CF Species Research Articles

Palladium-catalyzed difluorocarbene transfer enables access to enantioenriched chiral spirooxindoles

AbstractWe disclose herein an unprecedented Pd-catalyzed difluorocarbene transfer reaction, which assembles a series of structurally interesting chiral spiro ketones with generally over 90% ee. Commercially available BrCF2CO2K serves as the difluorocarbene precursor, which is harnessed as a user-friendly and safe carbonyl source in this transformation. Preliminary mechanistic studies exclude the formation of free CO in the reaction process, and importantly, we also find that BrCF2CO2K outcompete gaseous CO and several common CO surrogates in this asymmetric process. The reaction mechanism, including the in-situ progressive release of the difluorocarbene, the rapid migratory insertion of ArPd(II) = CF2 species, and subsequent defluorination hydrolysis by water to introduce the carbonyl group, accounts for the overall high efficiency and uniqueness. This work clearly showcases the advantage and potential of the difluorocarbene in synthesis and supplies a mechanistically distinct route for asymmetric carbonylative cyclization reactions.

Recent Advances in the Synthesis and Transformation of Carbamoyl Fluorides, Fluoroformates, and Their Analogues.

Carbamoyl fluorides, fluoroformates, and their analogues are a class of important compounds and have been evidenced as versatile building blocks for the preparation of useful molecules in organic chemistry. While major achievements were made in the synthesis of carbamoyl fluorides, fluoroformates, and their analogues in the last half of 20th century, an increasing number of reports have focused on using O/S/Se=CF2 species or their equivalents as the fluorocarbonylation reagents for the direct construction of these compounds from the parent heteroatom-nucleophiles in recent years. This review mainly summarizes the advances in the synthesis and typical application of carbamoyl fluorides, fluoroformates, and their analogues by the halide exchanges and fluorocarbonylation reactions since 1980.

Use of X-Ray Photoelectron Spectroscopy as an analytical tool for the study of contamination by Teflon in the synthesis of Mo(VI)Cl2O2Bipy/TiO2 catalysts

The preparation of catalysts can involve various sources of contamination, which can seriously affect the quality of the prepared materials. In the present work, a case of fluorine contamination in a set of catalyst samples was studied, in which using the X-Ray Photoelectron Spectroscopy (XPS) technique, it was evidenced by the F 1s signal that this element was present in the form of Teflon, since its binding energy corresponded mainly to the CF2 species. Furthermore, using the C 1s signal, it was also possible to corroborate the presence of the CF2 group, which is associated with the main component of the Teflon carbon chains. The use of this information made it possible to identify that the solvent dehydration procedure (previous step to obtaining the catalysts) could lead to contamination with Teflon since it involved various accessories with Teflon, organic solvents and high temperature; the Teflon tape and the magnetic stirrer being the possible sources of contamination.

Unraveling the Mechanism of Maskless Nanopatterning of Black Silicon by CF4/H2 Plasma Reactive-Ion Etching.

The process of deep texturization of the crystalline silicon surface is intimately related to its promising diverse applications, such as bactericidal surfaces for integrated lab-on-chip devices and absorptive optical layers (black silicon—BSi). Surface structuring by a maskless texturization appeals as a cost-effective approach, which is up-scalable for large-area production. In the case of silicon, it occurs by means of reactive plasma processes (RIE—reactive-ion etching) using fluorocarbon CF4 and H2 as reaction gases, leading to self-assembled cylindrical and pyramidal nanopillars. The mechanism of silicon erosion has been widely studied and described as it is for the masked RIE process. However, the onset of the erosion and the reaction kinetics leading to defined maskless patterning have not been unraveled to date. In this work, we specifically tackle this issue by analyzing the results of three different RIE recipes, specifically designed for the purpose. The mechanism of surface self-nanopatterning is revealed by deeply investigating the physical chemistry of the etching process at the nanoscale and the evolution of surface morphology. We monitored the progress in surface patterning and the composition of the etching plasma at different times during the RIE process. We confirm that nanopattering issues from a net erosion, as contributed by chemical etching, physical sputtering, and by the synergistic plasma effect. We propose a qualitative model to explain the onset, the evolution, and the stopping of the process. As the RIE process is started, a high density of surface defects is initially created at the free silicon surface by energetic ion sputtering. Contextually, a polymeric overlayer is synthesized on the Si surface, as thick as 5 nm on average, and self-aggregates into nanoclusters. The latter phenomenon can be explained by considering that the initial creation of surface defects increases the activation energy for surface diffusion of deposited CF and CF2 species and prevents them from aggregating into a continuous Volmer–Weber polymeric film. The clusterization of the polymer provides the self-masking effect since the beginning, which eventually triggers surface patterning. Once started, the maskless texturing proceeds in analogy with the masked case, that is, by combined chemical etching and ion sputtering, and ceases because of the loss of ion energy. In the case of CF4/H2 RIE processes at 10% of H2 and by supplying 200 W of RF power for 20 min, nanopillars of 200 nm in height and 100 nm in width were formed. We therefore propose that a precise assessment of surface defect formation and density in dependence on the initial RIE process parameters can be the key to open a full control of outcomes of maskless patterning.

