Novel Behavior of Thiiranium Radical Cation Intermediates. Reactions of Dimethyl Disulfide with Alkenes in the Presence of Pd(OAc)₂

Abstract: Reaction of dimethyl disulfide (1) with cyclohexene (2a) in AcOH in the presence of Pd(OAc)₂ yields trans-1-acetoxy-2-methylcyclohexane (3a). The equivalent reactions with hex-1-ene (2b) and 2-methylpent-1-ene (2c) or 1-methylcyclohex-1-ene (2d) preferentially give anti-Markovnikov and Markovnikov adducts 4 and 3, respectively, by acetoxymethylthiolation of the alkene. The Markovnikov regioselectivity 3b/4b for the reaction with 2b is higher than that for the reaction using AgOAc instead of Pd(OAc)₂, which proceeds via a thiiranium ion. Addition of a polar solvent (MeCN or MeNO₂) to the reactions with 2b or 2c using Pd(OAc)₂ abnormally decreases the Markovnikov regioselectivity. The total yield of 3 and 4 increases with an increased concentration of AcOH. Compounds 3 and 4 are also formed and the reactions in MeCN or MeNO₂ not containing AcOH. A solution of Pd(OAc)₂ in 1 exhibits λ_max 380 nm (log ε 3.6) assigned to the absorption of a relatively stable sulfonium salt. These indicate that the reactions using Pd(OAc)₂ proceed by S_N2 ringopening of a new type of thiiranium radical cations paired with −OAc via the sulfonium salts. The insensitivity of the 3/4 ratios to the reaction time at 25-60°C in the reactions with 2c-d shows the ring-opening to be controlled kinetically, but the increased ratio with reaction time at 116°C in the reaction with 2b suggests that the ring-opening is thermodynamically governed. The reaction product with 2d also undergoes a skeletal rearrangement to a thietanium radical cation to give 1-acetoxymethyl-2-methylthiocyclohexane.