinated compounds has become one of the most challenging topics in modern organic chemistry.3,4 Several methodologies and reagents have been established for the asymmetric fluorination, trifluoromethylation, and perfluoroalkylation reactions.5 However, the progress on assembling chiral difluoro-methylene structural scaffolds via asymmetric catalytic difluorination has been slow.6 The enantioselective Reformatsky reaction of bromodifluoroacetate with aldehydes was disclosed by Braun7 and co-workers in 1995, affording the corresponding α, α-difluoro-β-hydroxy esters in up to 84% ee and moderate yields in the presence of an excess amount of (1R,2S)-N-methyl-ephedrine. The asymmetric Mukaiyama aldol reaction of difluoro-ketene silyl with various aldehydes afforded the corresponding products in good enantioselectivities (up to 97% ee) and excellent yields by using Masamune's8a or Kiyooka's catalysts.8bVery recently, Akiyama and his co-workers have established a successful catalytic enantioselective synthesis of β-amino-α,α-difluoro carbonyl compounds via the reaction of N-Boc-protected imines with difluoroenoxysilanes in the presence of chiral phosphoric acid.8c We also reported the zinc-catalyzed asymmetric difluorination reaction of readily available hydrazones with difluoroenoxysilane, providing the corresponding adducts in moderate yields and enantioselectivities.9 Herein, we would like to disclose the enantioselective Mannich-type reaction of difluoroenoxysilanes with hydrazones by using the catalytic complexes formed from zinc salt and novel chiral imidazoline–phosphine ligands. Our previous study revealed the use of Zn(OTf)2–oxazoline–phosphine ligand complexes (L5–L8, Fig. 1) as catalysts for the asymmetric difluorination of hydrazones with difluoroenoxysilane can lead to the formation of corresponding difluorination adducts in low yields and enantioselectivities (up to 51% ee with 23% isolated yield),9 while their chiral phosphine-anchored Schiff's base counterparts (L1–L4, Fig. 1) do nor show any effectiveness. Recently, we switched our strategy to the design and synthesis of N-containing heterocycle–phosphine ligands for Mannich-type difluorination reaction. Since the five-membered imidazoline ring can easily coordinate onto metal cations, which is similar to its oxazoline-based situation.10 We envisioned that they would offer an opportunity to render the asymmetric catalytic Mannich-type reaction of difluoroenoxysilanes with hydrazones. Following an efficient route developed by You and Kelly,11 we prepared a series of novel chiral imidazoline–phosphine ligands, L9–L14, based on a binaphthyl scaffold (Fig. 2). In addition, the structure of L9 has been unambiguously determined by X-ray diffraction analysis (see ESI†).
Fig. 1.
Selected chiral ligands for Mannich-type difluorinations.
Fig. 2.
Novel imidazoline–phosphine ligands L9–L14.
Our study was started with the use of hydrazone 1a.12,13 The initial reaction of hydrazone 1a with difluoroenoxysilane 2a14 was conducted in 1.0 mL of THF in the presence of Lewis acid, Zn(OTf)2 (10 mol%) together with imidazoline–phosphine ligand (Ra,S,S)-L9 (10 mol%) as the catalytic combination. We found that the reaction proceeded smoothly to give the desired product 3a in 76% yield and 62% ee (Table 1, entry 1), which was greater than the results of using Zn(OTf)2–oxazoline–phosphine ligand as reported previously.9 The use of Zn(NTf2)2 together with L9 showed no improvement in enantioselectivity (Table 1, entry 2). Inspired by the work performed by Mikami's group,15 