An aromatic C-F bond

Sadly then, the first attempt to retain a C — F bond in an organic fluoride, whilst a functional group was replaced, failed. A reaction far more significant in organo-fluorine chemistry than the planned one had been hidden by the misinterpretation however. The first recorded nucleophilic displacement of fluorine from an aromatic C —F bond had occurred a process to become of vast importance. 

The design of transition metal complexes capable of C—F bond activation for the functionalization of fluorocarbons has attracted attention recently. It has been known for several years that oxidative addition of an aromatic C—F bond takes place at tungsten(O) to yield stable tungsten(II) metallacycles, the cleaved carbon and fluorine atoms both finishing up bound to the metal centre (Eqn. (2)) [34-36]. 

Deprotonation of Tp"W(CO)2( 2-H2C=C(CH2R)) (R = H, Pr, Ph) at the P -sits results in the formation of anionic -allene species [Tp W(CO)2( ] -H2C=C=CHR)] , which react in turn with Mel to form Tp W(CO)2( -H2C=C(CHRMe))." Furthermore, the characteristic chemistry of dioxotungsten-vinyl complexes has been described, and sequential C-F oxidative addition (see Oxidative Addition) and migratory insertion of electron-deficient aUcynes at a tungsten center gives rise to novel ] -vinyl complexes (29) in which an aromatic C-F bond has been replaced by a C-C bond (equation 9). ... 

A photochemical route can also be used to effect the oxidative addition of a carbon-fluorine bond to rhodium(I). Such a conversion has been achieved by the photolysis of (>/ -cp )Rh(C2H4)(PMe3) in the presence of hexafluorobenzene, where the initial step involves displacement of C2H4 to give a / -complexed CeFe, followed by the photochemical oxidative addition of an aromatic C-F bond (Scheme 7.6). A similar tj -complexed aromatic moiety has been observed in the pulsed laser flash photolysis of Cr(CO)6 in benzene ... 

Aromatic fluorine bonds can be activated or deactivated by substituents on the ring. Experimental results show that the aromatic C-F bond is activated for reduction if there is an electron-withdrawing group (e. g., ester and carbonitrile groups) or electron-releasing groups (c.g., hydroxy and amino groups) ortho or para to the fluorine atom. Substituents in meta positions usually decrease the reactivity. 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

A nucleophilic mechanism can be applied in reductions with complex hydrides of highly fluori-nated aliphatic and alicyclic fluoroalkenes with electron-deficient C = C bonds the hydride anion adds as a strong nucleophilic agent to the more electrophilic carbon atom the intermediate anion can then lose a fluoride ion either from the original C = C bond, or from the allylic position finishing an SN2 displacement of the fluorine. Thus, the reductions of vinylic C-F bonds with hydrides proceed by a nucleophilic addition-elimination mechanism. Displacement of fluorine in highly fluorinated aromatic compounds proceeds by the same mechanism ... 

Aromatic fluorination chemistry has a remarkably long history, and the first successful synthesis of aryl C-F bonds was reported in 1870 [22], Significant developments in the area in the early part of the 20th century included the discovery of Balz-Schiemann reaction [23,24] involving diazotization of an aromatic amine in the presence of tetrafluoroboric acid and the reaction scheme is shown in Fig. 4. The above reaction produces large quantities of waste (such as NaBF4,... 

With the exception of a few special cases of mixed halogen compounds, the ease of reduction of carbon-halogen bonds follows the expected order, I>Br>CI>F, and becomes more facile as the number of halogen atoms on a given carbon atom increases. Accessible halfwave potentials range from-2.23 V (see) (C—Cl) to -0.3 V (see) (Br2C—Br)15. The isolated C—F bond cannot be reduced electrochemically below the solvent cut-off limit. However, when the C—F functionality is adjacent to an aromatic or carbonyl moiety, or is geminal, reduction often does occur. 

As is known, the greater the number of fluorine atoms in the organic molecule, the higher the positive charge on carbon atoms this is due to the difference in electronegativity between C and F. This effect, in turn, enhances the reactivity of fluorine atoms attached at the C=C or C-F bond in an aromatic ring. As a result, intramolecular nucleophilic cyclizations... 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

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