Catalytic trifluoromethylation of iodoarenes by use of 2-trifluoromethylated benzimidazoline as trifluoromethylating reagent.

The trifluoromethylation of iodoarenes was accomplished by use of a 2-trifluoromethylbenzimidazoline derivative as the trifluoromethylating reagent and a catalytic amount of Cu(I) in the presence of 2,2'-bipyridyl as the ligand. Through a mechanistic study, we found that the oxidative addition of the iodoarene to the Cu(I)–CF3 species is the rate-determining step.

Thermal Stability of Octadecyltrichlorosilane and Perfluorooctyltriethoxysilane Monolayers on SiO2.

Using in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements, the thermal behavior of octadecyltrichlorosilane (OTS) and 1H, 1H, 2H, and 2H-perfluorooctyltriethoxysilane (PTES) monolayers on SiO2 substrates has been investigated. OTS is thermally stable up to 573 K with vacuum annealing, whereas PTES starts decomposing at a moderate temperature between 373 K and 423 K. Vacuum annealing results in the decomposition of CF3 and CF2 species rather than desorption of the entire PTES molecule. In addition, our UPS results reveal that the work function (WF)of OTS remains the same after annealing; however WF of PTES decreases from ~5.62 eV to ~5.16 eV after annealing at 573 K.

Copper-Mediated Trifluoromethylation of Benzylic Csp3 -H Bonds.

Trifluoromethyl-containing compounds play a significant role in medicinal chemistry, materials and fine chemistry. Although direct C-H trifluoromethylation has been achieved on Csp2 -H bonds, direct conversion of Csp3 -H bonds to Csp3 -CF3 remains challenging. We report herein an efficient protocol for the selective trifluoromethylation of benzylic C-H bonds. This process is mediated by a combination CuIII -CF3 species and persulfate salts. A wide range of methylarenes can be selectively trifluoromethylated at the benzylic positions. A combination of experimental and theoretical mechanistic studies suggests that the reaction involves a radical intermediate and a CuIII -CF3 species as the CF3 transfer reagent.

Atomic layer deposition of aluminum fluoride using Al(CH3)3 and SF6 plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm−3 was found to be relatively close to the bulk value of 3.1 g cm−3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140–2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of <10−4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.

ChemInform Abstract: Fluoroform‐Derived CuCF3 for Trifluoromethylation of Terminal and TMS‐Protected Alkynes.

AbstractFluoroform derived air‐sensitive Cu(I)‐CF3 is oxidized to the much more electrophilic Cu(II)‐CF3 species upon treatment with air and the intermediate is directly employed for the trifluoromethylation process.

ChemInform Abstract: Electrophilic Trifluoromethylation of Carbonyl Compounds and Their Nitrogen Derivatives under Copper Catalysis

AbstractReview: [45 refs.

Fabrication of oleophobic fluorocarbon film by 13.56 MHz CH2F2/Ar plasma chemical vapor deposition

In order to control of the surface properties to create the oleophobic fluorocarbon film, this study reported the influence of low pressure CH2F2/Ar plasma processing on oil repellent thin film growth. Difluoromethane (CH2F2)/argon (Ar) gas mixture was used to generate low pressure plasma with radio frequency power at 13.56MHz for fabrication of oleophobic fluorocarbon coatings. The surface characteristics of the CH2F2/Ar plasma-deposited films were studied by static contact angle measurement (CA), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The effects of CH2F2/Ar plasma on the surface characteristics of the plasma-deposited films were investigated as a function of the CH2F2 content. The fluorocarbon coating prepared under optimized experimental parameters exhibited the glycerol contact angle greater than 140°, confirming the oleophobic property of the coating. XPS revealed that plasma-deposited films obtained contain mainly CH, CF, C―CFx, and CF2 species. In addition, AFM analysis shows that possible ion bombardment from CH2F2/Ar plasma can increase surface roughness. The combination of the low surface energy CFx species and the rough surface morphology successfully form the oleophobic CH2F2/Ar plasma-deposited film.