the use of Zn(OTf)2 and the chiral imidazoline–phosphine ligand L9 in a 1 : 2 ratio led to 78% ee with a slightly improved yield (Table 1, entry 3). However, 5 mol% of Zn (OTf)2 with 10 mol% of L9 did not show effectiveness in both the yield and enantioselectivity (Table 1, entry 4). Next, we examined different imidazoline–phosphine ligands, L10–L14. We found that ligands (Ra,R,R)-L10 and (Sa,S,S)-L11 afforded the desired Mannich-type adduct 3a in up to 81% yield and low enantioselectivities (Table 1, entries 5 and 6). Ligand (Sa,R,R)-L12 produced the opposite enantiomer of 3a in a similar yield and enantioselectivity, which was observed in the reaction of hydrazone 1a with difluoroenoxysilane 2a catalyzed by (Ra,S,S)-L9/Zn(OTf)2 (Table 1, entry 7). As compared to ligand (Ra,S,S)-L14, (Ra,S,S)-L13, bearing a di(3,5-dimethylphenyl)phosphine group, led to the corresponding product 3a in 78% yield and 87% ee (Table 1, entries 8 and 9). With the effective chiral ligand (Ra,S,S)-L13 in hand, we further optimized the condition by changing solvents, temperatures, additives, and zinc salts (Table 2). Since the ligand L13 and hydrazone 1a have low solubility in MTBE (tert-butyl methyl ether) and various alcohols, we combined THF with these solvents in 1 : 1 ratio. Interestingly, we found that when MeOH–THF (1 : 1) was employed, the corresponding adduct 3a was obtained in 88% yield along with 89% ee (Table 2, entry 2). However, the combination of MeOH with other solvents, such as MeCN, CH2Cl2 and toluene gave poor yield and enantioselectivity.9 Screening of the alcohols revealed that both MeOH–THF (1 : 1) and tBuOH–THF (1 : 1) resulted in higher enantioselectivity than the combinations of other alcohols (Table 2, entries 2–8). The ratios of MeOH–THF and tBuOH–THF were also examined, but lower ee was obtained, albeit a similar chemical yield was retained (Table 2, entries 9–13). When the reaction was performed at 5 °C, adduct 3a was obtained in 57% yield and 93% ee (Table 2, entry 14). Whereas, only a trace amount of 3a was formed when the reaction was performed at −5 °C (Table 2, entry 15). To further activate the hydrazone 1a at lower temperature, we utilized Zn (NTf2)2 instead of Zn(OTf)2 as Lewis activator. Pleasantly, product 3a was generated in 71% yield and 92% ee when the reaction was carried out at 5 °C in the presence of Zn(NTf2)2 (Table 2, entry 16). With the addition of 4 Å MS (50 mg), a higher enantioselectivity of 93% ee and yield (93%) were achieved (Table 2, entry 17).
Table 1.
Screening of ligands in the asymmetric catalytic Mannich-type reaction of hydrazone 1a with difluoroenoxysilane
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| Entrya | Zinc salt (x mol%) | Ligand (y mol%) | Yieldb (%) 3a | eec (%) 3a | Absolute configurationd |
| 1 | Zn(OTf)2 (10) | (Ra,S,S)-L9 (10) | 76 | 62 | S |
| 2 | Zn(NTf2)2 (10) | (Ra,S,S)-L9 (10) | 79 | 61 | S |
| 3 | Zn(OTf)2 (10) | (Ra,S,S)-L9 (20) | 82 | 78 | S |
| 4 | Zn(OTf)2 (5) | (Ra,S,S)-L9 (10) | 59 | 65 | S |
| 5 | Zn(OTf)2 (10) | (Ra,R,R)-L10 (20) | 81 | 13 | S |
| 6 | Zn(OTf)2 (10) | (Sa,S,S)-L11 (20) | 74 | 9 | R |
| 7 | Zn(OTf)2 (10) | (Sa,R,R)-L12 (20) | 71 | 77 | R |
| 8 | Zn(OTf)2 (10) | (Ra,S,S)-L13 (20) | 78 | 87 | S |
| 9 | Zn(OTf)2 (10) | (Ra,S,S)-L14 (20) | 67 | 82 | S |
Reaction conditions: 1a (0.10 mmol), 2a (0.30 mmol), zinc salt (x mol%), ligand (y mol%), THF (1.0 mL), and the reaction was carried out at 25 °C for 24 h.
Isolated yield after column chromatography.
Determined by chiral HPLC analysis.
Determined by X-ray diffraction.
