Guanidine-Urea Bifunctional Organocatalyst for Asymmetric Epoxidation of 1,3-Diarylenones with Hydrogen Peroxide

Abstract

A highly enantioselective catalytic epoxidation reaction to the electron-deficient α,β-unsaturated olefin moieties of diaryl­enones was achieved with high chemical yield by using aqueous hydrogen peroxide in the presence of a newly developed guanidine-urea bifunctional organocatalyst. These functional groups were suggested to perform cooperatively by interacting with guanidine-­hydrogen peroxide and urea-enones, respectively.

guanidine - urea - hydrogen peroxide - epoxidation - electron-deficient olefin

References and Notes

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14

Synthesis of Catalyst 1a and Spectral Data for 1a-e
To a solution of guanidine (S,S)-1f ^¹0a (258 mg, 0.317 mmol) in CH₂Cl₂ (3.0 mL) was added TFA (3.0 mL) at 0 ˚C. The reaction mixture was warmed to r.t. and stirred for 2 h. The resulting mixture was concentrated in vacuo to give diamine. To a solution of the diamine in THF (6.0 mL) was added phenyl isocyanate (0.21 mL, 1.90 mmol), and the mixture was stirred for 12 h. The resulting mixture was concentrated in vacuo, and the residue was purified by flash column chromatography on silica gel (n-hexane-EtOAc = 4:1 to 1:1, CHCl₃-MeOH = 9:1) to give 1a as a TFA salt (Scheme  [¹] ). The counteranion of 1a was exchanged into Cl^- by treatment with sat. aq NH₄Cl and EtOAc solution, and gave 1a as a HCl form in 81% yield from 1f (219 mg, 0.257 mmol).
Compound 1a: [α]_D ^²4 -41.2 (c 1.3, CHCl₃). ^¹H NMR (400 MHz, CD₃OD): δ = 7.33-7.10 (m, 18 H), 6.93 (t, J = 7.4 Hz, 2 H), 4.11 (br s, 2 H), 3.45-3.32 (m, 4 H), 3.16 (t, J = 7.3 Hz, 2 H), 3.03 (dd, J = 4.5, 14.1 Hz, 2 H), 2.79 (dd, J = 9.6, 13.7 Hz, 2 H), 1.59 (m, 2 H), 1.34-1.14 (m, 30 H), 0.88 (t, J = 7.3 Hz, 3 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 158.51, 156.33, 140.52, 138.97, 130.22, 129.83, 129.68, 127.78, 123.67, 120.18, 52.46 (br), 47.63 (br), 43.12, 39.32, 33.09, 30.78 (br), 30.69, 30.66, 30.49, 30.39, 29.75, 27.99, 23.75, 14.48. ESI-HRMS: m/z calcd for C₅₁H₇₄N₇O₂ [M + H⁺]: 816.5904; found: 816.5895.
Compound 1b: [α]_D ^²5 -11.3 (c 1.1, CHCl₃). ^¹H NMR (400 MHz, CD₃OD): δ = 7.94 (s, 4 H), 7.44 (s, 2 H), 7.31-7.14 (m, 10 H), 4.14 (br s, 2 H), 3.41 (d, J = 5.0 Hz, 4 H), 3.18 (t, J = 7.3 Hz, 2 H), 3.07 (dd, J = 4.0, 13.9 Hz, 2 H), 2.80 (dd, J = 9.5, 13.9 Hz, 2 H), 1.61 (m, 2 H), 1.34-1.08 (m, 30 H), 0.88 (t, J = 6.9 Hz, 3 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 157.82, 156.31, 142.99, 138.79, 133.13 (q, J _CF = 32.6 Hz), 130.14, 129.66, 127.81, 124.76 (d, J _CF = 271.3 Hz), 120.70, 118.98, 115.79, 52.66 (br) 47.46, 43.10, 39.20, 33.08, 30.75 (br), 30.61, 30.54, 30.47, 30.16, 29.64, 27.92, 23.74, 14.46. ESI-HRMS: m/z calcd for C₅₅H₇₀F₁₂N₇O₂ [M + H⁺]: 1088.5399; found: 1088.5370.
Compound 1c: [α]_D ^²6 -24.5 (c 1.4, CHCl₃). ^¹H NMR (400 MHz, CD₃OD): δ = 7.32-7.17 (m, 10 H), 6.97 (d, J = 9.6 Hz, 4 H), 6.46 (t, J = 9.2 Hz, 2 H), 4.10 (br s, 2 H), 3.39 (d, J = 5.5 Hz, 4 H), 3.18 (t, J = 7.3 Hz, 2 H), 3.07 (dd, J = 4.6, 13.8 Hz, 2 H), 2.79 (dd, J = 9.6, 14.2 Hz, 2 H), 1.63 (m, 2 H), 1.35-1.07 (m, 30 H), 0.88 (t, J = 6.4 Hz, 3 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 164.61 (dd, J _CF = 15.3, 243.4 Hz), 157.77, 156.22, 143.55 (t, J _CF = 13.5 Hz), 138.81, 130.18, 129.64, 127.77, 102.09 (dd, J _CF = 8.6, 21.1 Hz), 52.54 (br), 47.49 (br), 43.16, 39.22, 33.07, 30.78 (br), 30.71, 30.65, 30.48, 30.38, 29.76, 28.03, 23.74, 14.50. ESI-HRMS: m/z calcd for C₅₁H₇₀F₄N₇O₂ [M + H⁺]: 888.5527; found: 888.5572.
Compound 1d: [α]_D ^²5 -40.4 (c 1.1, CHCl₃). ^¹H NMR (400 MHz, CD₃OD): δ = 8.04 (s, 4 H), 7.47 (s, 2 H), 3.78 (br s, 2 H), 3.42 (dd, J = 13.8, 5.1 Hz, 2 H), 3.25 (m, 2 H), 3.17 (t, J = 7.4 Hz, 2 H), 1.94 (br, 2 H), 1.58 (m, 2 H), 1.33-1.08 (m, 30 H), 1.03 (d, J = 6.4 Hz, 6 H), 1.01 (d, J = 6.4 Hz, 6 H), 0.88 (t, J = 6.9 Hz, 3 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 158.30, 156.31, 143.15, 133.20 (q, J _CF = 32.6 Hz), 128.84, 124.78 (d, J _CF = 272.2 Hz), 120.73, 118.84, 115.68, 55.92 (br), 46.04, 43.03, 33.08, 31.04 (br), 30.75 (br), 30.70, 30.62, 30.52, 30.48, 30.19, 29.80, 27.87, 23.74, 20.19, 17.62 14.46. ESI-HRMS: m/z calcd for C₄₇H₇₀F₁₂N₇O₂ [M + H⁺]: 992.5399; found: 992.5373.
Compound 1e: [α]_D ^²6 -8.9 (c 1.1, CHCl₃). ^¹H NMR (400 MHz, CD₃OD): δ = 8.04 (s, 4 H), 7.46 (s, 2 H), 3.91 (br s, 2 H), 3.41-3.14 (m, 6 H), 1.64 (m, 2 H), 1.29 (d, J = 6.9 Hz, 6 H), 1.27-1.10 (m, 30 H), 0.87 (t, J = 6.9 Hz, 3 H). ^¹³C NMR (100 MHz, CD₃OD): δ = 157.80, 156.25, 143.11, 133.15 (q, J _CF = 32.6 Hz), 128.84, 124.79 (d, J _CF = 272.2 Hz), 120.73, 119.00, 115.72, 47.05 (br), 43.13, 33.08, 30.75 (br), 30.70, 30.61, 30.52, 30.48, 30.14, 29.63, 27.90, 23.74, 18.34, 14.46. ESI-HRMS: m/z calcd for C₄₃H₆₂F₁₂N₇O₂ [M + H⁺]: 936.4773; found: 936.4734.

