Lateral substituents fluorine

The decrease of the clearing point induced by lateral fluorination can be understood in terms of reduction of the length-to-breadth ratio of the liquid crystal molecule caused by any lateral substituent. As rule of thumb, for most compounds a decrease of the clearing temperature of 30-40 K is observed for each lateral fluorine atom. 

The 12-vertex carborane unit provides an effective shielding for lateral substitution of the core with fluorine atoms (Z = F in Figure 14.1a). Typically, a lateral substituent decreases the aspect ratio of the mesogenic molecule, which reduces the stability of the mesophase. It has been demonstrated that the NI in such derivatives approximately linearly depends on the size of the lateral substituent. Results demonstrated that lateral fluorination in series 33,34, and 58 (X=F) decreases the NI as compared to the respective unsubstituted compounds 30,29, and 58 (X=H), and the effect of phase... 

Stepwise fluorination of S02CH3 is thus seen to lead to a continuous increase in stepwise fluorination will be mentioned again later in connection with spectroscopically based substituent constants. 

Another approach to get new liquid crystals is the lateral fluorination of the stilbazole ligands,337 which is a common and highly effective tool to exert control over mesomorphism, crystal phase stability, and physical properties. Other modifications include the use of more alkoxy substituents and other alkyl sulfate anions.338-344 Ionic silver amino complexes also display liquid crystalline behavior at rather low temperatures they are of the form [Ag(NH2 -CJl +OJX (X = N03, n = 6,8,10,12,14 X = BF4, = 8,10,12,14).345... 

No simple rationalization for the observed values of the 19F resonances could be made for the peracetylated 2-deoxy-2-fluoro-D-gIuco-and -manno-pyranosyl fluorides, apart from the fact that the anomeric fluorine substituent always gives the lower field-resonance of the two198 this should, however, be compared with the explanation described287 (see later in this Section) for the observed differences in 19F... 

In 1988 Heinze and Burton reported a facile synthesis of various a,p,P-trifluorostyrenes. These trifluorostyrene compounds were reported to be imstable to cyclodimerization at room temperature when stored neat, especially the compounds that were iara-substituted with electron-donating substituents. They described the preparation of one compound, l,4-bis(trifiuorovinyl)benzene with the observation that the material gelled when allowed to stand neat overnight. They offered the explanation that the gel was a polymer network connected with fluorinated cyclobutanes. Burton later went on to utilize this dimerization reaction for the cross-linking of polyimide thermoplastics. ... 

The Ji-complexes formed between chromium(O), vanadium(O) or other transition metals, and mono- or poly-fluorobenzene show extreme sensitivity to heat and are explosive [1,2], Hexafluorobenzenenickel(O) exploded at 70°C [3], and presence of two or more fluorine substituents leads to unstable, very explosive chromium(O) complexes [1]. Apparently, the aryl fluorine atoms are quite labile, and on decomposition M—F bonds are formed very exothermically. Laboratory workers should be wary of such behaviour in any haloarenemetal Ji-complex of this type [1]. However, in later work, no indications of explosivity, or indeed of any complex formation, were seen [4]. Individually indexed compounds are ... 

A large number of synthetic pyrethroids with a variety of aromatic and aliphatic fluorine substituents have been commercialized, and will be discussed in the sections on fluorinated ether, aromatic fluorine, and fluorinated aliphatic groups, respectively. Fluvalinate (Mavrik ) [50] is a trifluoromethylphenyl pyrethroid, initially introduced by Zoecon (later Zandos Ag) and later replaced by tau-fluvalin-ate, which contains two of the four isomers of fluvalinate. Tau-fluvalinate (Apistan ) is a synthetic pyrethroid used for the topical treatment of honeybees against the parasitic mite Varroa jacobsoni. Mite resistance to tau-fluvalinate has been reported [51],... 

Many other structural variants were pursued, the most beneficial being the introduction of fluorine or methyl at the 6a position and, later, methyl at the 16a or 16(3 position. Considerable improvements in biological activity were also created by esterification of hydroxyl substituents at C-17 and C-21. Some aspects of the synthesis of the 16-methyl glucococorticoids are detailed later in a summary of some of the work undertaken by my colleagues in Schering. 

Aromatic compounds Nucleophilic displacement of halide ion from haloaromatic compounds containing other electronegative substituents is well known this process has been widely exploited for the synthesis of fluorinated aromatic compounds (Table 2.1) and is discussed later (see Chapter 9, Section 11). 

In 1958 I gave a course of lectures on aromatic character in Coulson s Theoretical Summer School in Oxford. After one of them I met Norman Paddock (later Professor on Chemistry in the University of British Columbia), then working in the research department of Albright and Wilson. The company s commercial interests included the cyclic phosphonitrilic halides (phosphazenes) that Paddock was characterizing. They have ring systems with alternate nitrogen and phosphorus atoms. The substituents on the pentavalent phosphorus atoms may be chlorines or fluorines or other atoms or groups. 

The strong effect on the mesophase sequence of lateral mono- and difluorination at different sites of an aromatic mesogenic core structure is illustrated by the data in Table 4.5 [40]. The relatively moderate mesogenic properties of material 14 were dramatically improved by introduction of two lateral fluorine substituents in the liquid crystal 22. Unfortunately, this target was achieved only by a purely empirical trial-and-error process requiring a tremendous amount of synthetic work. 

For a typical phenylcyclohexane-based mesogenic core structure, this results in a Ae of ca 4. The dielectric anisotropy is increased if one or two additional fluorine substituents are introduced ortho to the terminal group (Table 4.6). For larger systems with three or more phenylene moieties in their mesogenic core structure a further increase of the dielectric anisotropy is achieved by using the additional rings as a scaffold for attachment of further lateral fluorine substituents. 

Poland and Knutson [116] identified several structural features of the halogen-ated dibenzo-/)-dioxins that are associated with high levels of both toxicity and receptor binding. The molecules should be essentially planar and rectangular in shape, with at least three of the four lateral positions (2,3,7,8 see 5) bearing substituents however, at least one ring position should be unsubstituted. Activity decreases from bromine to chlorine to fluorine. 

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