Table 2.
Optimization of conditions of the zinc(II)-catalyzed asymmetric Mannich-type reaction
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| Entrya | Zinc salt | Solvent | T (°C) | Additive | Yieldb (%) 3a | eec (%) 3a | Absolute configurationd |
| 1 | Zn(OTf)2 | MTBE/THF = 1/1 | 25 | — | 23 | 82 | S |
| 2 | Zn(OTf)2 | MeOH/THF = 1/1 | 25 | — | 88 | 89 | S |
| 3 | Zn(OTf)2 | EtOH/THF = 1/1 | 25 | — | 92 | 59 | S |
| 4 | Zn(OTf)2 | iPrOH/THF = 1/1 | 25 | — | 84 | 53 | S |
| 5 | Zn(OTf)2 | tBuOH/THF = 1/1 | 25 | — | 49 | 90 | S |
| 6 | Zn(OTf)2 | CF3CH2OH/THF = 1/1 | 25 | — | 81 | 80 | S |
| 7 | Zn(OTf)2 | HFIP/THF = 1/1 | 25 | — | 60 | 71 | S |
| 8 | Zn(OTf)2 | t-amyl-OH/THF = 1/1 | 25 | — | trace | n.d.e | — |
| 9 | Zn(OTf)2 | tBuOH/THF = 1/5 | 25 | — | 23 | 86 | S |
| 10 | Zn(OTf)2 | tBuOH/THF = 1/10 | 25 | — | 31 | 83 | S |
| 11 | Zn(OTf)2 | MeOH/THF = 1/5 | 25 | — | 70 | 80 | S |
| 12 | Zn(OTf)2 | MeOH/THF = 1/10 | 25 | — | 71 | 81 | S |
| 13 | Zn(OTf)2 | MeOH/THF = 2/1 | 25 | — | 80 | 87 | S |
| 14 | Zn(OTf)2 | MeOH/THF = 1/1 | 5 | — | 57 | 93 | S |
| 15 | Zn(OTf)2 | MeOH/THF = 1/1 | −5 | — | trace | n.d. | — |
| 16 | Zn(NTf2)2 | MeOH/THF = 1/1 | 5 | — | 71 | 92 | S |
| 17 | Zn(NTf2)2 | MeOH/THF = 1/1 | 5 | MS4A | 79 | 93 | S |
Reaction conditions: 1a (0.10 mmol), 2a (0.30 mmol), zinc salt (10 mol%), L13 (20 mol%), solvent (1.0 mL) and MS4A (50 mg), the reaction was carried out for 24 h.
Isolated yield after column chromatography.
Determined by chiral HPLC analysis.
Determined by X-ray diffraction.
n.d. = not determined.
Having established the above optimal conditions, we next investigated the substrate scope of this reaction with the results summarized in Table 3 and Table 4. As for substrates 1b–1i, the reactions proceeded smoothly to afford the corresponding Mannich-type adducts 3b–3i in moderate to good yields (up to 87% yield) and good to excellent enantioselectivities (from 87% ee to 96% ee), regardless of electronic nature of the substituents on the aromatic rings (Table 3, entries 1–8). Using hydrazone bearing a 3-chlorobenzene group gave the corresponding product 3j in 71% yield and 94% ee (Table 3, entry 9). The naphthyl, furyl or thienyl-containing hydrazones 1k–1o afforded difluorination adducts 3k–3o in moderate to good yields and excellent enantioselectivities (up to 77% yield and 94% ee, Table 3, entries 10–14). It should be noted that the enantioselectivities of 3d, 3n and 3o were slightly improved when the reactions were carried out at 0 °C, but chemical yields were diminished due to the lower solubility of hydrazones at this temperature (Table 3, entries 3, 13 and 14).
Table 3.
