[917-73-7] · CH2O2S · Sulfene · (MW 78.10)
(reactive intermediate used for sulfonations and as a partner in [2 + 2], [3 + 2], [4 + 2], [8 + 2], and 1,3-dipolar cycloaddition reactions)
Solubility: sol CH2Cl2, THF, Et2O, MeCN.
Preparative Methods: prepared in situ by reaction of Methanesulfonyl Chloride and Triethylamine in a solvent such CH2Cl2. The sulfonyl chloride is usually added slowly to a solution of the substrate and Et3N.2 Alternatively, sulfene can be generated by the action of fluoride ion on 1-(trimethylsilyl)methanesulfonyl chloride (see (Trimethylsilyl)methanesulfonyl Chloride-Cesium Fluoride). This method has advantages over the amine method when reagents either have a low reactivity with sulfene or high reactivity with the amine.3
Handling, Storage, and Precautions: unstable at rt. It has been observed by IR after trapping at -196 °C.4 The methanesulfonyl chloride commonly used to generate sulfene is toxic, corrosive, a lachrymator, and moisture sensitive. Use in a fume hood.
Although sulfene is not stable, it is still useful for the preparation of methanesulfonate esters and other sulfonyl compounds. It will react with H2O, alcohols, amines, thiols, sulfonamides, imides, and oximes1a The most common use of sulfene is in the synthesis of mesylate esters of hydroxy compounds. These esters are typically prepared via the amine method (eq 1). These conditions will allow for the preparation of reactive mesylates provided the temperature is held at 0 °C.2
The simultaneous discovery that sulfene will undergo cycloaddition with an enamine was one of the first hints at its existence. The reaction with cyclohexanone enamine produced the four-membered ring sulfone (eq 2).5 The acyclic enamine from propionaldehyde also formed the cyclic sulfone (eq 3).6
There is no observed stereoselectivity in the reaction of sulfene with enamines of 4-t-butylcyclohexanone. This reaction produces a nearly 1:1 mixture of the two diastereomers (eq 4).7
An investigation of asymmetric induction in the sulfene-enamine condensation with an enamine derived from propionaldehyde and (R)-2-methylpyrrolidine was conducted. Following Hofmann elimination, the thiete 1,1-dioxide (eq 5) was obtained in 25% ee. The use of (R)-(+)-methyl-a-phenethylamine resulted in an opposite sense of induction but the product only had a 6% ee. Attempts to induce asymmetry by using an optically active tertiary amine to generate sulfene were unsuccessful.8
Sulfene generated using the amine protocol does not undergo reaction with simple alkenes or dienes. However, when cyclopentadiene was reacted with sulfene prepared using the newly developed fluorodesilylation method, the [4 + 2] cycloadduct was obtained in 64% yield (eq 6).3 The use of the sulfonic anhydride instead of the chloride gave a 75% yield of the cycloadduct.
Recently, an interesting reaction of sulfene with citreamicin h produced the cyclic b-hydroxy sulfonate in addition to forming the mesylate of the hydroxymethyl group (eq 7).9 A similar result was observed earlier in an attempt to mesylate the a-hydroxy ketone (eq 8). Instead of stopping at the simple mesylate, the intermediate reacted further to afford the unsaturated sultone.10
(Trimethylsilyl)methanesulfonyl Chloride-Cesium Fluoride.
Michael J. Taschner
The University of Akron, OH, USA