Abstract
Abstract: Reaction of dimethyl disulfide ( 1 ) with cyclohexene ( 2a ) in AcOH in the presenceof Pd(OAc) 2 yields trans -1-acetoxy-2-methylcyclohexane ( 3a ). The equivalent reactionswith hex-1-ene ( 2b ) and 2-methylpent-1-ene ( 2c ) or 1-methylcyclohex-1-ene ( 2d ) preferen-tially give anti -Markovnikov and Markovnikov adducts 4 and 3 , respectively, by ace-toxymethylthiolation of the alkene. The Markovnikov regioselectivity 3b / 4b for the reactionwith 2b is higher than that for the reaction using AgOAc instead of Pd(OAc) 2, which pro-ceeds via a thiiranium ion. Addition of a polar solvent (MeCN or MeNO 2 ) to the reactionswith 2b or 2c using Pd(OAc) 2 abnormally decreases the Markovnikov regioselectivity. Thetotal 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 MeNO 2 not containing AcOH. A solution ofPd(OAc) 2 in 1 exhibits λ max 380 nm (log e 3.6) assigned to the absorption of a relatively sta-ble sulfonium salt. These indicate that the reactions using Pd(OAc)
Highlights
Thiiranium ion intermediates have been generated by reactions of disulfides with alkenes in the presence of oxidants such as Pb(OAc)4 [1,2,3,4], Mn(OAc)3 [2,3,4], Fe(OAc)3 [2,3,4] or Cu(OAc)2 [5], by reactions of β-thiohalides with AgY [6] and by reactions of thiiranes with RY [6]
The intermediates which have soft counterions such as BF4−, PF6−, SbF6−, SbCl5−, FSO3− and CF3CO2− behave like carbenium ions [1] of a weakly bridged strong electrophile, undergoing a rapid ring-opening by the outer nucleophile at low temperature (−20-0oC) to give the observed predominant Markovnikov regioselectivity; in some cases, the skeletal rearrangement or stereoconversion of the thiiranium ions has been observed
We may infer that the reactions of RSCl with alkenes would generate not a σ-sulfurane but a strongly bridged thiiranium ion pairing tightly with hard Cl− [1], and the ring-opening by the pairing Cl− leads to the anti-Markovnikov regioselectivity without undergoing the stereoconversion and skeletal rearrangement
Summary
Thiiranium ion intermediates have been generated by reactions of disulfides with alkenes in the presence of oxidants such as Pb(OAc)4 [1,2,3,4], Mn(OAc)3 [2,3,4], Fe(OAc)3 [2,3,4] or Cu(OAc)2 [5], by reactions of β-thiohalides with AgY [6] and by reactions of thiiranes with RY [6]. Reactions of dimethyl disulfide with 2-alkyl and 2,2-dialkylalkenes in AcOH in the presence of Pd(OAc)2 lead to the anti-Markovnikov and Markovnikov regioselectivity products, respectively, for the addition to the alkenes.
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