Substrate scope of the Zn(NTf2)2-catalyzed asymmetric Mannich-type reaction of hydrazone with difluoroenoxysilane 2a
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| Entrya | Hydrazone 1 (R) | Yieldb (%) 3 | eec (%) 3 | Absolute configurationd |
| 1 | 1b (4-ClC6H4) | 3b, 75 | 95 | S |
| 2 | 1c (4-BrC6H4) | 3c, 79 | 96 | S |
| 3 | 1d (4-FC6H4) | 3d, 83/67e | 87/93e | S |
| 4 | 1e (4-MeC6H4) | 3e, 73 | 92 | S |
| 5 | 1f (4-MeOC6H4) | 3f, 87 | 94 | S |
| 6f | 1g (4-NO2C6H4) | 3g,79 | 90 | S |
| 7 | 1h (4-CNC6H4) | 3h, 67 | 91 | S |
| 8 | 1i (4-CF3C6H4) | 3i, 69 | 90 | S |
| 9 | 1j (3-ClC6H4) | 3j, 71 | 94 | S |
| 10 | 1k (1-Naphthyl) | 3k, 69 | 91 | S |
| 11 | 1l (2-Naphthyl) | 3l, 72 | 94 | S |
| 12 | 1m (2-Furyl) | 3m, 77 | 57 | S |
| 13 | 1n (2-Thienyl) | 3n, 59/51e | 83/90e | S |
| 14 | 1o (3-Thienyl) | 3o, 79/69e | 87/94e | S |
| 15 | 1p [CH3(CH2)6] | 3p, 53 | 60 | S |
| 16 | 1q (Cyclohexyl) | 3q, 52 | 78 | S |
Reaction conditions: 1 (0.10 mmol), 2a (0.30 mmol), Zn(NTf2)2 (10 mol%), L13 (20 mol%), THF (0.50 mL), MeOH (0.50 mL) and MS4A (50 mg), the reaction was carried out at 5 °C for 24 h.
Isolated yield after column chromatography.
Determined by chiral HPLC analysis.
Determined by the X-ray diffraction.
The reaction was carried out at 0 °C.
The reaction was carried out at 25 °C.
Table 4.
The scope of Zn(NTf2)2-catalyzed asymmetric Mannich-type reaction of aromatic hydrazones with difluoroenoxysilane
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| Entrya | Hydrazone 1 or 4 (R1/2) | Difluoroenoxysilane 2 (R3) | Yieldb (%) 3 | eec (%) 3 | Absolute configurationd |
| 1 | 4a (C6H5/4-MeOC6H4) | 2a (C6H5) | 5a, 81 | 94 | S |
| 2 | 4b (C6H5/4-NO2C6H4) | 2a (C6H5) | 5b, 59e/54f | 86/92f | S |
| 3 | 4c (4-BrC6H4/4-BrC6H4) | 2a (C6HS) | 5c, 71e/52f | 82/95f | S |
| 4 | 4d (C6H5/4-C6H5C6H4) | 2a (C6H5) | 5d, 72e/52f | 89/96f | S |
| 5 | 1b (4-ClC6H4/C6H5) | 2b (4-MeC6H4) | 5e, 88/59g | 88/92g | S |
| 6 | 1b (4-ClC6H4/C6H5) | 2c (2-Thienyl) | 5f, 81/50g | 51/76g | S |
Reaction conditions: 1 or 4 (0.10 mmol), 2 (0.30 mmol), Zn(NTf2)2 (10 mol%), L13 (20 mol%), THF (0.50 mL), MeOH (0.50 mL) and MS 4A (50 mg), the reaction was carried out at 5 °C for 24 h.
Isolated yield after column chromatography.
Determined by chiral HPLC analysis.
Determined by the X-ray diffraction.
The reaction was carried out at 25 °C.
The reaction was carried out at 10 °C and 5.0 equiv of 2a was utilized.
The reaction was carried out at −5 °C and 5.0 equiv of 2b or 2c was utilized.
