Evidence for a S_N2-type pathway in the exchange of phosphines at a [PhSe]⁺centre

Abstract

A range of thio- and seleno-phosphonium cationic complexes [RE(PR'3)](+)[X](-) (R = Me, Ph; E = S, Se; X = GaCl4, SbF6) have been synthesised and structurally characterised. Reaction of [PhSPPh3][GaCl4] and [PhSePPh3][GaCl4] with P(t)Bu3 results in the ready transfer of the "RS(+)" and "RSe(+)" fragments from PPh3 to the stronger electron donor P(t)Bu3. NMR experiments combined with an Eyring analysis on the corresponding degenerate phosphine exchange reaction allowed the thermodynamic values for the phosphine exchange reaction of the sulfur cation (ΔH(‡) 18.7 ± 12.0 kJ mol(-1); ΔS(‡) -99.3 ± 36.3 J mol(-1) K(-1)) to be compared with the corresponding values (ΔH(‡) 2.4 ± 1.1 kJ mol(-1) and ΔS(‡) -58.1 ± 5.0 J mol(-1) K(-1)) for the [PhSePPh3](+) system. Importantly, the large negative entropy of activation and linear dependence on the rate of exchange are compatible with an SN2-type exchange process. This conclusion is supported by DFT calculations which confirm that the phosphine exchange process occurs via an associative mechanism. The rate of exchange was found to increase from sulfur to selenium and those with aryl substituents underwent exchange faster than those with alkyl substituents.

Rhenium(V) Complexes with Selenolato‐ and Tellurolato‐Substituted Schiff Bases – Released PPh3as a Facile Reductant

“…The phosphine attacks the coordinated diselenide with formation of an intermediate, which may be assigned to {Ph 3 PSe(C 6 H 4 )‐2‐N=CH(C 6 H 4 )‐2‐OH} + Cl – , and the liberation of a {L Se } 2– ligand. The recorded chemical shift of 30 ppm of this potential intermediate is in the range where also the 31 P resonances of the related organoseleno‐phosphonium species {Ph 3 PSePh}Br (37 ppm), {Bu 3 PSeMe}I (50 ppm), {Ph 3 PSePh}[GaCl 4 ] (38 ppm) and {Ph 3 PSeMe}(BF 4 ) (36 ppm) are found. The reduction of disulfides and diselenides with phosphines with formation of phosphonium‐chalcogenolate ion pairs or bis(organochalcogen)phosphoranes has been observed before .…”

Sulfonyl Chlorides as Thiol Surrogates for Carbon–Sulfur Bond Formation: One-Pot Synthesis of Thioethers and Thioesters

“…Triphenylphosphine (PPh 3 ) is a widely used mild reductant to generate the thiols required for carbon–sulfur bond formation . Reduction of sulfonyl chlorides by PPh 3 generates a putative thiophosphonium salt, which yields the free thiol upon aqueous workup (Figure C). ,, We hypothesized it might be possible to intercept the thiophosphonium intermediate in situ with a carboxylic acid or alcohol. The released thiolate could then trap the activated intermediates to generate the corresponding thioester or thioether (Figure D).…”