Cody Ross Pitts

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.

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.

Direct, Catalytic Monofluorination of sp3 C–H Bonds: A Radical-Based Mechanism with Ionic Selectivity

Recently, our group unveiled a system in which an unusual interplay between copper(I) and Selectfluor effects mild, catalytic sp(3) C-H fluorination. Herein, we report a detailed reaction mechanism based on exhaustive EPR, (19)F NMR, UV-vis, electrochemical, kinetic, synthetic, and computational studies that, to our surprise, was revealed to be a radical chain mechanism in which copper acts as an initiator. Furthermore, we offer an explanation for the notable but curious preference for monofluorination by ascribing an ionic character to the transition state.

Triethylborane-Initiated Radical Chain Fluorination: A Synthetic Method Derived from Mechanistic Insight

We offer a mild, metal-free sp(3) C-H fluorination alternative using Selectfluor and a substoichiometric amount of triethylborane--an established radical initiator in the presence of O2. This radical-chain-based synthetic method is particularly noteworthy as an offspring of the insight gained from a mechanistic study of copper-promoted aliphatic fluorination, constructively turning O2 from an enemy to an ally. Furthermore, BEt3/O2 is a preferred initiator in industrial processes, as it is economical, is low in toxicity, and lends way to easier workup.

Multiple Enone-Directed Reactivity Modes Lead to the Selective Photochemical Fluorination of Polycyclic Terpenoid Derivatives

In the realm of aliphatic fluorination, the problem of reactivity has been very successfully addressed in recent years. In contrast, the associated problem of selectivity, that is, directing fluorination to specific sites in complex molecules, remains a great, fundamental challenge. In this report, we show that the enone functional group, upon photoexcitation, provides a solution. Based solely on orientation of the oxygen atom, site-selective photochemical fluorination is achieved on steroids and bioactive polycycles with up to 65 different sp3 C-H bonds. We have also found that γ-, β-, homoallylic, and allylic fluorination are all possible and predictable through the theoretical modes reported herein. Lastly, we present a preliminary mechanistic hypothesis characterized by intramolecular hydrogen atom transfer, radical fluorination, and ultimate restoration of the enone. In all, these results provide a leap forward in the design of selective fluorination of complex substrates that should be relevant to drug discovery, where fluorine plays a prominent role.

Intermolecular Aliphatic C–F···H–C Interaction in the Presence of “Stronger” Hydrogen Bond Acceptors: Crystallographic, Computational, and IR Studies

An unprecedented intermolecular aliphatic C-F···H-C interaction was observed in the X-ray crystal structure of a fluorinated triterpenoid. Despite the notion of fluorine being a poor acceptor, computational and IR studies revealed this interaction to be a weak to moderate hydrogen bond with a C-H stretch vibration frequency blue-shifted by 14 cm-1 and d(F-H) = 2.13 Å. In addition, the aliphatic C-F bond is the preferred acceptor in the presence of multiple, traditionally stronger oxygen-based hydrogen bond acceptors.

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 (19 F, 1 H, 13 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. As the culminating experiment, a Saunders isotopic perturbation test confirmed the symmetric structure. Congruent with the trend in other types of onium ions, the calculated charge at fluorine moves in a more positive (less negative) direction from the neutral. It is this important trend that explains in part the extraordinary historical difficulty in making theoretical predictions of fluoronium ions come true in solution, and why it takes fluorine captured in a cage to produce, finally, a stable ion and complete the historical arc of the organic halonium ion story.

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.

Catalyzed and Promoted Aliphatic Fluorination

In the last six years, the direct functionalization of aliphatic C-H (and C-C) bonds through user-friendly, radical-based fluorination reactions has emerged as an exciting research area in fluorine chemistry. Considering the historical narratives about the challenges of developing practical radical fluorination in organic frameworks, notable advancements in controlling both reactivity and selectivity have been achieved during this time. As one of the participants in the field, herein, we a provide brief account of research efforts in our laboratory from the initial discovery of radical monofluorination on unactivated C-H bonds in 2012 to more useful strategies to install fluorine on biologically relevant molecules through directed fluorination methods. In addition, accompanying mechanistic studies that have helped guide reaction design are highlighted in context.

Sensitized Aliphatic Fluorination Directed by Terpenoidal Enones: A “Visible Light” Approach

In our continued effort to address the challenges of selective sp3 C-H fluorination on complex molecules, we report a sensitized aliphatic fluorination directed by terpenoidal enones using catalytic benzil and visible light (white LEDs). This sensitized approach is mild, simple to set up, and an economical alternative to our previous protocol based on direct excitation using UV light in a specialized apparatus. Additionally, the amenability of this protocol to photochemical flow conditions and preliminary evidence for electron-transfer processes are highlighted.