Electrochemical and Chemical Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

AbstractThe electroreduction of the halofluoromethanes CF3Br, CF2Br2 and CF2BrCl has been studied in high‐pressure stainless steel autoclaves at different cathodes [Pt, steel (V2A, V4A), glassy carbon (GC)] and in various solvent‐supporting electrolyte systems (SSE), e.g. DMF/[Bu4N]Br, NMP/[Bu4N]BF4 etc. The reduction potentials for CF3Br increase from Pt (–1.6 V) < V2A (–1.8 V) < GC (–2.1 V) and are lower for CF2Br2 and CF2BrCl suggesting a reductive cleavage of C‐X bonds as the first step. CF2Br2 and CF2BrCl show a two‐step reduction in accord with the C–X bond energies (C–F > C–Cl > C–Br) and the “Perfluoro‐effect”. The electrolysis of CF3Br in different SSE‐systems with sacrificial zinc or cadmium anodes has been reinvestigated with our experimental set‐up to elucidate the influence of the experimental conditions on the type and ratio of the products. The observed products CF3MBr·42L and (CF3)2M·42L (M = Zn, Cd; L = DMF or AN) are the same as in the previous investigations, but are obtained in different ratios, as a rule caused by a parallel chemical corrosion of the respective anodes. By using aluminium as sacrificial anode no CF3Al compounds are formed. The CF3 species generated by electroreduction of CF3Br react with the solvents via hydrogen abstraction and formation of CF3H. The current yield with respect to the dissolution of the Al anode reaches 120 % indicating a considerable chemical corrosion in addition to the anodic oxidation. This result enabled a one‐pot trifluoromethylation reaction of NMP as organic carbonyl substrate and solvent with CF3Br and aluminium powder (ratio 3 : 2) at higher temperatures (> 70 °C). The complete reaction of CF3Br to give CF3H and 1‐methyl‐2‐trifluoromethyl‐4,5‐dihydropyrrol allowed the isolation of the latter by vacuum condensation and distillation in 45 % yield, rel. to the CF3Br used. Gallium and indium were also applied as sacrificial anodes in combination with CF3Br as substrate. In both cases, anodic current yields of about 280 % indicated an extreme chemical corrosion together with cathodic metal depositions corresponding to the cathodic current yield. These deposits – in contrast to those of Zn and Cd – do not react with CF3Br in Grignard‐type conversions to CF3Ga and CF3In compounds. So, the observed products (CF3)nMBr3–n·L (M = Ga, In; n 1‐3; L = DMF, NMP) are obviously formed by chemical corrosion of the electro‐activated anodes. Finally, electrochemical and chemical trifluoromethylations were successfully carried out, using R3SiCl (R = Me, Vi, Ph), Me3M′Cl (M′ = Ge, Sn) and aluminium anodes or Al‐powder. The products were characterized either after isolation or in the product solutions by NMR‐spectroscopic investigations.

Atomic simulation of SiC etching by energetic SiF3

The authors present results from molecular-dynamics simulations of SiF3 impact on SiC (100) surfaces at normal incidence and over a range of energies of 10, 50, and 150eV. The surface temperatures are set to 300K for all energies and 600K for 150eV. The uptake of Si atoms is sensitive to the incident energy and temperature, while the uptake of F atoms is not very sensitive to the incident energy and temperature. The simulation results show that the etching yield of Si is higher than that of C. After 30 ML (monolayers) fluence, SiF3 does not etch SiC. The F-containing reaction layer is sensitive to the incident energy. The thickness of the reaction layer increases with the incident energy. In the reaction layer, SiF, SiF2, CF, and CF2 species are dominant. In etch products, atomic F etch products are dominant. Si atoms in SiC are mainly sputtered as SiFx (x=1–4). C atoms in SiC are sputtered as larger SixCyFZ species.

