Reactivity of Aluminium chlorofluoride (ACF) towards C−F bond activations and C−F bond formations

Der Fokus dieser Dissertation lag in der Untersuchung des Potentials von Aluminium-chlorofluorid (ACF) als Katalysator für die Synthese von fluorierten Verbindungen. Insbesondere die C−F-Aktivierung von verschiedenen polyfluorierten Stoffen wurde untersucht, welches die Effizienz des festen Lewis-Säure-Katalysators für diesen Reaktionstyp zeigte. Das potente Treibhausgas 2-Chlor-1,1,1,2-tetrafluorpropan wurde erfolgreich in das dehydrofluorierte Produkt. Weiterhin wurden Umsetzungen von Pentafluorpropan-Isomeren wie z.B. 1,1,1,3,3-Pentafluorpropan, 1,1,1,2,2-Pentafluorpropan und 1,1,1,2,3-Pentafluorpropan mit ACF als Katalysator untersucht. Es konnte gezeigt werden, dass die Aktivierung der primären CH2F-Gruppe in schnell stattfindet und dabei keine Wasserstoffquelle erfordert. Im Kontrast dazu, wurde für die Aktivierung von CF2-Gruppen eine Wasserstoffquelle wie etwa HSiEt3 benötigt und resultierte in der Bildung eines Produktgemischs. Alternativ wurden Hydrofluorierungsreaktionen von mehreren Substraten durch die Synthese und den Einsatz eines neuen Materials erreicht, welches auf der Immobilisierung von HF auf der Oberfläche von ACF beruht. Dieses HF-ACF wurde unter der Verwendung von vielfältigen Charakterisierungsmethoden umfassend untersucht. Die innere Struktur des Festkörpers, wurden mit MAS-NMR-Spektroskopie, FTIR, Inelastische Neutronenstreuung, XRD und Thermoanalyse analysiert Dadurch konnte gezeigt werden, dass eine geringfügige Reorganisation des bulks zu einer besser geordneten Matrix und die Bildung einer mit der ACF-Oberfläche wechselwirkenden Polyfluorid-Struktur vorliegt. Zur Bestimmung der Oberflächengröße wurde das BET-Modell genutzt und zur Analyse der Porengröße wurde die NLDFT verwendet. Abschließend wurden verschiedene Probeverbindungen an der Oberfläche des HF-ACFs adsorbiert um die Azidität der Oberfläche zu bestimmen und es konnte gezeigt werden, dass eine signifikante Reduktion der Lewis- und Brønsted-Azidität vorliegt.The main focus of this thesis lies in the study of the potential of aluminum chlorofluoride (ACF) as a catalyst for the synthesis of fluorinated compounds. In particular, C−F bond activations of various polyfluorinated compounds were studied, showing the efficiency of this solid Lewis acid catalyst for this type of reaction. The potent greenhouse gas 2-chloro-1,1,1,2-tetrafluoropropane was successfully transformed into the dehydrofluorination product 2-chloro-3,3,3-trifluoropropene under mild conditions. Similarly, transformation of pentafluoropropane isomers, such as 1,1,1,3,3-pentafluoropropane, 1,1,1,2,2- pentafluoropropane and 1,1,1,2,3-pentafluoropropane was also investigated using ACF as a catalyst. It was evidenced that the primary CH2F group present in 1,1,1,2,3-pentafluoropropane was easily activated without the need for a hydrogen source. In contrast, to activate CF2 groups, a hydrogen source such as HSiEt3 was required, generating a variety of products. Alternatively, successful hydrofluorination reactions of several substrates were conducted by synthesizing a new material, based on the loading of hydrogen fluoride (HF) at the surface of ACF. This HF-loaded ACF was deeply studied using a wide range of characterization methods. For the bulk, MAS NMR spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), Inelastic Neutron Scattering (INS), Powder X-Ray Diffraction (P-XRD), and thermoanalysis were performed, revealing a slight reorganization of the bulk towards a better-ordered matrix and the formation of polyfluoride structure interacting with the surface of ACF. The BET model was used for the surface area determination, and the pore size analysis was established using the non local density functional theory (NLDFT). Finally, various probe molecules were adsorbed at the surface of HF-loaded ACF to determine the acidity of the surface, revealing a significantly reduced Lewis and Brønsted acidity

C–F bond functionalisation using main group reagents

Fluorinated compounds have greatly increased our quality of life. They have found application in nearly every industry. Among their many uses they find applications as aerosols, in polymeric materials, as solvents and surfactants whilst they are particularly relied upon for refrigeration purposes. However, the fluorine industry is not currently sustainable. Most organofluorine compounds can be considered ‘single-use’ and the majority are lost into the atmosphere as fluorinated gases such as hydrofluorocarbons. The desirable characteristics of organofluorine compounds are also their detriment. They are particularly inert to decomposition and are therefore persistent in the environment. The emission of fluorocarbons into the atmosphere is a significant contributor to climate change and environmental pollution. The recycling of fluorinated compounds therefore represents a timely challenge to synthetic chemists. Due to the increasing incorporation of fluorine into complex molecules such as pharmaceuticals and agrochemicals, the upgrading of fluorine-dense hydrofluorocarbons and hydrofluoroolefins (HFOs and HFCs) to fluorine containing reactive building blocks is an attractive method to close the fluorine cycle. In this context, we demonstrate methods to selectively activate sp2 and sp3C–F bonds in fluorocarbons using main group compounds. We have developed efficient methods to chemically upgrade industrially relevant HFOs and HFCs to simple-bench stable silicon compounds. Furthermore, we have advanced the understanding of how to activate strong C–F bonds, by interrogating the reaction mechanisms using computational calculations (DFT).Open Acces

