Quest for a Symmetric [C–F–C]+ Fluoronium Ion in Solution: A Winding Path to Ultimate Success

ConspectusIn this Account, we chronicle our tortuous but ultimately fruitful quest to synthesize a [C–F–C]+ fluoronium ion in solution, thus providing the last piece of the organic halonium ion puzzle. Inspiration for the project can be traced all the way back to the graduate career of the corresponding author, wherein the analogy between a [C–H–C]+ “hydrido” bridge and a hypothetical [C–F–C]+ bridge was first noted. The earliest attempt to construct a bicyclo[5.3.3]tridecane-based fluoronium ion (based on the analogous hydrido bridged cation) proved to be synthetically difficult. A subsequent attempt involving a 1,8-substituted naphthalene ring was theoretically naive in retrospect, and it resulted in a classical benzylic carbocation instead. A biphenyl-based substrate, although computationally sound, proved to be kinetically untenable. At last, after some tweaking (including a dead-end detour into a fluoraadamantane skeleton), we finally achieved success with a highly rigid, semicage precursor based on ...