Previous research showed that aliphatic hydrazones, such as 1t [R = CH3(CH2)3], was inefficient in the Zn(OTf)2-promoted Mannich-type reaction of hydrazones with difluoroenoxysilanes.8c,9 However, under this new Zn(NTf2)2/L13-based catalytic system, aliphatic hydrazones showed effectiveness leading to moderate yields and enantioselectivities (Table 3, entries 15 and 16). More examples are provided in the Supporting Information (Table S1).†
Hydrazones having a different R2 group were subjected to the reaction with several difluoroenoxysilanes 2 under the optimized conditions. When R2 is a 4-methoxyphenyl group, the reaction proceeded smoothly leading to the corresponding adduct 5a in 81% yield and 94% ee (Table 4, entry 1). As revealed in Table 4, when hydrazones 4b–4d were used as substrates, the corresponding adducts were obtained in moderate to good yields (59–72%) and enantioselectivity (up to 89% ee) regardless of the electronic nature of R2 group (Table 4, entries 2–4). Lowering the reaction temperature from r.t. to 10 °C resulted in the corresponding products 5b–5d in 92–96% ee but in lower yields due to the low solubility of hydrazones (Table 4, entries 2–4). Adduct 5c was obtained in 95% ee, which enabled successful formation of single crystals for the X-ray diffraction analysis which has proven to be (S)-absolute configuration (Fig. SI-1 in the ESI†). In addition, the reaction of hydrazone 1b with difluoroenoxysilane 2b or 2c occurred smoothly to give the corresponding adducts 5e and 5f in good yields (up to 88%) and moderate to good enantioselectivities (up to 88% ee) (Table 4, entries 5 and 6). Furthermore, when the reaction was carried out at −5 °C, 5e and 5f were formed in 59% yield/92% ee and 50% yield/76% ee, respectively (Table 4, entries 5 and 6). The reaction of 4c with 2a was also performed on a 1.0 mmol scale (536 mg), giving 5c in 63% yield and 95% ee.
To obtain direct evidence of the coordination pattern between the imidazoline–phosphine ligand and Zn(II) salt, we conducted 1H NMR and 31P NMR spectroscopic studies of (Ra,S,S)-L13 by using a 1 : 1 mixture of L13 : Zn(OTf)2 and 2 : 1 mixture of L13/Zn(OTf)2 in CDCl3 at ambient temperature (Fig. SI-2†). The phosphorus signal of L13 appeared at δ −14.87 ppm. For a 1 : 1 mixture of L13 : Zn(OTf)2, the phosphorus signal of L13 appeared at δ −14.84 ppm along with an additional signal at δ −1.88 ppm. The signal at δ −1.88 ppm was strengthened in the 2 : 1 mixture of L13 and Zn(OTf)2, as compared to that of the 1 : 1 mixture of L13 and Zn(OTf)2. These results indicate that the 2 : 1 mixture of imidazoline-phosphine ligands and zinc(II) would be able to generate the active species for the present catalytic system.
Finally, it should be noted that adduct 3a has been easily transformed to the heterocyclic compound 7 upon treatment with NaBH4 and triphosgene in a good yield of 73% (Scheme 1, see ESI†).
Scheme 1.
Transformation of 3a
In conclusion, we have established a novel catalytic system of using chiral zinc(II)–imidazoline–phosphine complexes as catalysts for the asymmetric Mannich-type difluorinations of hydra-zones with difluoroenoxysilanes. The reaction can smoothly occur under mild conditions, affording the corresponding Mannich-type adducts in moderate to good yields and excellent enantioselectivities. The catalytic complexes was studied by direct NMR observations, which indicated that the ratio 2 : 1 of imidazoline–phosphine ligand with zinc(II) salt was sufficient to give the active species for this asymmetric difluorination reaction.
Supplementary Material
Acknowledgements
We thank the National Basic Research Program of China (973)-2010CB833302, and the National Natural Science Foundation of China for financial support (20928001, 21072206, 20472096, 20872162, 20672127, 20821002 and 20732008), Robert A. Welch Foundation (D-1361) and NIH (R21DA031860-01).
Footnotes
Electronic supplementary information (ESI) available: Spectroscopic data and chiral HPLC traces of the compounds shown in Tables 1–4, the detailed descriptions of experimental procedures and the crystal structures of L9 and 5c. CCDC 785244 and 798183. For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/c2ob07022g
Notes and references
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