Ligand-induced reactivity of β-diketiminate magnesium complexes for regioselective functionalization of fluoroarenes via C-H or C-F bond activations

Using β-diketiminate Mg(II) complexes containing either alkyl, aryl or amide groups, the regioselective functionalization of a wide range of fluoroarenes is accomplished but in uniquely different ways. Overcoming common limitations of traditional s-block bases, kinetically activated [(DippNacnac)Mg(TMP)] (1) deprotonates these molecules at room temperature, trapping sensitive fluoroaryl anions that can then engage in Negishi cross-coupling; whereas [(DippNacnac)Mg(R)THF] (R = nBu, Ph, benzofuryl) have proved to be effective reagents for C–F bond alkylation/arylation via pyridine directed C–F bond cleavage

Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR₃)<sub style="vertical-align: sub;">2</sub>(CO)(H)<sub style="vertical-align: sub;">2</sub>]: predicted scope and regioselectivities

Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F6-nHn (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)(2)(CO)(H)(2)]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-sigma-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F center dot center dot center dot HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F6-nHn substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)(2)(CO)(H)(2)] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.</p

Mechanism of C−F Reductive Elimination from Palladium(IV) Fluorides

The first systematic mechanism study of C−F reductive elimination from a transition metal complex is described. C−F bond formation from three different Pd(IV) fluoride complexes was mechanistically evaluated. The experimental data suggest that reductive elimination occurs from cationic Pd(IV) fluoride complexes via a dissociative mechanism. The ancillary pyridyl-sulfonamide ligand plays a crucial role for C−F reductive elimination, likely due to a κ^3 coordination mode, in which an oxygen atom of the sulfonyl group coordinates to Pd. The pyridyl-sulfonamide can support Pd(IV) and has the appropriate geometry and electronic structure to induce reductive elimination

Stabilizing edge fluorination in graphene nanoribbons

The on-surface synthesis of edge-functionalized graphene nanoribbons (GNRs) is challenged by the stability of the functional groups throughout the thermal reaction steps of the synthetic pathway. Edge fluorination is a particularly critical case in which the interaction with the catalytic substrate and intermediate products can induce the complete cleavage of the otherwise strong C-F bonds before the formation of the GNR. Here, we demonstrate how a rational design of the precursor can stabilize the functional group, enabling the synthesis of edge-fluorinated GNRs. The survival of the functionalization is demonstrated by tracking the structural and chemical transformations occurring at each reaction step with complementary X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements. In contrast to previous attempts, we find that the C-F bond survives the cyclodehydrogenation of the intermediate polymers, leaving a thermal window where GNRs withhold more than 80% of the fluorine atoms. We attribute this enhanced stability of the C-F bond to the particular structure of our precursor, which prevents the cleavage of the C-F bond by avoiding interaction with the residual hydrogen originated in the cyclodehydrogenation. This structural protection of the linking bond could be implemented in the synthesis of other sp2-functionalized GNRs

Strengthening of the C-F Bond in Fumaryl Fluoride with Superacids

The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D;M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4H3F2O2](+)[MF6](-) (M=As, Sb) and [C4H2X2F2O2](2+)([MF6](-))(2) (X=H, D;M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low-temperature vibrational spectroscopy. Low-temperature NMR spectroscopy and single-crystal X-ray structure analyses were carried out for [C4H3F2O2](+)[SbF6](-) as well as for [C4H4F2O2](2+)([MF6](-))(2) (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4H4F2O2 . 2 HF](2+) and [C4H3F2O2 . HF](+) at the B3LYP/aug-cc-pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge-related properties of the diprotonated species. The C-F bond lengths in the protonated dication are considerably reduced on account of the +R effect

Formal Insertion of Alkenes Into C(sp3)−F Bonds Mediated by Fluorine-Hydrogen Bonding

C−F Insertion reactions represent an attractive approach to prepare valuable fluorinated compounds. The high strength of C−F bonds and the low reactivity of the fluoride released upon C−F bond cleavage, however, mean that examples of such processes are extremely scarce in the literature. Here we report a reaction system that overcomes these challenges using hydrogen bond donors that both activate C−F bonds and allow for downstream reactions with fluoride. In the presence of hexafluoroisopropanol, benzyl and propargyl fluorides undergo efficient formal C−F bond insertion across α-fluorinated styrenes. This process, which does not require any additional fluorinating reagent, occurs under mild conditions and delivers products featuring the gem-difluoro motif, which is attracting increasing interest in medicinal chemistry. Moreover, readily available organic bromides can be engaged directly in a one-pot process that avoids the isolation of organic fluorides