A closer look at the reactivity between N-heterocyclic carbenes and fluoroalkenes

What's Next for First Row Fluorometallacycles?

Hydrocarbon-derived metallacycles have been identified as key intermediates in a host of catalyzed transformations of unsaturated organic substrates. In contrast, our knowledge of analogous reactivity of fluorometallacycles is underdeveloped and largely confined to first row metals. Our summary of recent advances aims to inform young investigators of the exciting challenges offered by this pursuit.

Regioselective formation of fluorinated metallacycles from fluoroalkenes and an electron-rich Ni(0) difluorocarbene

The reactivity of an electron-rich Ni-difluorocarbene complex, Ni=CF2(dppe)[P(OMe)3] (1), with a variety of fluorinated alkenes was investigated (dppe = 1,2-bis(diphenylphosphino)ethane). Reactions of 1 with perfluoro(methyl vinyl ether) and chlorotrifluoroethylene (CTFE) give regiospecific formation of metallacyclobutanes, in which the carbene C attacks the most electron-rich carbon of the fluoroalkene. Further reaction of the CTFE-derived metallacycle in the presence of tetrahydrofuran affords a single isomer of NiCl(σ-CF2CF=CFH) (dppe), 8, proposed to be formed by Ni-mediated Cα-Cl activation followed by hydride abstraction and loss of HF. Although 1 also undergoes a cyclization reaction with trifluoroethylene (TrFE), instability of the presumed nickelacyclobutane affords the C3 fluoroalkene, F2C=CH(CF3), and subsequent formation of isomeric metallacyclopentanes from two additional TrFEs. Alternatively, reaction of 1 with hexafluoropropene forms an unexpected Ni–CF3 σ-perfluoroallyl complex.

Activation of C–F, Si–F, and S–F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues

Reactions involving C–F, Si–F, and S–F bond cleavage with N-heterocyclic carbenes and isoelectronic species are reviewed. Most examples involve activation of aromatic C–F bond via an SNAr pathway and nucleophilic substitution of fluorine in electron-deficient olefins. The mechanism of the C–F bond activation depends on the reaction partners and the reaction can proceed via addition–elimination, oxidative addition (concerted or stepwise) or metathesis. The adducts formed upon substitution find applications in organic synthesis, as ligands and as stable radical precursors, but in most cases, their full potential remains unexplored.

1 Introduction

1.1 The C–F Bond

1.2 C–F Bond Activation: A Short Summary

1.3 C–F Bond Activation: A Special Case of SNAr

1.4 N-Heterocyclic Carbenes (NHCs)

1.5 The Purpose of this Article

2 C–F bond Activation in Acyl Fluorides

3 Activation of Vinylic C–F Bonds

4 Activation of Aromatic C–F Bonds

5 X–F Bond Activation (X = S or Si)

6 C–F Bond Activation by Main Group Compounds Isoelectronic with NHCs

7 Conclusions and Outlook

Selective Copper Complex-Catalyzed Hydrodefluorination of Fluoroalkenes and Allyl Fluorides: A Tale of Two Mechanisms

The transition to more economically friendly small-chain fluorinated groups is leading to a resurgence in the synthesis and reactivity of fluoroalkenes. One versatile method to obtain a variety of commercially relevant hydrofluoroalkenes involves the catalytic hydrodefluorination (HDF) of fluoroalkenes using silanes. In this work it is shown that copper hydride complexes of tertiary phosphorus ligands (L) can be tuned to achieve selective multiple HDF of fluoroalkenes. In one example, HDF of the hexafluoropropene dimer affords a single isomer of heptafluoro-2-methylpentene in which five fluorines have been selectively replaced with hydrogens. DFT computational studies suggest a distinct HDF mechanisms for L₂CuH (bidentate or bulky monodentate phosphines) and L₃CuH (small cone angle monodentate phosphines) catalysts, allowing for stereocontrol of the HDF of trifluoroethylene.