[2525-42-0] · C9H8O2S · Phenylsulfonylallene · (MW 180.24)
Alternate Name: 1-phenylsulfonyl-1,2-propadiene.
Physical Data: mp 44-45 °C (isopropyl ether-light petroleum).6
Analysis of Reagent Purity: can be distinguished from the isomeric alkynes by NMR: allene resonances, 5.46 (d, 2H) and 6.30 (t, 1H) in CDCl3.
Preparative Methods: prepared via oxidation of the corresponding thioether2 or by reaction of Propargyl Alcohol with phenylsulfinyl chloride in high yield.6 In the latter case the product arises from a 2,3-sigmatropic shift of the intermediate sulfinate ester. It can be also obtained from the reaction of the isomeric alkynes, 1- and 3-phenylsulfonylpropyne, with bases.7a
The title reagent (or the compound tolylsulfonylallene [17534-15-5]) is thermodynamically more stable than either of the two related isomeric alkynes (1) or (2) and thus can be obtained from these under a variety of reaction conditions, among which the most practical appears to be elution over basic alumina.2 When exposed to a basic reagent, (1) or (2) first isomerize to the corresponding allene.
Sulfur, oxygen, and nitrogen nucleophiles react at the central carbon atom of the allene to produce an alkene that is normally the kinetic product, which isomerizes to the thermodynamically more stable alkene, the trans-2-substituted 1-phenylsulfonylpropene (eq 1).2
(-)-Ephedrine reacts with the allene to give the enamine that cyclizes stereospecifically to the 1,3-oxazolidine (eq 2).7 Allyl oxides8a and the conjugated bases of hydroxylamines8b give adducts that undergo carbanionic Claisen rearrangement. In the example shown in eq 3, this sequence has been used for the synthesis of 2-substituted indoles. Carbon nucleophiles also give addition products at the central carbon atom that eventually isomerize to the more stable isomer. A case in point is the addition of dimethyl malonate (eq 4).9 The reaction with bis(phenylsulfonyl)methane in the presence of traces of base affords the expected Michael-type adduct in only 6%. The major product in this case was the isomeric adduct shown (eq 4). It was demonstrated that the latter product forms via an initial addition of the phenylsulfonyl anion present in traces in the reaction mixture.9
The intermediate a-sulfonyl carbanion generated in the Michael addition can be trapped with electrophilic alkenes, eventually leading to functionalized five-membered rings (eq 5).9 The reaction with enamines of cyclic ketones (acyclic enamines give [2 + 2] cycloadducts)5 produces adducts that can be transformed in a simple way into dienes (eq 6).10
The title reagent, because of its markedly lowered LUMO energy level compared with allene, displays rich cycloaddition chemistry and can be used as the synthetic equivalent of the parent allene after reductive desulfonylation of the adducts.
Diels-Alder reactivity occurs with reactive cyclic dienes such as cyclopentadienes or activated electron-rich acyclic 1,3-dienes.3 With cyclic dienes it furnishes mixtures of endo and exo adducts of which the endo form usually predominates. The adducts can be subjected to desulfonylation to mimic the addition of allene3b or transformed with bases into a-sulfonyl anions that can be reacted with electrophiles to give a variety of products that can eventually lead to trienes (eq 7).3b The adduct with furan, upon hydrogenation, provides a simple entry into substituted cyclohexenols (eq 8).3c Higher order cycloadditions occur with tropone and derivatives (eq 9).3b
[2 + 2] Cycloadditions with enamines of acyclic ketones and some alkenes give cyclobutanes.5 In the latter case, the cycloaddition was proved to be catalyzed by Ethylaluminum Dichloride (eq 10).5b This is one of the rare cases in which a sulfonyl-substituted alkene has been shown to be activated by a Lewis acid.
The large field of dipolar cycloadditions include reaction with diazoalkanes,11 nitrones,12 and nitrile oxides13 among others.14 The reactive site is always the double bond a to the phenylsulfonyl group,4 at variance with the lower oxidized molecules (i.e. phenylthio- and phenylsulfinylallene). Among the several examples of dipolar cycloadditions of the title reagent, those worth mentioning are the preparation of benzazepines with N-phenyl-substituted nitrones (eq 11)12b,c and of a,b-unsaturated carbonyl compounds with alkyl-substituted ones (eq 11).12d
Electrophilic addition of bromine or iodine to phenylsulfonylallene gives 2,3-dihalo-1-phenylsulfonyl-1-propenes, which display a rich chemistry.15a,b An example is reported (eq 12).15a Similarly useful are the adducts of thiophenol, PhSO2CH2C(SPh)=CH2,15c and phenylsulfinic acid, PhSO2CH2C(SO2Ph)=CH2.15d
Ottorino De Lucchi & Fabrizio Fabris
Università di Venezia, Italy