The influence of fluorine substitution

The influence of fluorine substitution on the torsional potential of 1,2-dithioglyoxal has been examined by Toro-Labbd140. However, this study has been carried out at a minimal basis set level, which yields quite poor molecular geometries. This implies that the trends shown for the torsional potentials are only of a qualitative value. The work of Cimiraglia and coworkers141 on variation-perturbation CIPSI calculations of the first excited states of 1,2-dithiete and 1,2-dithiin permits one to rationalize the main features of their electronic absorption spectra. 

Two other molecules indicating the influence of fluorine substitution on i orbital participation are SJN4H4 and N4S4F4 tsee Chapter 16). Telrusulfur letraimide is isoelectronic with the molecule and so the structure... 

H and 13C NMR Data. The examples in Scheme 3.3 provide insight into expected proton and carbon chemical shift and coupling constant data for primary alkyl fluorides. It can be seen that the influence on both proton and carbon chemical shifts diminishes rapidly as one moves away from the site of fluorine substitution. 

An interesting paper concerning the influence of fluorine and chlorine substitution on the aromaticity of carbocyclic systems appeared quite recently B. A. Hess and L. J. Schaad, hr. J. Chem. 17, 155 (1978). 

Clearly, attack at the (3-position maximises the influence of fluorine substituents. These same approaches can be used to account for the orientation of substitution in other systems (Figure 9.29). 

The chemistry of phospholes and related phospholide anion complexes remains a very active area, which also continues to attract the attentions of the theoretical community. Among recent theoretical contributions are a consideration of the stability, structure and bonding in lithium- and beryl-lium-pentaphospholide systems, the aromaticity of the pentaphospholide anion (and its arsenic analogue) as probed by ring currents,the remarkable influence of fluorine-substitution (either at phosphorus or at a ring carbon) on the electronic and thermochemical properties of phospholes,and the effects of methyl and vinyl substitution at various positions on the geometries, relative stabilities and Diels-Alder reactivities of phospholes. An ab initio approach has been used to reinterprete some spectral and thermochemical properties of IH-phospholes. The synthesis and reactivity of phospholes of reduced... 

The confonnation of fluoroalkane derivatives is of considerable contemporary interest. The influence of fluorine on the conformation of vicinally substituted compounds is based on a stabilizing gauche interaction that relies upon hyperconjugation between a C-H bond and the low lying a orbital of the C-F bond. The energy of this interaction is estimated to be 0.8 kcal mol", modest but... 

Van der Ham DMF, Harrison GFS, Spaans A, Van der Mer D (1975) Electrochemical reduction of aza-aromatics. Part V Influence of fluorine substitution on the electron affinities. Reel Trav Chim Pay B 94 168-173... 

Schwarzer A, Weber E (2008) Influence of fluorine substitution on the crystal packing of A-phenylmaleimides and corresponding phthalimides. Cryst Growth Des 8 2862... 

The strong para-directing influence of fluorine in Fnedel-Crafts acylation is illustrated by the reaction of fluorotoluenes with acetyl chloride Treatment of 2-fluorotoluene, 3-fluorotoluene, and 2-fluoro-3-methyltoluene with acetyl chloride and aluminum chloride gives, respectively, 91, 82, and 80% substitution para to fluorine [27]... 

There are limited related data available dealing with the influence of P-Br or I substitution, but the few that are given in Scheme 4.11 indicate that fluorines are also shielded by P-bromine or iodine. 

A comparison of the yields of para-substituted [18F]fluoroarenes indicate that (i) acetyl hypofluorite is an inferior fluorination agent for the fluorodegermylation reaction and (ii) the aromatic substituents have considerable influence over the reactivity of the fluorination. A decrease in the fluorodegermylation yield was observed with electron-withdrawing aromatic substituents. The electrophilic aromatic degermylation reaction is thought to proceed via a cr-complex intermediate (Scheme 3). It has been hypothesized that the yield of aryl fluoride is influenced to some extent by the aromatic substituents ability to stabilize the cr-complex intermediate. 

In the range of leaving groups, fluorine has been recognized as a valuable substituent that has practically no heavy atom effect and that in many cases is smoothly replaced under the influence of irradiation (Brasem et al., 1972). With 2-fluoro-4-nitroanisole it even proved capable of efficient substitution by the weak nucleophile water, a reaction that has not been equalled by any other substituent. Curiously, the photosubstitution of fluorine by cyanide is generally less efficient than that by other nucleophiles. 

Under the influence of a Lewis acid, Cjq can be arylated via a Eriedel-Crafts reaction (see Section 8.6). A similar reaction occurs with CggEjg where ipso substitution of fluorine atoms takes place [68]. The fluorofullerene is treated with FeClj in benzene for two weeks (Scheme 9.8). Electrophilic substitution leads to arylated C5QFjg Ph with n = 1-3. Probably due to steric reasons the 5-substitution is not observed in this arylation. 

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