From 1,2-Difluorobenzene

This survey has highlighted the application of a broad range of modem synthetic methods for the preparation of derivatives of quinoxaline as an important class of benzoheterocycle. Undoubtedly, among the methods for synthesizing quinoxaline derivatives, the most widely used are those based on the condensation of 1,2-DABs with a-dicarbonyl compounds (Hinsberg-Komer reaction), mostly a-diketones, or their equivalents. 

As can be seen from the data presented in this chapter, there have recendy appeared many original and interesting methods for the synthesis of quinoxalines, which are difficult to obtain or in general are imobtainable using the Hinsberg-Komer reaction. These new methods are based on the reactions of a wide variety of compounds that deserve further mention. 

The Kaufmann method-pyrazinoannulation-uses aniline or its ring-substituted derivatives with l,l,2-trichloro-2-nitroethene, with the formation of quinoxaline-2 (l//)-on -oxide derivatives. 

The Tanimori method involves reactions of 2-haloanilines with a variety of a-amino acids leading to the formation of quinoxaline-2(lfl)-ones. 

The Kalinski method uses the Ugi(4CR) reaction of 2-bromoanilines, ketones/aldehydes, a carboxylic acid and an isocyanide in polar protic solvents, and the Pd-catalyzed intramolecular N-arylamidation with the formation of A -alkyl and A -acyl derivatives of quinoxalin-2-ones. 

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Protonation of dihuorobenzenes yields benzenonium ions. That from 1,2-difluorobenzene (2) shows a temperature-dependent spectrum, indicating... 

XB2u — XAg system of 1,4-difluorobenzene, with a weak overlapping sequence band labelled Bj. (Reproduced, with permission, from Cvitas, T. and Hollas, J. M., Mol. Phys., 18, 793, 1970)... 

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

In this paper we present the relative energies of the isomers of the phenylenediamines, dihydroxybenzenes and difluorobenzenes from ab initio calculations using large basis sets and including correlation corrections at the MP-2 level. These calculations were done at the geometry optimized structures. We also include zero-point energy corrections based on our calculated force fields. 

Figure 11. SNIFTIRS spectra from a Pt electrode in IM HClOi, + 0.5 mM p-difluorobenzene. Modulation from +0.2V to +0.4V.

The 13C NMR spectra of the three isomeric difluorobenzenes are distinctive and interpretable upon inspection, with the 1,2-isomer exhibiting three signals, the 1,3-isomer four, and the 1,4-isomer two. (Figs. 3.23-3.25). The signal observed in each of these 13C spectra at -128.3 ppm derives from the solvent, benzene-d6. 

Wittig and Harle were the first to use tetrahaloarenes as diaryne equivalents.3 From 1,4-dibromo-2,5-difluorobenzene, butyllithium and furan (in THF) they isolated the mono- and bis-adducts shown. 

Xylene (extractive) from p-, m-, o-xylene, ethylbenzene KBaX, KBaY 1,2-Difluorobenzene, 1,3 -difluorobenzene [139, 140]... 

In keeping with the simplicity of its structure, fluconazole can be synthesized in two steps from commercially available starting materials (Scheme 5.3). Metallation of 1-bromo-2,4-difluorobenzene with butyllithium in ether gave the corresponding aryllithium. 

Figure 5 Free energies for attachment to solutes MeSt— methylst5rene. Sty—styrene, Bph— biphenyl, CO2, Pyr—pyrimidine, Tph— triphenylene, Dfb—p-difluorobenzene, Tol— toluene. But— 1,3-butadiene, Pyz—pyrazine in TMS, 2,2,4-trimethylpentane, and -hexane at 298 K and in supercritical ethane at 310 K. (From Refs. 90-99.)...

Dinitro-l,3-difluorohenzene (DNDFB) or 2,4-Dinitro-l-difluorobenzene (DNFFB) (called 4.6-Difluor-1.3-dinitro-benzol in Ger), prisms (from ale), mp 73-75° readily sol in hot ale less sol in cold ale can be prepd by nitrating either 1,3-difluorobenzene or 1-nitr o-2,4-di fluoro benzene with mixed acids (Ref 1) or by other methods (See Refs)... 

First, pellets of the diazonium tetrafluoroborate are spread on a cylindrical nickel roll, internally heated with Dowtherm.202Iftheroll is heated 100 to 150°C above the decomposition temperature, the diazonium tetrafluoroborate is decomposed within 10 to 25 seconds. If the temperature is nearer to the decomposition point, decomposition requires around 30 minutes. The roll speed is adapted to the reaction time. The roll surface is regenerated by a doctor blade which scratches off the residues. In this way, 4-chloro-l,2-difluorobenzene was obtained in 36 % yield from 4-chloro-2-fluorobenzenediazonium tetrafluoroborate at 290-310 C within 10 to 25 seconds. 

Difluorobenzene from [FLUORINE COMPOUNDS, ORGANIC - FLUORINATED AROMATIC COMPOUNDS] (Vol 11)... 

From these data it can be pointed out that for a given size of the clusters the proton affinity of water is smaller than for the other solvents consequently, for fluorobenzene/methanol or para-difluorobenzene/water systems, a proton affinity of 205/215 kcal mol-1 seems to be the limit of the reaction process (it is reached for two molecules of methanol and three molecules of water). 

In closing this section, we note that although the Koopmans picture is a simplification of the ionization dynamics, it provides a very useful zeroth order picture from which to consider the TRPES results. Any potential failure of this independent electron picture can always be experimentally tested directly through variation of the photoionization laser frequency resonance structures should lead to variations in the form of the spectra with electron kinetic energy, although the effect of resonances is more likely to be prominent in PAD measurements, and indeed an observation of a shape resonance in p-difluorobenzene has been reported [153, 154]. 

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, from Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

Carbon-boron clusters ( carboranes ) have been shown to react with Cr-coordinated halo-arenes. For example, reaction of 2 equiv. of LiC2BjoHio(CH3) with para-difluorobenzene-tricarbonylchromium complex in refluxing THF results in the displacement of both fluoride substituents from the arene ring to yield the para-phenylene compound, albeit in just 9 % yield owing to the effect of the steric bulk of the carborane [55]. 

Several interesting fluoroaromatic derivatives, e.g, 3, can be obtained in good to excellent yield by fluorodenitration with potassium fluoride in sulfolane using phthaloyl fluoride (2) or tetrafluorophthaloyl fluoride as nitrite ion traps. 1,2-Difluorobenzenes 4 are formed from 1 -fluoro-... 

A process for the preparation of fluorobcnzencs comprises the heating of fluorobenzaldehydes in the presence of a catalyst. Suitable catalysts are transition metals from the B groups 1, 11. VI. VII and VIII. The best catalytical properties seem to be held by rhodium and the metals of the platinum group, e.g. formation of 1.3-difluorobenzene (5). The reaction maybe carried out in homogeneous solution with soluble rhodium catalysts (Wilkinson s catalyst) or in heterogeneous phase with the catalyst fixed on a carrier. ... 

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