Deposition of Super‐Hydrophobic and Oleophobic Fluorocarbon Films in Radio Frequency Glow Discharges

AbstractFluorocarbon films using a monomer, 1H, 1H, 2H‐perfluoro–1‐dodecene were deposited in a continuous radio frequency (RF) glow discharge, the process was carried out in a parallel‐plate RF discharge onto stainless steel reactor in order to produce coating with a water‐and oil–repellent surface. Fourier‐Transform Infrared spectroscopy (FT‐IR) and X‐ray Photoelectron Spectroscopy (XPS) revealed that the films obtained contain mainly perfluoromethylene (CF2) species. Film wettability was tested using water and hydrocarbon liquids for contact angle measurements, furthermore surface energy was also calculated. Oil‐repellency was found to increase as the amount of CF2 species increases in the film structure. Film morphology was studied by Atomic Force Microscopy (AFM), films showing an usual morphology from that typical of Plasma Polymerised Fluorocarbon (PPFC) films. The combination of the low surface energy coating and the surface morphology produces materials which are both water and oil repellency.

Studies of film deposition in fluorocarbon plasmas employing a small gap structure

A small gap structure was designed to examine surface chemistry aspects of film deposition for fluorocarbon (FC) plasmas produced using both inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) systems. The small gap structure provides a completely shadowed region without direct ion bombardment. Neutrals diffuse into this region and form a fluorocarbon layer. The lack of ion bombardment increases the retention of the chemical structure of the FC film precursors. The surface chemistry of FC film deposited in this region is compared with film deposited in the region exposed to direct ion bombardment. For films deposited in the exposed region, x-ray photoelectron spectroscopy analysis shows that CF2 is the dominant chemical bond for pure C4F8 in both ICP and CCP systems. For C4F8∕Ar discharges, C–C bonding is dominant for the polymer formed in the ICP system, whereas CF2 species are dominant for films deposited in the CCP system. In the completely shadowed region, CF2 bonding is dominant fo...

Vacuum beam studies of fluorocarbon radicals and argon ions on Si and SiO2 surfaces

Si and SiO2 were exposed to c-C4F8 with and without Ar+, and to a mixture of characterized CxFy radical and stable species with and without Ar+. The mixture of CxFy radical and stable species was created from c-C4F8 and Ar plasma and included CF, CF2, CF3, and various heavy CxFy species. The neutral fluorocarbon flux to Ar+ flux and the energy of the Ar+ were varied. During the exposure, etch/deposition rates were measured and the flux of CxFy species leaving the surface for various conditions were qualitatively determined. The following were observed: (1) c-C4F8 is an etchant with Ar+ bombardment; (2) the CF, CF2, and CF3 species flux cannot account for the observed mass increase during depositing conditions; (3) CF2 and CF3 species are net products during etching conditions; and (4) the flux of large CxFy species leaving the surface is smaller during etching conditions than for depositing conditions. These observations imply that large CxFy species play a significant role in the surface chemistry.

Deposition of plasma polymerized perfluoromethylene-dominated films showing oil-repellency

Plasma polymerized fluorocarbon films were deposited onto polyethylene (PE) substrates to increase oil-repellency of PE. Depositions were performed using the monomer, 1H,1H,2H-perfluoro-1-dodecene in a parallel-plate, radio frequency (rf) reactor, with variable continuous-wave power ranging from 2 to 160 W. The film deposition rate and morphology were strongly dependent on the applied rf power. Most importantly, the chemical structure of the deposited films was also altered, resulting in changes in contact angles of various liquids and the surface energy. Films deposited at low power were composed mainly of perfluoromethylene (CF2) species (up to 67.2%), as shown by x-ray photoelectron spectroscopy (XPS). With an increase in rf power, CF2 content in the film decreased as further fragmentation of the monomer occurred. For each deposition at varying rf powers, even at powers as low as 2 W, the C=C and C–H bonds in the monomer were dissociated by the plasma and not incorporated into the films, as shown by Fourier transform infrared spectroscopy. Oil-repellency, as shown by increased contact angles of hydrocarbon liquids, was found to increase as the amount of CF2 species increased in the film structure. A low critical surface energy (2.7 mJ/m2) was calculated for the film deposited with only 2 W of rf power. Adhesion of the plasma-polymerized films to the PE was also evaluated and found to be poor for films with a high concentration of CF2 species, where cohesive failure within the film occurred. However, adhesion increased as a function of rf power, where the film structure showed more cross-linking. There was a compromise between producing a film with high oleophobicity (oil-repellency) while maintaining adhesion of the film to PE, as some disruption of the CF2 chains in the films was necessary for cohesion through cross-linking.

