Conversion of a (sp3)C–F bond of alkyl fluorides to (sp3)C–X (X = Cl, C, H, O, S, Se, Te, N) bonds using organoaluminium reagents

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

Cross- and Multi-Coupling Reactions Using Monofluoroalkanes

Carbon-fluorine bonds are stable and have demonstrated sluggishness against various chemical manipulations. However, selective transformations of C-F bonds can be achieved by developing appropriate conditions as useful synthetic methods in organic chemistry. This review focuses on C-C bond formation at monofluorinated sp3 -hybridized carbons via C-F bond cleavage, including cross-coupling and multi-component coupling reactions. The C-F bond cleavage mechanisms on the sp3 -hybridized carbon centers can be primarily categorized into three types: Lewis acids promoted F atom elimination to generate carbocation intermediates; nucleophilic substitution with metal or carbon nucleophiles supported by the activation of C-F bonds by coordination of Lewis acids; and the cleavage of C-F bonds via a single electron transfer. The characteristic features of alkyl fluorides, in comparison with other (pseudo)halides as promising electrophilic coupling counterparts, are also discussed.

Insights into the Mechanism of Aluminum-Catalyzed Halodefluorination

Aluminum has been reported to catalyze halodefluorination reactions, where aliphatic fluorine is substituted with a heavier halogen. Although it is known that stoichiometric aluminum halide can perform this reaction, the role of catalytic aluminum halide and organyl alane reagents is not well understood. We investigate the mechanism of the halodefluorination reaction using catalytic aluminum halide and stoichiometric trimethylsilyl halide. We explore the use of B(C₆F₅)₃ as a catalyst to benchmark pathways where aluminum acts either as a Lewis acid catalyst in cooperation with trimethylsilyl halide or as an independent halodefluorination reagent which is subsequently regenerated by trimethylsilyl halide. Computational and experimental results indicate that aluminum acts as an independent halodefluorination reagent and that reactivity trends observed between different halide reagents can be attributed to relative barriers in halide delivery to the organic fragment, which is the rate-limiting step in both the aluminum halide- and B(C₆F₅)₃-catalyzed pathways.

Vessel effects in organic chemical reactions; a century-old, overlooked phenomenon

One of the most intriguing aspects of synthetic chemistry is the interplay of numerous dependent and independent variables en route to achieve a successful, high-yielding chemical transformation. The experienced synthetic chemist will probe many of these variables during reaction development and optimization, which will routinely involve investigation of reaction temperature, solvent, stoichiometry, concentration, time, choice of catalyst, addition sequence or quenching conditions just to name some commonly addressed variables. Remarkably, little attention is typically given to the choice of reaction vessel material as the surface of common laboratory borosilicate glassware is, incorrectly, assumed to be chemically inert. When reviewing the scientific literature, careful consideration of the vessel material is typically only given during the use of well-known glass-etching reagents such as HF, which is typically only handled in HF-resistant, polyfluorinated polymer vessels. However, there are examples of chemical transformations that do not involve such reagents but are still clearly influenced by the choice of reaction vessel material. In the following review, we wish to condense the most significant examples of vessel effects during chemical transformations as well as observations of container-dependent stability of certain molecules. While the primary focus is on synthetic organic chemistry, relevant examples from inorganic chemistry, polymerization reactions, atmospheric chemistry and prebiotic chemistry are also covered.

A Review on the Halodefluorination of Aliphatic Fluorides

Halodefluorination of alkyl fluorides using group 13 metal halides has been known for quite some time (first reported by Newman in 1938) and is often utilized in its crude stoichiometric form to substitute fluorine with heavier halogens. However, recently halodefluorination has undergone many developments. The reaction can be effected with a range of metal halide sources (including s-block, f-block, and p-block metals), and has been developed into a catalytic process. Furthermore, methods for monoselective halodefluorination in polyfluorocarbons have been developed, allowing exchange of only a single fluorine with a heavier halogen. The reaction has also found use in cascade processes, where the final product may not even contain a halide, but where the conversion of fluorine to a more reactive halogen is a pivotal reaction step in the cascade. This review provides a summary of the developments in the reaction from its inception until now.

