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.
Modern pentafluorosulfanyl (SF5) chemistry has an Achilles heel: synthetic accessibility. Herein, we present the first approach to aryl‐SF4Cl compounds (key intermediates in state‐of‐the‐art aryl‐SF5 synthesis) that overcomes the reliance on hazardous fluorinating reagents and/or gas reagents (e.g. Cl2) by employing easy‐to‐handle trichloroisocyanuric acid, potassium fluoride, and catalytic amounts of acid. These simple, mild conditions allow direct access to aryl‐SF4Cl intermediates that either have not been or cannot be demonstrated using previous methods. Furthermore, the same approach provides access to aryl‐SF3 and aryl‐SeF3 compounds, which extend the applications of this chemistry beyond arene SF5‐functionalization, and demonstrate its ability to address a more general oxidative fluorination problem.
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.
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 sp 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.
We have discovered a highly regioselective aminofluorination of cyclopropanes. Remarkably, four unique sets of conditions-two photochemical, two purely chemical-generated the same aminofluorinated adducts in good to excellent yields. The multiple, diverse ways in which the reaction could be initiated provided valuable clues that led to the proposal of a "unifying" chain propagation mechanism beyond initiation, tied by a common intermediate. In all, the proposed mechanism herein is substantiated by product distribution studies, kinetic analyses, LFERs, Rehm-Weller estimations of ΔGET, competition experiments, KIEs, fluorescence data, and DFT calculations. From a more physical standpoint, transient-absorption experiments have allowed direct spectroscopic observation of radical ion intermediates (previously only postulated or probed indirectly in photochemical fluorination systems) and, consequently, have provided kinetic support for chain propagation. Lastly, calculations suggest that solvent may play an important role in the cyclopropane ring-opening step.
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.
The TeF5 group is significantly underexplored as a highly fluorinated substituent on an organic framework, despite it being a larger congener of the acclaimed SF5 group. In fact, only one aryl‐TeF5 compound (phenyl‐TeF5) has been reported to date, synthesized using XeF2. Our recently developed mild TCICA/KF approach to oxidative fluorination provides an affordable and scalable alternative to XeF2. Using this method, we report a scope of extensively characterized aryl‐TeF5 compounds, along with the first SC‐XRD data on this compound class. The methodology was also extended to the synthesis and structural study of heretofore unknown aryl‐TeF4CF3 compounds. Additionally, preliminary reactivity studies unveiled some inconsistencies with previous literature regarding phenyl‐TeF5. Although our studies conclude that the arene‐based TeF5 (and TeF4CF3) group is not quite as robust as the SF5 group, we find that the TeF5 group is more stable than previously thought, thus opening a door to explore new applications of this motif.
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