Challenging the Bredt's rule in an acid catalyzed cationic cyclization to get bicyclo[3.3.1]nonane derivatives.

Vinyl carbocations are long known in chemistry but remain less explored than common sp²-hybridized carbocations, and their synthetic applications are limited. To expand understanding of cation chemistry and strained molecules, we designed a cationic cyclization reaction to transform simple alkynol derivatives into vinyl cations strategically positioning an aromatic ring. Unexpectedly, in the presence of 1,4-cyclohexadiene (as hydride donor) and hexafluoroisopropanol (HFIP), the reaction proceeds through a formal reductive Friedel-Crafts pathway, affording bicyclo[3.3.1]nonane derivatives. While this outcome suggests intermediates that appear to violate Bredt's rule, mechanistic and computational studies indicate HFIP plays a central role in an alternative pathway. Notably, computations reveal that unreported anti-Bredt bicyclo[3.3.1]non-1(9)-ene frameworks are unexpectedly stable, thereby opening new possibilities for the development of synthetic strategies to access them. These findings not only broaden vinyl cation reactivity but also highlight new opportunities for synthesizing three-dimensional scaffolds as potential naphthalene isosteres, while reinforcing fundamental textbook concepts.