1 Introduction

2 Stoichiometric Halodefluorination

2.1 Group 13 Halodefluorination Reagents

2.2 Other Metal Halide Mediated Halodefluorination

3 Catalytic Halodefluorination

4 Monoselective Halodefluorination

5 Cascade Reactions Involving Halodefluorination

6 Summary and Outlook

C-F bond activation under transition-metal-free conditions

The unique properties of fluorine-containing organic compounds make fluorine substitution attractive for the development of pharmaceuticals and various specialty materials, which have inspired the evolution of diverse C-F bond activation techniques. Although many advances have been made in functionalizations of activated C-F bonds utilizing transition metal complexes, there are fewer approaches available for nonactivated C-F bonds due to the difficulty in oxidative addition of transition metals to the inert C-F bonds. In this regard, using Lewis acid to abstract the fluoride and light/radical initiator to generate the radical intermediate have emerged as powerful tools for activating those inert C-F bonds. Meanwhile, these transition-metal-free processes are greener, economical, and for the pharmaceutical industry, without heavy metal residues. This review provides an overview of recent C-F bond activations and functionalizations under transition-metal-free conditions. The key mechanisms involved are demonstrated and discussed in detail. Finally, a brief discussion on the existing limitations of this field and our perspective are presented.

Iron-Catalyzed Halogen Exchange of Trifluoromethyl Arenes*

The facile production of ArCF₂ X and ArCX₃ from ArCF₃ using catalytic iron(III)halides is reported, which constitutes the first iron-catalyzed halogen exchange for non-aromatic C-F bonds. Theoretical calculations suggest direct activation of C-F bonds by iron coordination. ArCX₃ and ArCF₂ X products of the reaction are synthetically valuable due to their diversification potential. In particular, chloro- and bromodifluoromethyl arenes (ArCF₂ Cl, ArCF₂ Br respectively) provide access to a myriad of difluoromethyl arene derivatives (ArCF₂ R). To optimize for mono-halogen exchange, a statistical method called Design of Experiments was used. Optimized parameters were successfully applied to electron rich and electron deficient aromatic substrates, and to the late stage diversification of flufenoxuron, a commercial insecticide. These methods are highly practical, being run at convenient temperatures and using inexpensive common reagents.

Selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes from unactivated gem-difluoroalkanes controlled by aluminum-based Lewis acids

The highly selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes via the cleavage of inert C(sp³)–F bonds in unactivated gem-difluoroalkanes using readily available and inexpensive aluminum-based Lewis acids of low toxicity is reported. The selectivity of this reaction can be controlled by modifying the substituents on the central aluminum atom of the promoter. An intramolecular cascade Friedel-Crafts alkylation of unactivated gem-difluorocarbons can be achieved using a stoichiometric amount of AlCl₃. The subsequent synthesis of alkenyl chlorides via F/Cl exchange followed by an elimination can be accomplished using AlEt₂Cl as a fluoride scavenger and halogen source. The defluorinative elimination of acyclic and cyclic gem-difluorocarbons to give monofluoroalkenes can be achieved using AlEt₃.

Magnesium-mediated arylation of amines via C–F bond activation of fluoroarenes

Exploiting the high nucleophilic power of structurally defined β-diketiminate stabilised magnesium amides, a new method for amine arylation via rapid C–F bond activation of fluoroarenes is introduced.

Selective Monodefluorination and Wittig Functionalization of gem-Difluoromethyl Groups to Generate Monofluoroalkenes

Monodefluorination of gem-difluoromethyl groups is achieved using a frustrated Lewis pair (FLP) approach. Triarylphosphines and group 13 Lewis acids were surveyed as FLP components, with the combination of P( o-Tol)₃ and B(C₆F₅)₃ found to provide the best results, although the reaction is feasible with more economical components (PPh₃ and BF₃·OEt₂). The α-fluoroalkylphosphonium products arising from the reaction were of lower activity, in regard to further fluoride abstraction, as compared to difluoride starting materials, leading to highly selective monodefluorination. The activated substrates were subject to Wittig reaction protocols to generate a variety of monofluoroalkenes in moderate to high yields.

