(S)-Dimethylsulfonium(p-tolylsulfinyl)methylide: A New Chiral Sulfonium Ylide and its Use in Asymmetric Epoxidation

Abstract

The title optically active sulfonium ylide was prepared by methylation of easily available (-)-(S)-p-tolylmethylthiomethylsulfoxide and subsequent deprotonation. Its reaction with aldehydes gave α,β-epoxy sulfoxides with full enantioselectivity and moderate diastereoselectivity. The steric course of acid-catalyzed rearrangement of sulfinyl oxiranes is briefly discussed.

Key words

asymmetric synthesis - chiral auxiliaries - ylides - epoxidation - sulfoxides

References and Notes

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8

Procedure for Preparation of Sulfonium Salt 2. To 3 mmol (0.6 g) of (-)-(S)-p-tolylmethylthiomethyl-sulfoxide excess of MeI (0.5 mL) was added under an argon atmosphere and mixture was cooled to 0 °C. Next, 0.65 g of AgBF₄ was added in small portions and vigorous stirring. The temperature was maintained for 0.5 h, then 5 mL of Et₂O was added and stirring was continued at r.t. overnight. After evaporation of Et₂O the residue was treated with 20 mL of H₂O-acetone mixture (1:1) and filtrated. Then, acetone was evaporated, water solution was extracted with 2 × 3 mL of CHCl₃ to remove impurities, and H₂O was evaporated affording 9 g of crystalline salt 2; mp 114-116 °C, [α]_D ²² -285.9 (c 1.2, acetone). ¹H NMR (200 MHz, CD₃COCD₃): δ = 2.43 (3 H, s, CH₃PhS), 3.05 (3 H, s, CH₃S), 3.16 (3 H, s, CH₃S), 4.83 and 5.10 (2 H, AB system, J = 13.2 Hz, SCH₂S), 7.49 and 7.73 (4 H, AB system J = 8.4 Hz, Ar). ¹³C NMR (50 MHz): δ = 22.0, 27.3, 27.5, 64.0, 126.0, 131.9, 139.3, 144.9. Anal. Calcd for C₁₀H₁₅BF₄OS₂: C, 39.75; H, 5.00. Found: C, 39.64; H, 4.96.

11

¹H NMR (200 MHz) spectrum of the ylide 1 generated in DMSO-d ₆ showed signals at δ = 2.32 (3 H, s, CH₃Ph), 2.57 (3 H, s, CH₃S), 2.62 (3 H, s, CH₃S), 3.04 (1 H, SCHS), 7.23 and 7.44 (4 H, aromatic).

14

Typical Procedure for Epoxidation. To a solution of 1 mmol (0.3 g) of (S)-dimethylsulfonium- (p-tolylsulfinyl)methylide in 5 mL of dry DMSO or 4-5 mL of MeCN, 0.52 g (1.1 mmol) of NaH was added at r.t. under an argon atmosphere. The resulting mixture was stirred for 30 min. Then, the precipitate was filtered off and 1 mmol (0.19 g) of p-bromobenzaldehyde in 1 mL of appropriate solvent was added. After stirring at r.t. for 2 h, the reaction was quenched with aq NH₄Cl solution (20 mL), extracted with hexane (4 × 5 mL) and dried over MgSO₄. After filtration, the solvent was evaporated under reduced pressure affording 4b in 82% of yield as a mixture of trans/cis isomers in ratio 1.5:1 ratio (DMSO), 2.5:1 (MeCN). Separation of isomers was achieved by chromatography on basic alumina (hexane-acetone, 50: 1).
(2 S ,3 S , S _S )-3-( p -Bromophenyl)-2-( p -tolylsulfinyl)oxirane ( trans -4b): [α]_D ²² +52.1 (c 0.6, acetone). ¹H NMR (200 MHz, CDCl₃): δ = 2.42 (3 H, s, CH₃PhS), 3.94 (1 H, d, J = 1.6 Hz, CHPh), 4.54 (1 H, d, J = 1.6 Hz, CHS), 7.11 and 7.45 (4 H, AB system J = 8.4 Hz, Ar), 7.36 and 7.59 (4 H, AB system J = 8.2 Hz, Ar). ¹³C NMR (50 MHz): δ = 21.5, 29.6, 53.8, 75.7, 123.1, 124.6, 127.5, 130.3, 131.9, 133.0, 137.0, 142.7.
(2 S ,3 R , S _S )-3-( p -Bromophenyl-2-( p -tolylsulfinyl)oxirane ( cis -4b): [α]_D ²² -117.1 (c 0.4, acetone). ¹H NMR (200 MHz, CDCl₃): δ = 2.45 (3 H, s, CH₃PhS); 4.08 (1 H, d, J = 3.3 Hz, CHPh), 4.40 (1 H, d, J = 3.3 Hz, CHS), 7.35 and 7.59 (4 H, AB system, J = 8.4 Hz, Ar), 7.39 and 7.62 (4 H, AB system, J = 8.2 Hz, Ar).

16

The ee was determined by chiral HPLC on Chirobiotic V column using i-PrOH-hexane (2.5:97.5) as eluent with a flow rate of 0.4 mL/min. For (R)-5b the second enantiomer was not detected. Desulfinylation of the crude mixture of 4b (trans/cis 2.5:1) under the same conditions afforded 5b with 42% ee. The R enantiomer eluted at 187 s and the S enantiomer at 209 s.

19

¹H NMR of the crude sample obtained by rearrangement of (2S,3S)-4b after 3 d shows the presence of both isomers of 6b in 2:1 ratio.

20

The mechanistic details of this reaction are under current investigation.