Vinylic cations from solvolysis. 30. Bromide exchange as a reactivity-selectivity probe in vinylic solvolysis

Rearrangement studies with 14C. XLVI. Isotopic scrambling studies on the acetolysis of trianisylvinyl bromide labeled with 13C and with 14C

“…Such linear behavior thus suggests that the free acetate ion may be chiefly responsible for the observed decrease in scrambling with increasing added NaOAc concentration; that is, under the conditions employed in the present work, the nucleophile involved in the product-forming capture reaction may chiefly be the acetate ion rather than HOAc or the Na+OAc-ion-pair. This finding thus constitutes a further confirmation of previous conclusions that in the solvolysis of certain vinylic substrates including 1-Br in HOAc-NaOAc, the capturing nucleophile is the acetate ion, as discussed recently by Rappoport and co-workers (18).…”

“…It has been pointed out that while ion-pairs from the acetolysis of triarylvinyl substrates may be involved in the return process, their reactions are intramolecular and they are unreactive toward external nucleophiles (18,20). It has also been suggested that while the presence of.added NaOAc can decrease the extent of scrambling that takes place in the dissociated triarylvinyl cation, the scrambling that occurs in the ion-pair stage may be insensitive to added NaOAc (7,21,22).…”

Substituent Effects on the Photogeneration and Selectivity of Triaryl Vinyl Cations

“…The thermal nucleophilic substitution of 1dBr occurs via the S N 2 (C + ) mechanism, , a rarely observed variation on the S N 1 mechanism in which the cation−nucleophile reaction is rate determining under solvolytic conditions. The activation data (Δ H ⧧ is larger for the reaction of the stabilized tris( p -methoxyphenyl) vinyl cation 1d + with the weakly nucleophilic solvent acetic acid than for the ionic combination reactions of 1d + with Br − and OAc − ) show that the same happens photochemically.…”

“…These ions are also thermally accessible, albeit only at high reaction temperatures . The nature of photogenerated triaryl vinyl cations has been shown to be the same as that of the ions involved in thermal nucleophilic substitution reactions, i.e., free (not ion-paired), “cold” (thermally relaxed, i.e., solvated) vinyl cations: the extent of β-aryl rearrangement as well as the selectivity toward different nucleophiles are the same in photochemically and thermally induced reactions. Also, triaryl vinyl cations have been observed in flash photolysis studies by both optical absorption and electrical conductivity measurements. , …”

“…This procedure probes the selectivity of the photochemically formed triaryl vinyl cations 1 + toward selected pairs of nucleophiles, including the (labeled) common halide ion. The method was initially introduced to deal with the common ion rate depression which often hampers thermal kinetic studies of triaryl vinyl cation formation. , It has become a powerful supplement to the more common mechanistic tools to investigate the mechanistic pathway from substrate to nucleophilic photosubstitution product. It also provides insight into the selectivity of carbocations of the type 1 + toward different kinds of nucleophiles.…”