Novel C-S Bond Formation Through α’,β-Elimination of tert-Butyl Sulfoxonium Ylides: A Facile Approach to Chiral Sulfoxides

Abstract

Chiral sulfoxides were prepared in excellent enantio­selectivity through α′,β-elimination of the tert-butyl substituted sulf­oxonium ylide intermediates in situ generated from diazoacetates and (R)-N-tert-butylsulfinyl aldimines in the presence of a copper(I) catalyst.

Key words

chiral sulfoxides - sulfoxonium ylide - α′,β-elimination

References and Notes

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14

Typical Procedure for the Synthesis of Compounds 2 and 3 To a refluxing CH₂Cl₂ (3 mL) solution of Rh₂(OAc)₄ (2.6 mg, 3 mol%), (R)-(tert-butylsulfinyl)benzene [(R)-1a, 36.4 mg, 0.2 mmol] was added ethyl diazoacetate (68.4 mg, 0.6 mmol) in CH₂Cl₂ (3 mL) over 1 h via a syringe pump. After the addition was completed, the reaction mixture was cooled to r.t. Solvent was removed, and the crude product was purified by a flash column chromatography on silica gel eluting with 20% EtOAc-light PE to give (S)-2a (18 mg) in 43% yield.

15

Analytical Data of ( S )-Ethyl 2-(Phenylsulfinyl)acetate [( S )-2a] TLC: R _f  = 0.15 (PE-EtOAc, 5:1); [α]_D ^²0 -131 (c 1, EtOAc); 90% ee, determined by HPLC [Daicel Chiralpak AD-H, flow rate 0.4 mL/min, hexane-2-PrOH = 90:10, 254 nm; t _R(major) = 37.2 min and t _R(minor) = 40.0 min]. ^¹H NMR (300 MHz, CDCl₃): δ = 1.20 (t, J = 7.1 Hz, 3 H), 3.68 (d, J = 13.6 Hz, 1 H), 3.87 (d, J = 13.6 Hz, 1 H), 4.16 (q, J = 7.1 Hz, 2 H), 7.52-7.70 (m, 5 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 13.98, 61.70, 62.01, 124.17, 129.36, 131.75, 143.07, 164.68 ppm. HRMS (EI): m/z calcd for C₁₀H₁₂NaO₃S [M + Na]⁺: 235.0405; found: 235.0399.

16

Analytical Data of ( S )-Ethyl 2-(Phenylsulfinyl)acetate [( S )-2b]
TLC: R _f  = 0.13 (PE-EtOAc, 5:1); [α]_D ^²0 -120 (c 1, EtOAc); 99% ee, determined by HPLC [Daicel Chiralpak AD-H, flow rate 0.4 mL/min, hexane-2-PrOH = 90:10, 254 nm, t _R(major) = 42.35 min and t _R(minor) = 45.58 min]. ^¹H NMR (300 MHz, CDCl₃): δ = 1.26 (t, J = 7.2 Hz, 3 H), 3.67 (d, J = 13.5 Hz, 1 H), 3.85 (d, J = 10.8 Hz, 1 H), 3.86 (s, 3 H), 4.21 (q, J = 7.1 Hz, 2 H), 7.03-7.20 (m, 4 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 14.02, 55.60, 61.90, 62.04, 108.41, 116.18, 118.23, 130.31, 144.61, 160.53, 164.73 ppm. HRMS (EI): m/z calcd for C₁₁H₁₄NaO₄S [M + Na]⁺: 265.0510; found: 265.0505.

17

Analytical Data of ( R , S )-Ethyl 2-(Phenylsulfinyl)acetate [( R , S )-3] TLC: R _f  = 0.26 (PE-EtOAc, 5:1); [α]_D ^²0 -110 (c 1, EtOAc); 97.5% ee, determined by HPLC [Daicel Chiralpak AD-H, flow rate 0.4 mL/min, hexane-2-PrOH = 90:10, 254 nm; t _R(major) = 44.7 min and t _R(minor) = 50.9 min]. ^¹H NMR (300 MHz, CDCl₃): δ = 3.84 (s, 3 H), 4.55 (s, 1 H), 3.87 (d, J = 13.6 Hz, 1 H), 4.16 (q, J = 7.1 Hz, 2 H), 7.04-7.49 (m, 10 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 61.70, 78.02, 125.19, 128.50, 128.57, 129.22, 129.37, 129.94, 131.70, 141.18, 168.01 ppm. HRMS (EI): m/z calcd for C₁₅H₁₄NaO₃S [M + Na]⁺: 297.0561; found: 297.0556.

18

Typical Procedure for the Synthesis of Compounds 5 and 6 To a refluxing CH₂Cl₂ (3 mL) solution of CuPF₆ (MeCN)₄ (2.2 mg, 3 mol%), (S)-N-(4-nitrobenzylidene)-2-methyl-propane-2-sulfinamide (4c, 50.8 mg, 0.2 mmol) was added ethyl diazoacetate (68.4 mg, 0.6 mmol) in CH₂Cl₂ (3 mL) over 1 h via a syringe pump. After the addition was completed, the reaction mixture was cooled to r.t. Solvent was removed, and the crude product was purified by a flash column chromatography on silica gel eluting with 20% EtOAc-light PE to give (R)-5c (24 mg) in 42% yield.

19

Analytical Data of ( R )- N -(4-Nitrobenzylidene)-2-ethoxy-2-oxomethanesulfinamide [( R ) - 5c] TLC: R _f  = 0.12 (PE-EtOAc, 5:1); [α]_D ^²0 +84.5 (c 1, EtOAc); 100% ee, determined by HPLC [Daicel Chiralpak AS, flow rate 0.6 mL/min, hexane-2-PrOH = 100:30, 254 nm; t _R(major) = 25.72 min and t _R(minor) = 19.09 min]. ^¹H NMR (300 MHz, CDCl₃): δ = 1.30 (t, J = 7.1 Hz, 3 H), 3.76 (d, J = 13.6 Hz, 1 H), 3.90 (d, J = 13.6 Hz, 1 H), 4.31 (q, J = 7.1 Hz, 2 H), 8.05 (d, J = 8.8 Hz, 2 H), 8.36 (d, J = 8.8 Hz, 2 H), 8.76 (s, 1 H) ppm. ^¹³C NMR (75 MHz, CDCl₃): δ = 14.15, 59.98, 62.24, 124.26, 130.39, 132.32, 138.32, 161.13, 164.06 ppm. HRMS (EI): m/z calcd C₁₁H₁₂N₂NaO₅S [M + Na]⁺: 307.0365; found: 307.0359.

20

Crystal Structure Data for ( R , R )- N -(4-Nitrobenzylidene)-2-ethoxy-2-oxomethane-1-benzylsulfinamide [(R , R )-6a] C₁₆H₁₄N₂O₅S, Mw = 346.35, light yellow, orthorhombic, P2, a = 5.70380 (10), b = 8.2382 (2), c = 34.4784 (8) Å, α = 90.00, β = 90.00, γ = 90.00, V = 1620.11 (6) Å^³, Z = 4, T = 296 (2) K, ρ _calcd = 1.420 Mg˙m, F(000) = 720, λ = 0.71073 Å, µ = 0.229 mm^-¹, R(F) = 0.0299 and wR(F)2 = 0.0792 for 2722 observed reflections, I > 2σ, 2.36˚ < q < 24.99˚. CCDC 722774 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax [phone]; or deposit@ ccdc.cam.ac.uk].