Nickel-catalyzed coupling reaction of alkyl halides with aryl Grignard reagents in the presence of 1,3-butadiene: mechanistic studies of four-component coupling and competing cross-coupling reactions

We describe the mechanism, substituent effects, and origins of the selectivity of the nickel-catalyzed four-component coupling reactions of alkyl fluorides, aryl Grignard reagents, and two molecules of 1,3-butadiene that affords a 1,6-octadiene carbon framework bearing alkyl and aryl groups at the 3- and 8-positions, respectively, and the competing cross-coupling reaction. Both the four-component coupling reaction and the cross-coupling reaction are triggered by the formation of anionic nickel complexes, which are generated by the oxidative dimerization of two molecules of 1,3-butadiene on Ni(0) and the subsequent complexation with the aryl Grignard reagents. The C-C bond formation of the alkyl fluorides with the γ-carbon of the anionic nickel complexes leads to the four-component coupling product, whereas the cross-coupling product is yielded via nucleophilic attack of the Ni center toward the alkyl fluorides. These steps are found to be the rate-determining and selectivity-determining steps of the whole catalytic cycle, in which the C-F bond of the alkyl fluorides is activated by the Mg cation rather than a Li or Zn cation. ortho-Substituents of the aryl Grignard reagents suppressed the cross-coupling reaction leading to the selective formation of the four-component products. Such steric effects of the ortho-substituents were clearly demonstrated by crystal structure characterizations of ate complexes and DFT calculations. The electronic effects of the para-substituent of the aryl Grignard reagents on both the selectivity and reaction rates are thoroughly discussed. The present mechanistic study offers new insight into anionic complexes, which are proposed as the key intermediates in catalytic transformations even though detailed mechanisms are not established in many cases, and demonstrates their synthetic utility as promising intermediates for C-C bond forming reactions, providing useful information for developing efficient and straightforward multicomponent reactions.

NHC-catalyzed [4+2] cycloaddition reactions for the synthesis of 3′-spirocyclic oxindoles via a C–F bond cleavage protocol

A chiral NHC-catalyzed cycloaddition of γ-fluoroenals is developed. The nucleophilic γ-carbon generated via C–F bond cleavage undergoes highly enantioselective cycloaddition (up to >99% ee) to isatins and provides 3′-spirocyclic oxindoles in good yields (up to 91%).

Catalytic halodefluorination of aliphatic carbon–fluorine bonds

A variety of halosilanes, in conjunction with aluminum catalysts, convert fluorocarbons into higher halocarbons.

Consecutive C-F bond activation and C-F bond formation of heteroaromatics at rhodium: the peculiar role of FSi(OEt)3

C-F activation of 2,3,5,6-tetrafluoropyridine at [Rh{Si(OEt)3}(PEt3)3] (1) yields [Rh{2-(3,5,6-C5F3HN)}(PEt3)3] (2) and FSi(OEt)3, but in an unprecedented consecutive reaction FSi(OEt)3 acts as a fluoride source to give [Rh(4-C5F4N)(PEt3)3] (4) by regeneration of the C-F bond and C-H activation. Analogous refluorination steps were observed for other 2-pyridyl rhodium complexes. NMR spectroscopic studies revealed a delicate balance between the feasibility for C-F bond formation accompanied by a C-H activation and the occurrence of competing reactions such as hydrodefluorinations induced by the intermediary presence of H2.

Regio- and stereoselective multisubstituted olefin synthesis via hydro/carboalumination of alkynes and subsequent iron-catalysed cross-coupling reaction with alkyl halides

A new synthetic route towards multisubstituted olefins was developed based on the direct cross coupling of alkenyl aluminium reagents, prepared by hydro- and carboalumination, with alkyl halides in the presence of an iron catalyst.