Fluorine atom catalyst

During hydrogenation of vinylic chlorides, both saturation of the double bond and hydrogenolysis of the C-Cl bond take place.433 35 Also, selective hydrogenolysis of chlorine occurs in the presence of fluorine atoms on a Pd/C catalyst (Scheme 4.125).436... 

The heterobimetallic asymmetric catalyst, Sm-Li-(/ )-BINOL, catalyzes the nitro-aldol reaction of ot,ot-difluoroaldehydes with nitromethane in a good enantioselective manner, as shown in Eq. 3.78. In general, catalytic asymmetric syntheses of fluorine containing compounds have been rather difficult. The S configuration of the nitro-aldol adduct of Eq. 3.78 shows that the nitronate reacts preferentially on the Si face of aldehydes in the presence of (R)-LLB. In general, (R)-LLB causes attack on the Re face. Thus, enantiotopic face selection for a,a-difluoroaldehydes is opposite to that for nonfluorinated aldehydes. The stereoselectivity for a,a-difluoroaldehydes is identical to that of (3-alkoxyaldehydes, as shown in Scheme 3.19, suggesting that the fluorine atoms at the a-position have a great influence on enantioface selection. 

A very impressive example of the synthetic utility of this chemistry is the one-pot enantioselective double G-H activation reaction of 86 to generate chiral spiran 87 (Equation (73)).172 In this case, the phthalimide catalyst Rh2(enantiotopically selective aromatic C-H insertions of diazo ketoesters (Equation (74)).216 Moreover, dirhodium(n) tetrakisIA-tetrafluorophthaloyl- )-/ /-leucinate], Rh2(hydrogen atoms of the parent dirhodium(n) complex are substituted by fluorine atoms, dramatically enhances the reactivity and enantioselectivity (up to 97% ee). Catalysis... 

The fluorination of carbon nanotubes was first reported by Margrave and is traditionally performed by using elemental fluorine in the presence of small amounts of HF which serves as catalyst [24]. The loading turns out to be very high, with one fluorine atom every two carbon atoms. 

Other Addition Reactions.—At variance with previous reports, the palladium-catalysed reduction of 9a-fluoro-ll/3-hydroxy-A -and-A -corticosteroids proceeded stereospecifically to give the 5/8-isomers. The 9a-fluorine atom appears to be responsible for an increased folding of ring A towards the a-face, thus exposing the -face to the catalyst. 

Coates and co-workers reported on bis(phenoxy-ketimine) Ti complexes, which can also be catalysts for living ethylene polymerization at temperatures between 0 and 50 These complexes do not have fluorine atoms as the... 

Intramolecular migration of fluorine and chlorine atoms is described in Section 5. The rearrangement of fluorine and chlorine atoms likewise can occur intermolecularly between a number of chlorofluorocarbon molecules. This disproportionation (dismutation) takes place catalyti-cally or thermally. With aluminum trichloride as a catalyst, for example, enrichment of the fluorine atoms takes place in one molecule 1 and enrichment of the chlorine atoms in the other 2. 

As can be seen from Table 3, the product distribution is different due to the catalyst. Chromium(III) oxide leads, in the case of l,l,2-trichloro-l,2,2-trifluorocthanc and 1,2-dichlo-ro-l,l,2.2-tetrafiuoroethane, to products with roughly the same amounts of fluorine atoms attached to both carbons whereas aluminum trifluoridc leads to products with a high fluori-nation degree on one carbon and a low one on the other carbon. Similar results are obtained with an aluminum trifluoride catalyst prepared from aluminum oxide and chlorofluoroethanes.25... 

The reaction involves the replacement of the halogen atoms in nonpolar halide molecules by fluorine atoms. The fluorinating agent is usually resublimed antimony (III) fluoride. If necessary, a catalyst, such as antimony(V) chloride, chlorine, or bromine, may be used. 

Xenon difluoride is able to transfer a fluorine atom to organic molecules without any catalyst, with very reactive organic molecules or at higher temperatures or under photochemical conditions. However, reactions are usually performed in the presence of various types of catalysts, but in some cases a new xenon compound FXeYL is formed (Scheme 1), which can be a source of fluorine atoms, or of a YL or Y group for transfer to an organic molecule. On the other hand, decomposition of the FXeYL molecule results in the desired fluorinated products (FYL, FL or FY). Fluoro-substituted xenon derivatives (FXeYL) can also be further transformed to disubstituted derivatives, which are usually less stable at room temperature, but can also be excellent sources of YL, Y or L groups for transfer to various organic molecules. The next possibility is that xenon difluoride converts an... 

Thus, AgF3 is a more effective fluorine atom source than molecular fluorine, with which xenon does not react at ordinary temperatures and pressures in the absence of catalyst. It is more surprising that even AgFj is able to oxidize xenon since Ag(III) in the electron rich environment of an anion must be less electronegative than the Ag(III) in AgF3 ... 

This result clearly supports the inference that more active catalysts can be prepared by placing highly electronegative fluorine atoms around the active catalyst site, which in turn activates the nucleophilic monomers more easily. 

Thus, AgFj is a more effective fluorine atom source than molecular F2, with which Xe does not interact, in the absence of catalysts, under ordinary pressures and temperatures. It was, however, more surprising to find that even anionic Ag(lII) will oxidize xenon spontaneously according to... 

The anion in the salts A[Nb(OCH(CF3)2)g] (A = Li, Ph3C) has a single negative charge shared by 36 peripheral fluorine atoms. Consequently, it is weakly coordinating and the Li+ salt exhibits useful properties as a Lewis acid catalyst.302 Related salts of [M(OC6F5)6] are also available.303... 

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