Remarkably Facile Solvolyses of Triflates via Carbocationic Processes in Dimethyl Sulfoxide

Abstract

A number of triflates have been shown to undergo clean pseudo-first-order solvolysis reactions in DMSO-d(6) to give products derived from carbocationic intermediates. Thus, t-BuCH(OTf)CO-t-Bu (5) and t-BuCH(2)OTf (9) react readily in DMSO-d(6) at 25 degrees C to give a rearranged oxosulfonium salts, and subsequent alkene products where methyl migration to the incipient cationic center occurs. t-BuCH(OTf)CO(2)CH(3) (14) gives analogous rearranged products, and 1-methylcyclopropyl triflate (21) gives a ring-opened allylic oxosulfonium salt. These triflates react primarily via k(Delta) pathways. 6-Methylbicyclo[3.1.0]hex-6-yl triflate (23), bicyclo[2.2.1hept-1-yl triflate (24), 1,6-methano[10]annulen-11-yl triflate (25), (CH(3))(2)C(OTf)CO(2)CH(3) (26), and (CH(3))(2)CCN(OTf) (29) all react in DMSO-d(6) to give carbocation-derived products. PhCH(OTf)CF(3) (33) and substituted analogues also react readily in DMSO-d(6), and the Hammett rho(+) value is -3.7. This suggests a "borderline" mechanism where the transition state has substantial charge development. The primary feature of these solvolyses is the high reactivity of all of these triflates in DMSO-d(6). Thus, these triflates are all more reactive in DMSO-d(6) than in HOAc, and for most, rates are faster than in CF(3)CH(2)OH. Triflates 5, 21, 29, and 33 are 10(8)-10(9) times more reactive in DMSO-d(6) than the corresponding mesylates. It is suggested that the decreased need for electrophilic solvation of triflate anion, and the high cation solvating ability of DMSO, are the reasons for the high triflate reactivity in DMSO-d(6).

Mechanistic Aspects of the Bismuth‐Catalysed Oxidation of Epoxides to α‐Diketones

“…We assume that it corresponds to a sulfonium intermediate, formed after the nucleophilic ring opening of the activated epoxide by DMSO, namely trans ‐2‐[(dimethylsulfonio)oxy]cyclohexanolate. The presence of the sulfonium intermediate in the reaction mixture is consistent with previous studies on the acid‐catalysed oxidation of epoxides by DMSO (Figure 2) described by Swern et al,16 and has also been reported in the solvolysis of triflates by DMSO 18…”

Transacylation in Ferrocenoyl-Purines. NMR and Computational Study of the Isomerization Mechanism

“…We were unable to isolate or spectroscopically detect the intermediate I′, but the existence of the oxosulfonium ion was confirmed earlier. 40,41 The similar was observed in DMSOassisted solvolysis of triflates in which the intermediacy of oxosulfonium ion was proposed. 42 By using DFT methods (B3LYP and M06L), we located the structure I′ as a genuine minimum at the corresponding potential energy surface.…”

Carbocation-Forming Reactions in Ionic Liquids

“…Certain triflates were especially prone to give carbocation chemistry in DMSO. 3 We are therefore interested in the ability of novel or unusual solvents to support carbocationic intermediates.…”