One-step Oxidative Monofluorination of Electron-Deficient Sulfoxides to Access Highly Lewis Acidic Sulfur(VI) Cations

The strongly oxidizing, powerful electrophilic fluorination reagent [FXe][OTf] is shown to effect direct oxidative monofluorination of sulfoxides. This one-step, chloride promoter-free methodology provides access to so far inaccessible, yet highly desirable strongly Lewis acidic fluorosulfoxonium cations from electron-deficient and/or sterically demanding sulfoxides that are shown to be practically unreactive towards the previously reported XeF₂ /NEt₄ Cl system. Experimental and density functional theory studies have been conducted to assess the Lewis acidities of the prepared sulfur(VI) cations. Preliminary results obtained with chiral sulfoxides provide early insights into the mechanism of these fluorination reactions.

Bifunctional Fluorophosphonium Triflates as Intramolecular Frustrated Lewis Pairs: Reversible CO2 Sequestration and Binding of Carbonyls, Nitriles and Acetylenes

Electrophilic fluorophosphonium triflates bearing pyridyl (3[OTf]) or imidazolyl (4[OTf])-substituents act as intramolecular frustrated Lewis pairs (FLPs) and reversibly form 1 : 1 adducts with CO2 (5+ and 6+ ). An unusual and labile spirocyclic tetrahedral intermediate (72+ ) is observed in CO2 -pressurized (0.5-2.0 bar) solutions of cation 4+ at low temperatures, as demonstrated by variable-temperature NMR studies, which were confirmed crystallographically. In addition, cations 3+ and 4+ actively bind carbonyls, nitriles and acetylenes by 1,3-dipolar cycloaddition, as shown by selected examples.

The formation of cage phosphoranes and their rearrangements in the reactions of substituted 2-(3-oxo-3-phenyl)ethoxybenzo[d]-1,3,2-dioxaphospholes with perfluorodiacetyl

The Kukhtin-Ramirez reaction of 2-(3-oxo-3-phenyl)ethoxy-benzo[d]-1,3,2-dioxaphospholes with perfluorodiacetyl was monitored by NMR methods. To our surprise the initial stage involved a kinetically controlled [4+4]-cycloaddition with the formation of a cage phosphorane containing a 2',5',8',9'-tetraoxa-2λ⁵-phosphaspiro[benzo[d][1,3,2]dioxaphosphole-2,1'-bicyclo[4.2.1]nonan]-3'-ene (compound 5) scaffold. Intermediate 5 then converts to spirophosphorane-4',5'-bis(trifluoromethyl)-2λ⁵-spiro[benzo[d] [1,3,2]dioxaphosphole-2-yl-2,2'-[1,3,2] dioxaphosphole (compound 4). Compound 4 further rearranges into a cage phosphorane derivative containing a [2,5]epoxybenzo[d][1,3,6,2]trioxaphosphocine] (compound 3) backbone.

Electrophilic fluorosulfoxonium cations as hidden Brønsted acid catalysts in (n + 2) annulations of strained cycloalkanes

A fluorosulfoxonium cation and triflic acid were shown to promote (n + 2) annulations of donor–acceptor cyclopropanes or -butanes with 1,2-dipoles, with different activity and selectivity but a presumed similar role as (hidden) Brønsted acids.