Difluorobenzene rings

Figure 7. Dihedral angle (in degrees) associated with the rotation of the difluorobenzene ring versus time in picoseconds (each tic represents 5 picoseconds).

The frequency of rotation of the difluorobenzene ring was determined to be 1.8 x 10 Hz. We also obtained a similar plot for the rotational dihedral angle of the dichlorobenzene ring versus time. The results are found in Figure 8. For the dichlorobenzene ring, the frequency of rotation is estimated to be 3.6 X 10" Hz. The difference in the calculated frequencies between the two dihalo-substituted benzene rings demonstrates that different frequencies of electromagnetic radiation are required to stimulate rotation for each. 

Carboxylic acids and their anhydrides acy late a variety of benzene derivatives, fused ring systems, and heterocyclic compounds. An improved procedure for the preparation of l,4-difluoroanthracene-9,10-dione involves reacting phthalic anhydride and 1,4-difluorobenzene to prepare an intermediate carboxylic acid [35] Intramolecular acylation in polyphosphonc acid completes the synthesis (equahon 24). 

Figure 1. SNIFTIRS difference spectra of the b3u ring bending node of p-difluorobenzene at a platinum electrode as a function of modulation potential.

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]. 

At 400°C and without a catalyst, chlorine reacts in its atomic form and causes a free-radical substitution. Of all the bonds to carbons of the benzene ring, the bond between carbon and nitrogen is the weakest (292 kJ, 70 kcal/mol) compared to carbon-hydrogen bond (415 kJ, 99 kcal/mol) and carbon-fluorine bond (443 kJ, 106 kcal/mol). Therefore, atomic chlorine cleaves the bond between carbon and nitrogen and replaces the nitro group. The product is l-chloro-3,4-difluorobenzene (compound Q) [36. ... 

Fig. II.3. Effect of fluorine substitutions on the out-of-plane minus average in-plane component of the magnetic susceptibility are shown for several aromatic rings. The difference between the Cotton-Mouton and the rotational Zeeman effect data is probably due to the neglect of the field dependence of the electric polarizability in the analysis of the Cotton-Mouton data. Note that the difference in the results for 1,2- and 1,3-difluorobenzene indicates that the ring current quenching effects of substituents strongly depend on their position...

Ort/io-selectivity is generally observed in the reactions of 2,4-dichloro- and 2,4-difluoro-nitrobenzene with alkoxide and thiophenoxide ions [199]. Also in less activated systems, nucleophiles generated from phenols and thiophenols with potassium fluoride-alumina and 18-crown-6-polyether will react in DMSO with cyano- or nitro-substituted fluoro- or chloro-benzenes [200]. Interestingly, the reaction of difluorobenzenes with two diffoent alcohols can occur sequentially. Introduction of the first etho" function deactivates the ring, and use of more forcing conditions allows substitution of the second fluorine [201]. Consecutive displacements of fluorine and nitro groups have been observed in the reaction of ort/io-fluoronitrobenzene with chiral acyl bicyclic lactones in a highly enantioselective synthesis of spirooxindoles [202]. 

Okamoto, H., Kimura, M., Satake, K., and Morosawa, S., Synthesis and adiabatic photochemistry of a 1,4-difluorobenzene-naphthalene biplanemer. Bull. Chem. Soc. Jpn., 66, 2436,1993. Nishimura, J., Nakamura, Y., Hayashida, Y., and Kudo, T, Stereocontrol in cyclophane synthesis a photochemical method to overlap aromatic rings, Acc. Chem. Res., 33, 679, 2000. 

---