P-Tolyl Vinyl Sulfoxide

Abstract

[36832-47-0] C9H10OS (MW 166.26) InChI = 1S/C9H10OS/c1-3-11(10)9-6-4-8(2)5-7-9/h3-7H,1H2,2H3 InChIKey = ODAVDFDCUDDZPI-UHFFFAOYSA-N (a vinyl sulfoxide,2 typically used in enantiomerically pure form in conjugate additions,3 cycloadditions,4 and Pummerer-type reactions,5 and to prepare vinyl anions6) Physical Data: bp 110 °C/1.0 mmHg; 79–80 °C/0.04 mmHg; (R)-(+)-enantiomer: [α]26D +421° (c 0.15, EtOH). Solubility: sol most moderately polar to polar solvents. Preparative Method: (R)-(+)-p-tolyl vinyl sulfoxide is usually prepared via Andersen sulfinylation,7 beginning with (1R,2S,5R,SS)-(−)-menthyl p-toluenesulfinate, which is prepared as follows.8 A two-neck round-bottom flask, equipped with N2 inlet and magnetic stirring bar, is purged with N2 and then charged with 2.75 equiv of Thionyl Chloride. Under a N2 atmosphere, anhydrous sodium p-toluenesulfinate (see Sodium Benzenesulfinate) is added in portions over 1 h, with the addition of dry benzene as needed to facilitate stirring. After stirring for an additional 1.75 h, additional benzene is added and the mixture transferred to a one-neck round-bottom flask. The volatiles are removed by rotary evaporation, with occasional addition of benzene to ensure that all excess SOCl2 has been removed. The flask, containing crude p-toluenesulfinyl chloride, is equipped with a magnetic stirring bar and constant pressure addition funnel. Ether is added and the resultant suspension is cooled to 0 °C. 1.0 Equiv of (1R,2S,5R)-(−)-menthol in excess pyridine is added over a few minutes, and the reaction is stirred overnight. The product is isolated by addition of ice, extraction, washing of the ether layer with 20% aq HCl, and drying (Na2SO4 + K2CO3). Filtration and concentration by rotary evaporation, followed by crystallization from acetone at −20 °C, affords the ester, mp 105–106 °C, [α]D25 −199.4° (c 1.5, acetone). The ester is converted into (R)-(+)-p-tolyl vinyl sulfoxide in the following manner.9 A two-neck round-bottom flask equipped with constant pressure addition funnel, N2 inlet, and magnetic stirring bar is charged with a suspension of 1.3 equiv of (1R,2S,5R,SS)-(−)-menthyl p-toluenesulfinate in dry THF under N2. Cooling to −78 °C is followed by dropwise addition of 1.0 equiv of Vinylmagnesium Bromide in THF and slow warming to room temperature. Satd aq NH4Cl is added; separation is followed by drying (MgSO4) and concentration by rotary evaporation. Distillation under reduced pressure affords the sulfoxide. The corresponding (S)-(+)-p-tolyl vinyl sulfoxide may be prepared from (1S,2R,5S,RS)-(+)-menthyl p-toluenesulfinate. Alternative preparations via TBAF-mediated desilylation of (R)-(+)-p-tolyl α-trimethylsilylvinyl sulfone10 and hydrogenation of (R)-(+)-ethynyl p-tolyl sulfoxide2 are possible. Although rarely used, (RS)-p-tolyl vinyl sulfoxide should be available in a manner analogous to the preparation of (RS)-phenyl vinyl sulfoxide, by oxidation of the sulfide.11 Handling, Storage, and Precautions: sulfoxides are known to undergo thermal racemization.12