Ring-Opening Carbonyl-Olefin Metathesis of Cyclobutenes.

The ring-opening carbonyl-olefin metathesis of cyclobutenes to furnish γ,δ-unsaturated aldehydes-formal Claisen rearrangement products-is reported. The bistrifluoroacetic acid salt of 2,3-diazabicyclo[2.2.

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

In 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.

Spectroscopic Characterization of a [C-F-C] Fluoronium Ion in Solution.

We report the first spectroscopic evidence for a [C-F-C] fluoronium ion in solution. Extensive NMR studies ( F, H, C) characterize a symmetric cage-like species in which fluorine exhibits substantial covalent bonding to each of the two carbon atoms involved in the three-center interaction. Experimental NMR data comport well with calculated values to lend credence to the structural assignment.

Fluorine in a C-F Bond as the Key to Cage Formation.

Cage molecules have long been employed to trap reactive or transient species, as their rigid nature allows them to enforce situations that otherwise would not persist. In this Minireview, we discuss our use of rigid cage structures to investigate the close noncovalent interactions of fluorine with other functional groups and determine how mutual proximity affects both physical properties and reactivity. Unusual covalent interactions of fluorine are also explored: the cage can close to form the first solution-phase C-F-C fluoronium ion.

Through-Space Activation Can Override Substituent Effects in Electrophilic Aromatic Substitution.

Electrophilic aromatic substitution (EAS) represents one of the most important classes of reactions in all of chemistry. One of the "iron laws" of EAS is that an electron-rich aromatic ring will react more rapidly than an electron-poor ring with suitable electrophiles. In this report, we present unique examples of electron-deficient arenes instead undergoing preferential substitution in intramolecular competition with more electron-rich rings.

Positioning a Carbon-Fluorine Bond over the π Cloud of an Aromatic Ring: A Different Type of Arene Activation.

It is known that the fluoro group has only a small effect on the rates of electrophilic aromatic substitutions. Imagine instead a carbon-fluorine (C-F) bond positioned tightly over the π cloud of an aryl ring-such an orthogonal, noncovalent arrangement could instead stabilize a positively charged arene intermediate or transition state, giving rise to novel electrophilic aromatic substitution chemistry. Herein, we report the synthesis and study of molecule 1, containing a rigid C-F⋅⋅⋅Ar interaction that plays a prominent role in both its reaction chemistry and spectroscopy.

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](+) 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.

Synthesis of a Tight Intramolecular OH···Olefin Interaction, Probed by IR, (1)H NMR, and Quantum Chemistry.

We have synthesized a molecule containing a tight hydrogen-bonding interaction between an alcohol and a nonconjugated π-system. The strength of this hydrogen bond results in a large red shift, nearly 189 cm(-1), on the alcohol stretching frequency in the IR spectrum in comparison to a free alcohol control. The interaction is notable in that it possesses a better defined intramolecular hydrogen bond compared to the usual molecules for which it is noted, such as syn-7-norbornenol.

Metal-catalyzed benzylic fluorination as a synthetic equivalent to 1,4-conjugate addition of fluoride.

We explore in detail the iron-catalyzed benzylic fluorination of substrates containing aromatic rings and electron-withdrawing groups positioned β to one another, thus providing direct access to β-fluorinated adducts. This operationally convenient process can be thought of not only as a contribution to the timely problem of benzylic fluorination but also as a functional equivalent to a conjugate addition of fluoride, furnishing products in moderate to good yields and in excellent selectivity.

Iron(II)-catalyzed benzylic fluorination.

Direct C-F functionalization of benzylic sp(3) C-H bonds is a synthetic challenge that has yet to be propitiously overcome. A mild, one-pot synthesis of monofluorinated benzylic substrates is reported with commercially available iron(II) acetylacetonate and Selectfluor in good to excellent yields and selectivity. A convenient route to β-fluorinated products of 3-aryl ketones is also highlighted, providing a synthetic equivalent to the difficult to accomplish conjugate addition of fluoride to α,β-unsaturated ketones.

A polycomponent metal-catalyzed aliphatic, allylic, and benzylic fluorination.

A group effort: Reported is the title reaction using a polycomponent catalytic system involving commercially available Selectfluor, a putative radical precursor N-hydroxyphthalimide, an anionic phase-transfer catalyst (KB(C(6)F(5))(4)), and a copper(I) bis(imine). The catalyst system formed leads to monofluorinated compounds selectively (see example) without the necessity for an excess of the alkane substrate.