Fluorobenzene complex

For the 1-1 ammoniajl-chloro,2-fluorobenzene complex, the SN reaction leads quantitatively to the 2-fluoroanilinium (Riehn et al. 1992) ... 

The tricarbonylchromium entity promotes ipso-Smiles rearrangement of the O-phenyl derivatives of ephedrine and pseudoephedrine to the N-phenyl derivatives [32]. Indeed, treatment of the sodium alkoxide derived from (1 R,2S)-ephedrine with fluorobenzene complex... 

A synthesis of a-arylamino acids has been reported using Schiff-base anions 61 derived from amino esters, which are arylated with fluorobenzene complex la to give a-aryl imino esters 62 in 48-76 % yield (R = H, Me, CH2Ph) [49]. Using the same procedure, these authors reported a convenient synthesis of optically pure a-aryl amino acid precursors 64 by enantioselective substitution of fluorobenzene complex la using the Schiff base of L-alanine methyl ester with f 1 R.2R.5 R -2-hydroxy-3-pinanone 63 in the presence of LDA (Scheme 30) [50]. 

Chromium tricarbonyl-complexed aryl fluorides undergo nucleophilic substitution reactions. The substitution is not a straightforward SNAr mechanism as can be seen using, for example, 4-methoxy-l-fluorobenzene complex (71). Reaction of (71) with acetylide (72) gives a 1 2 mixture of the 1,2 and 1,3 products (73) and (74) (Scheme 115). Other leaving groups include halogens, alkoxides, and amines. Indazoles can be prepared by reaction with hydrazine followed by acidic deprotection-decomplexation (Scheme 116). 

Intermolecular Nucleophilic Substitution with Heteroatom Nucleophiles. A patent issued in 1965 claims substitution for fluoride on fluorobenzene-Cr(CO)3 in dimethyl sulfoxide (DMSO) by a long list of nucleophiles including alkoxides (from simple alcohols, cholesterol, ethylene glycol, pinacol, and dihydroxyacetone), carboxylates, amines, and carbanions (from triphenyhnethane, indene, cyclohexanone, acetone, cyclopentadiene, phenylacetylene, acetic acid, and propiolic acid). In the reaction of methoxide with halobenzene-Cr(CO)3, the fluorobenzene complex is ca. 2000 times more reactive than the chlorobenzene complex. The difference is taken as evidence for a rate-limiting attack on the arene ligand followed by fast loss of halide the concentration of the cyclohexadienyl anion complex does not build up. In the reaction of fluorobenzene-Cr(CO)3 with amine nucleophiles, the coordinated aniline product appears rapidly at 25 °C, and a carefiil mechanistic study suggests that the loss of halide is now rate limiting. 

The most reactive of the common arene-metal complexes has been the least developed with regard to nucleophilic substitution for halide. It is only an improved, gentler method of complexation that allowed isolation of the fluorobenzene complex with Mn(CO)3+. The chlorobenzene analog is easily prepared and undergoes substitution with alkoxy. 

However, the frontier orbital picture based on the free arene does not account for nearly exclusive meta selectivity in addition to anisole-Cr(CO)3 the LUMO for anisole shows essentially the same pattern as for toluene. With a strong resonance electron donor, the traditional electronic picture (deactivation of the ortho and para positions) is sufficient to account for the observed meta selectivity. In this case the balance of charge control and orbital control is pushed toward charge control by strong polarization. The same argument applies to the aniline and fluorobenzene complexes. 

A most dramatic difference is the case of fluoride [4,129]. In a competition between carboxamide and fluoro substituents arranged ortho on a benzene ligand, LDA rapidly removes the proton ortho to F with complete selectivity [4]. In another example, amide dominates over a m-methoxide but with low selectivity [24]. The collective results suggest that conformational effects are signiflcant (best conformation of the Cr(CO)3 tripod), and that inductive effects are relatively more important than speciflc coordination of the base to a ring ortho substituent (compared to the uncomplexed arene). The high inductive effect of F is critical in directing deprotonation of the fluorobenzene complexes. 

Amunugama, R. and Rodgers, M.T. (2002) Influence of substituents on cation- r interactions. 2. Absolute binding energies of alkali metal cation-fluorobenzene complexes determined by threshold collision-induced dissociation and theoretical studies. J. Phys. Chem. A, 106, 9092-9103. 

The hexafluorophosphates are among the most stable halogen complexes known. The highly symmetrical PF ion is stable to boiling aqueous alkaH and is decomposed only slowly in acidic solutions at ambient temperatures. It does hydrolyze rapidly at elevated temperatures in acid (85). The stabiHty of the PF 3 ion can be compared to the isoelectronic SF. The hexafluorophosphates can be decomposed yielding PF although in many cases only at temperatures where the PF reacts with the metal containers. Benzenediazonium hexafluorophosphate can be decomposed to PF, N2, and fluorobenzene at 120°C (86) and is a convenient source for laboratory amounts of PF as well as a frequently used catalyst. 

Partially fluorinated cryptands were synthesized from l,3-f>fs(bromomethyl)-2-fluorobenzene and diaza-18-crown-6 or benzodiaza-18-crown-6 in good yields these ligands formed very stable Ag(I) complexes <96CB1211>. 

Halogen bonding is also observed with electron-poor bromides, and so attempts were made to form complexes between stilbazole and fcromopenta-fluorobenzene. We were never able to find evidence that such a complex formed and indeed, heating crystallised samples only reproduced the thermal behaviour of the stilbazoles themselves. Thus, any halogen bonding is supposed weak (there was no observable colour change in the stilbazole) and unable to sustain the complex at temperatures much above ambient. 

The same paper also described an analogous complex of 1,4-dibromotetra-fluorobenzene, 17-8, whose molecular structure, as obtained by single crystal methods, is shown as Fig. 17. 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

The main interaction of the complex of equilibrium 31 is probably a hydrogen bonding interaction between nitrofluorobenzenes and some amines. These complexes are more stable when the nitro groups are in position 2 of fluorobenzene than when they are in position 4. A reasonable explanation of this trend is the interaction of the ortho nitro group with the amine shown in 110, in which a second hydrogen bond (between the amino and the nitro groups) enhances the interaction. 

The -complexes formed between chromium(O), vanadium(O) or other transition metals, and mono- or poly-fluorobenzene show extreme sensitivity to heat and are... 

Acidity is evidently the only factor directing lithiation when coordination to the heteroatom is electronically or geometrically impossible. For example, fluorobenzene is slowly lithiated by BuLi-TMEDA at —50°C, despite the unlikelihood of a strong F-Li complex forming (in contrast with anisole, no PhF-BuLi complex is discernible by NMR ). 

Other fluoroaromatic compounds such as fluorobenzene, trifluorotoluene and their derivatives may be elaborated to more complex aromatic compounds by directed metallation reactions (in itself, the subject of another large review [322-324]). Wakselman and co-workers described the conversion of 3-tri-fluoromethylphenol into 2-(trifluoromethyl)-l,3-cyclopentadienone, an intermediate they used to synthesise angularly trifluoromethylated steroid analogues [325]. The reaction, which involved an interesting ring contraction reaction, occurred with rather low efficiency (Eq. 122) [326]. 

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