Interaction of MSn (MRu, Rh, Pd) dimers with CH2 and CF2: A density functional study

AbstractThe importance of Sn to improve the performance of noble metal‐based catalysts has been extensively discussed in the literature. However, a detailed discussion about the effect of Sn on the electronic structure of the metals has not been reported so far. In this work, density functional theory (DFT) calculations have been performed on the M2 and MSn (MRu, Rh, Pd, Sn) dimers and their interaction with CH2 and CF2 carbenes. The electronic structure, geometry, and harmonic frequencies are reported and compared with the available experimental data and to the previous published high level ab initio and DFT calculations. The calculated values of M2 are in good agreement with the experimental and theoretical results. The exception is the binding energy of the Sn dimer, which can be different from the experimental estimates as much as 16 kcal · mol−1. It has been stressed that this difficulty is related to the atomic energy reference used to estimate the binding energy. Transition metal elements are still a difficult task for DFT methods. Calculations on MSnCH2 and MSnCF2 species with different multiplicities and conformations have been performed. Bonding analyses on the MSn metal dimers showed that carbenes prefer to adsorb in bridge sites favoring a donating/backdonating mechanism of interaction that is enhanced with the presence of the Sn atom. The MSnCH2 binding energy is predicted to be 40% smaller than the MSnCF2 binding energy, following the same trends observed for the first‐row transition‐metal MCH2 and MCF2 complexes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 164–176, 2003

Preparation and Properties of Trifluoromethylated Aryl Derivatives of Germanium, Tin, and Lead. High-Yield Syntheses of Trifluoromethylbenzene

Ligand exchange reactions of Cd(CF3)2·glyme with a number of aryl-containing Pb, Sn, and Ge halides, acetates, and thioethers were examined in THF or CHCl3. With an excess of trifluoromethylating agent and relatively lengthy reaction times, complete displacement of the halides or halide equivalents from the group 14 centers was observed, and the new compounds PbPh(CF3)3, 51%, PbPh2(CF3)2, 61%, PbPh3CF3, 81%, SnPh2(CF3)2, 55%, SnPh3CF3, 80%, and GePh3CF3, 72%, were isolated and characterized. With shorter reaction times and/or with the trifluoromethylating agent as the limiting reactant, partial substitution predominated, and the new species PbPh2(Cl)CF3, 78%, PbPh2(O2CCH3)CF3, 89%, PbPh(O2CCH3)2CF3, 63%, and PbPh(O2CCH3)(CF3)2, 64%, were separated. In addition, two preliminary studies of the reactivity of aryl−group-IVA compounds were carried out. In the first, ligand exchanges of PbPh3CF3 with a representative main group and transition metal complex were probed; Sb(CF3)3, 61%, and Pd(PEt3)2(Cl)CF3, 62%, were generated. In the second, the feasibility of forming aryl−CF3 species in high yields via CuCl-mediated M−Ph cleavages was examined. In these reactions PhCF3 was isolated from M2Ph6 in yields of 95% (M = Pb), 96% (M = Sn), and 58% (M = Ge).

Pulsed plasma-enhanced chemical vapor deposition from CH2F2, C2H2F4, and CHClF2

Pulsed plasma enhanced chemical vapor deposition films have been grown from 1,1,2,2-C2H2F4, CH2F2, and CHClF2. C 1s x-ray photoelectron spectroscopy indicates a prevalence of C¯–CF species in the films from C2H2F4 and CH2F2, whereas CF2 species dominate the films from CHClF2. The CFx species distributions for the films are largely controlled by the competition between CF2-producing and HF elimination reactions in the pulsed plasmas. Dominance by HF elimination produces films with high C¯–CF and CF concentrations (e.g., CH2F2), whereas dominance by CF2-producing reactions leads to films with higher CF2 concentrations (e.g., CHClF2). The % CF3 in the film is lowest for the precursor having the lowest F:H ratio, CH2F2. Little or no hydrogen was detected in the deposited films. Pulsed plasma films from all three precursors gave dielectric constants of 2.4, with loss tangents on the order of 10−2. Dielectric measurements of pulsed plasma films from hexafluoropropylene oxide gave a dielectric constant of 2.0±0.1 with a loss tangent of 0.009.