Perfluorination complexes

Table 17 Bond Lengths (A) for Perfluorinated Complexes of the Form...

Carbenium ions can also be generated in situ from, for example, Lewis acids and alkyl esters or halides. This involves reversible ion formation as discussed in detail in the next section. Alternatively, carbenium ions can be generated by reaction of alkyl halides with silver salts with complex anions such as AgSbF6 and AgBF4. This method is useful for generating perfluorinated complex anions because most alkyl fluorides are not ionized by Lewis acids. 

Figure 2.17 ORTEP view of the [Er(L )3(OP(C6F5)3)2] molecule [23c]. (Reprinted with permission from A. Monguzzi, R. Tubino, E. Meinardi et al, Novel Er + perfluorinated complexes for broadband sensitized near infrared emission, Chemistry of Materials, 21, 128-135, 2009. 2009 American Chemical Society.)...

The MAO-activated complexes 188-191 have been also tested in ethylene/propylene co-polymerization.1139 The perfluorinated complexes 188 and 189 resulted in somewhat higher activities and molecular masses than the corresponding non-fluorinated complexes 190 and 191 (188 4/w = 64 000 189 / / w 193 000 vs. 190 Mw = 4000 191 Mw = 9000). Moreover, the fluorinated complexes 188 and 189 incorporate propylene better than the non-fluorinated complexes 190 and 191 ((188 P = 12.9 mol% 189 P = 9.5mol% vs. 190 P = 6.0mol%, 191 P = 7.2mol%). This finding has been correlated to the higher electrophilicity of the metal in the fluorinated complexes.1... 

This reaction has often reached explosive proportions in the laboratory. Several methods were devised for controlling it between 1940 and 1965. For fluorination of hydrocarbons of low (1—6 carbon atoms) molecular weight at room temperature or below by these methods, yields as high as 80% of perfluorinated products were reported together with partially fluorinated species (9—11). However, fluorination reactions in that eta involving elemental fluorine with complex hydrocarbons at elevated temperatures led to appreciable cleavage of the carbon—carbon bonds and the yields invariably were only a few percent. 

Fluorination of aliphatic ethers at gentle conditions with cobalt trifluoride or potassium tetrafluorocobaltate do not give perfluorinated products and cause only negligible cleavage of the ether bond. Complex mixtures are formed from ethyl methyl ether and from diethyl ether [9] (equations 16 and 17)... 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Nafion-H (144), a perfluorinated resin-sulfonic acid, is an efficient Bronsted-acid catalyst which has two advantages it requires only catalytic amounts since it forms reversible complexes, and it avoids the destruction and separation of the catalyst upon completion of the reaction [94], Thus in the presence of Nafion-H, 1,4-benzoquinone and isoprene give the Diels-Alder adduct in 80% yield at 25 °C, and 1,3-cyclohexadiene reacts with acrolein at 25 °C affording 88 % of cycloadduct after 40 h, while the uncatalyzed reactions give very low yields after boiling for 1 h or at 100 °C for 3.5 h respectively [95], Other examples are given in Table 4.24. In the acid-catalyzed reactions that use highly reactive dienes such as isoprene and 2,3-dimethylbutadiene, polymerization of alkenes usually occurs with Nafion-H, no polymerization was observed. 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

A biomimetic oxidation with perfluorinated porphyrin complexes [(F20TPP) FeCl] showed high catalytic activity with secondary alcohols with over 97% yield in all cases [144]. Furthermore, this catalyst is able to oxidize a broad range of alcohols under mild conditions with wCPBA as terminal oxidant. Here, an a-hydroxyalkyl radical species is proposed as central intermediate. 

Actually, a similar approach was used in studying the oxidative addition of methane to an iridium complex. Hydrocarbon solvents would have reacted faster than methane with the photochemically produced unsaturated iridium species, therefore J.K. Hoyano et al chose perfluorinated hexane as being an inert solvent. The elevated pressure was necessary in order to increase the concentration of the methane in the solution sufficiently to shift equilibrium (15) to the right /20/. 

As a result of their low redox potentials [173], bis(phthalocyaninato) lanthanide complexes are often inadvertently reduced or oxidized, and they are also very sensitive to acids and bases. In order to solve these problems, Veciana et al. achieved certain success on designing a series of novel compounds with characteristics that would give them improved redox stability. Electroactive ligands based on phthalo-cyaninato tetra dicarboximide [175] or perfluorinated phthalocyanine [176] were used to assemble the double-decker lanthanide complexes, with the effect of stabilizing the negative charge of the anionic state of the compounds, which resulted in a strong shift of 0.7 V of their first oxidation potentials. 

Figure 6.15. Perfluorinated =-diketonate complex used for a variety of oxidation reactions (M = Ni)[55] or...

Oxidation reactions are not limited to those that occur at a carbon centre. The perfluorinated Ni(F-acac)2-benzene-CgFi7Br system described above was also active for the oxidation of sulfides to sulfoxides and sulfones [28], A sacrificial aldehyde is required as co-reductant, but the reaction may be tuned by changing the quantity of this aldehyde. If 1.6 equivalents of aldehyde are used, the sulfoxide is obtained, whereas higher quantities (5 equivalents) lead to sulfones. Fluorous-soluble transition metal porphyrin complexes also catalyse the oxidation of sulfides in the presence of oxygen and 2,2-dimethylpropanal [29],... 

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

A complex reaction between perfluoro 2-methylpent-2-ene and carboxylic acids in the presence of potassium carbonate and Aliquat produces the acid fluoride, as the major product, with variable amounts of the 3-acyloxy perfluorinated alkene [37]. The procedure has little value for the synthesis of either compound. 

Membrane research is a rather diverse field, exploiting perfluorinated iono-mers, hydrocarbon and aromatic polymers, and acid-base polymer complexes. Polyether and polyketo polymers with statistically sulfonated phenylene groups such as sPEK, sPEEK, and sPEEKK or polymers on the basis of benzimidazole have been tested as well. Recent reviews on membrane synthesis and experimental characterization can be found in the literature. ... 

An important step towards a possible application of these compounds in technical syntheses of chemicals was the successful demonstration of a ther-momorphic reversible immobihzation of perfluorinated catalysts on teflon or other solid fluorous matrices, which might be practiced in industrial low-scale applications, e.g., of pharmaceutical intermediates in the case of quantitative recovery of the organometalHc compound. The facile separation due to their physicochemical behavior and the constant good performance in coupHng reactions of the involved perfluorinated pincer complex makes this system attractive for further investigations. 

As mentioned in the previous section, there are good reasons to search for new reaction conditions for Heck and related reactions, which permit catalyst recovery, the use of less toxic solvents, and simpler product recovery. The use of liquid or supercritical (SC) CO2 addresses all of these issues [171]. Until recently, however, the use of supercritical COj had been limited to organometallic Pd complexes functionalized with perfluorinated ligands [172-174], due to the limited solubility of metal colloids in CO2, and often required the use of water as a co-solvent [175]. The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings, but without the need for toxic ligands or solvents. 

Palladium(ll) complexes of imidazolin-2-ylidenes bearing perfluorinated side chains 46 were prepared and suggested to be useful in flourous biphasic catalysis as well as in supercritical C02. ° However, catalytic applications have not been reported yet. 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

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