L-fluoro-4-nitrobenzene

In sharp contrast to the KOCH3/CH3OH system, the KOt-Bu/t-BuOH combination is quite sensitive to the presence of cation-complexing agents, as was shown by Del Cima et al. (1973). Whereas on addition of ([20] + [21]), the rate of displacement of fluorine by reaction with KOt-Bu increased by only a factor of 3 in the case of l-fluoro-2-nitrobenzene, it increased by a factor of 1.4 x 103 for l-fluoro-4-nitrobenzene (Table 23). Consequently, the... 

For liquid-phase reactions it is known that they may be enhanced by applying higher pressures if the activation volumes are strongly negative, for example, with Diels-Alder reactions. At pressures up to 600 bar (6 x 10 Pa), Benito-Lopez et al. used fiber-based on-line UV-vis spectroscopy to monitor the nucleophilic aromatic substitution of l-fluoro-4-nitrobenzene with a ten-fold excess of pyrrolidine. ... 

The nucleophilic displacement of halogens in aromatic compounds by fluorine is aided by utilizing an appropriate catalyst. Polymer-supported aminopyridinium salts have been found to be versatile catalysts for the synthesis of aryl fluorides. The advantage of the catalyst is that it can be recycled and used again. l-Chloro-4-nitrobenzene (3) is converted to l-fluoro-4-nitrobenzene (4) in 71 % isolated yield using this method. The catalyst used has the structure 5.91... 

Tetrabutylphosphonium hydrogen difluoride [Bu4PF (HF)] and dihydrogen trifluoride [Bu4PF (HF)2] have been shown to be expedient reagents for nucleophilic fluorination of aromatic substrates containing a chlorine or bromine atom or a nitro group under mild conditions in non polar solvents. Thus, for example l-chloro-4-nitrobenzene (28) is converted to l-fluoro-4-nitrobenzene (29) in 90% yield.219... 

Decomposition of the diazonium trifluorotris(perfluoroalkyl)phosphates proceeds mildly, without formation of tars (since no strong Lewis acid evolves), and with high yields of the substituted fluorobenzenes. For example, l-fluoro-4-nitrobenzene is obtained in a far better yield (82%) than from 4-nitrobenzenediazonium hexafluorophosphate or tetrafluoroborate [cf. formation of 2 (R = 4-N02)]. The coproduced difluorotris(perfluoroalkyl)-A5-phosphanes are not soluble in the resulting fluorobenzene derivatives and, because of their high density, form a lower layer which is readily separated. Thus, these difluoro-25-phosphanes can be regenerated quantitatively and used repeatedly. 

Fluorine atoms in electron-deficient benzene derivatives, such as l-fluoro-2-nitroben-zene,149-150 l-fluoro-4-nitrobenzene.151 153 2-fluorobenzaldchyde,154"156 2-fluoroaceto-phenone,155156 4-fluoroacetophenonc,157 2-fluorobenzophenone.156 2- and 4-fluorobenzoni-trile,158 and 4-fluorophenyl trifluoromethyl sulfone,152,159 are easily substituted by N-nucleophiles. Even fluorine in electron-deficient sandwich complexes, such as tricarbonyl(t/6-fluoro-benzene)chromium (3), is substituted by amines.160161... 

Several chemical methods have been devised for identifying the N-terminal amino acid. They all take advantage of the fact that the N-terminal amino group is free and can act as a nucleophile. The a-amino groups of aU the other amino acids are part of amide linkages, are not free, and are much less nucleophilic. Sanger s method for N-terminal residue analysis involves treating a peptide with l-fluoro-4-nitrobenzene, which is very reactive toward nucleophilic aromatic substitution. 

Reaction of the B chain peptide with l-fluoro-4-nitrobenzene established that phenylalanine is the N terminus. 

Like pyridones, oxy-diazines are readily deprotonated under mild conditions, to give ambident anions which can be alkylated conveniently by phase-transfer methods, alkylation usually occurring at nitrogen. " M-Arylations of uracils also proceed in this way with, for example, l-fluoro-4-nitrobenzene."" 3-Pyridazinones alkylate cleanly on N-2 under phase-transfer conditions," but the regiochemistry of uracil alkylation is sometimes difficult to control (see also below). Uracils are sufficiently acidic to take part in Mitsunobu reactions." ... 

Polyimides containing polyoxyethylene units as flexibilizers were prepared (22) by reacting l-fluoro-4-nitrobenzene with ethylene oxide oligomers (n = 1 to 4 below) and then converting the nitro compound by means of hydrogenation into a diamine and these into the polyimides by reaction with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride. 

To a solution of 16.22 g (100 mmol) of 1,3,5-tris-hydroxybenzene (Aldrich Chemical Co.) and 44.59 g (316 mmol) of l-fluoro-4-nitrobenzene (Aldrich Chemical Co.) in a mixture of 100 mL V,V-methylacetamide (DMAc) and 100 mL toluene, 44.2 g (320 mmol) of potassium carbonate was added. The mixture was heated to gentle reflux with a Dean-Stark apparatus and mechanically stirred for 24 hours. At the end of the reaction, a mixture of 100 mL methanol and 100 mL of water was added with stirring. The reaction mbcture was chilled and the crystallized product filtered, washed with 1000 mL of water, followed by 300 mL of methanol. The product was redissolved in ethyl acetate, decolorized... 

As shown in Table 6, benzimidazole, pyrrole, pyrazole, and piperidine are also coupled with 1-chloro -nitrobenzene and l-fluoro-4-nitrobenzene to give the corresponding A-arylated products in excellent yields. 

As can be seen from Table 14, other nitrogen-containing heterocycles like benzimidazole, pyrrole, and pyrazole gave the corresponding iV-arylated products with l-chloro-4-nitrobenzene and l-fluoro-4-nitrobenzene in excellent yields. 

As can be seen from Table 19, other nitrogen-containing heterocycles like pyrrole, pyrazole, indole, and piperidine gave the corresponding A-arylated products with l-chloro-4-nitrobenzene and l-fluoro-4-nitrobenzene in excellent yields. Lower reaction rates but comparable yields were observed with resin supported 1 compared to its homogeneous counterpart. The resin supported 1 can be recovered by simple filtration and reused for three cycles with consistent activity. 

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