Scheme 1

15a Schreiner PR. Wittkopp A. Org. Lett.  2002,  4:  217 

15b Wittkopp A. Schreiner PR. Chem. Eur. J.  2003,  9:  407 

16

We recycled the catalyst 1b five times under the conditions of entry 11 in Table  [¹] . In these reactions, the yields and enantioselectivities were as follows: 2nd run: 95% with 90% ee; 3rd run: 99% with 90% ee; 4th run: 99% with 91% ee; and 5th run: 99% with 89% ee.

17a Lattanzi A. Org. Lett.  2005,  7:  2579 

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18

Typical Procedure for Asymmetric Epoxidation of 4a
A mixture of enone 4a (20.8 mg, 0.10 mmol) and guanidine-urea organocatalyst (S,S)-1b (5.6 mg, 0.005 mmol, 5 mol%) in toluene (0.95 mL) was cooled at -10 ˚C. To the mixture was added 1 M aq NaOH (0.050 mL, 0.050 mmol) and 30% aq H₂O₂ (0.051 mL, 0.50 mmol of H₂O₂). The mixture was stirred vigorously at -10 ˚C under argon atmosphere for 6 h. To the reaction mixture was added sat. aq NH₄Cl, and the organic layer was extracted with EtOAc. The combined organic extracts were dried over MgSO₄, filtered, and concentrated in vacuo. The residue was purified by flash column chromatography on silica gel (n-hexane-EtOAc = 100:1 to 10:1) to give epoxy ketone 5a (22.3 mg, 99%) and catalyst 1b was quantitatively recovered (5.6 mg, >99%). The ee and absolute configuration of the epoxy ketone 5a was determined by HPLC using a chiral column.
Spectral Data and HPLC Data for Epoxy Ketone 5a
[α]_D ^²4 -210.1 (c 0.83, CHCl₃). ^¹H NMR (400 MHz, CDCl₃): δ = 8.02 (d, J = 6.9 Hz, 2 H), 7.63 (t, J = 7.8 Hz, 1 H), 7.49 (t, J = 7.8 Hz, 2 H), 7.45-7.35 (m, 5 H), 4.31 (d, J = 1.8 Hz, 1 H), 4.08 (d, J = 1.8 Hz, 1 H). HPLC separation conditions: Chiralcel OD-H, 0.46 cm (ϕ) × 25 cm (L), hexane-2-PrOH = 98:2, 1.00 mL/min, t _R(minor) = 19.5 min (2S,3R); t _R(major) = 20.4 min (2R,3S).^¹7a

19

In the case of aliphatic substituted enones, enantioselec-tivities were moderate to low (ex. R^¹ = Me, R^² = Ph, 99% yield with 41% ee).

20

NMR studies were performed in C₆D₆.

21 Sohtome Y. Takemura N. Takagi R. Hashimoto Y. Nagasawa K. Tetrahedron  2008,  64:  9423 ; and references cited therein