1 research outputs found
Search for a Symmetrical C–F–C Fluoronium Ion in Solution: Kinetic Isotope Effects, Synthetic Labeling, and Computational, Solvent, and Rate Studies
Recently,
we reported evidence for the generation of a symmetrical fluoronium
ion (a [C–F–C]<sup>+</sup> interaction) in solution
from a cage-like precursor, relying heavily on a single isotopic-labeling
experiment. Paraphrasing the axiom that a strong claim must be met
by as much evidence as possible, we seek to expand upon our initial
findings with comprehensive labeling studies, rate measurements, kinetic
isotope effect (KIE) experiments, synthetic studies, and computations.
We also chronicle the development of the system, our thought process,
and how it evolved from a tantalizing indication of fluoronium ion
assistance in a dibromination reaction to the final, optimized system.
Our experiments show secondary KIE experiments that are fully consistent
with a transition state involving fluorine participation; this is
also confirmed by a significant remote isotope effect. Paired with
DFT calculations, the KIE experiments are indicative of the trapping
of a symmetrical intermediate. Additionally, starting with an epimeric <i>in</i>-triflate precursor that hydrolyzes through a putative
frontside S<sub>N</sub><i>i</i> mechanism involving fluorine
participation, KIE studies indicate that an identical intermediate
is trapped (the fluoronium ion). Studies also show that the rate-determining
step of the fluoronium forming S<sub>N</sub>1 reaction can be changed
on the basis of solvent and additives. We also report the synthesis
of a nonfluorinated control substrate to measure a relative anchimeric
role of the fluorine atom in hydrolysis versus ÎĽ-hydrido bridging.
After extensive testing, we can make the remarkable conclusion that
our system reacts solely through a “tunable” S<sub>N</sub>1 mechanism involving a fluoronium ion intermediate. Alternative
scenarios, such as S<sub>N</sub>2 reactivity, do not occur even under
forced conditions where they should be highly favored