Fluorinated compounds, fragmentation

In the course of their work on suitable materials for use as blood substitutes the group at the University of Padua have studied the electrochemical fluorination of cyclic tertiary amines [80]. The ECF of NW-diethylcyclohexylamine and N-ethyldicyclohexylamine gave the corresponding perfluoroamines together with several other compounds arising from incomplete fluorination and fragmentation reactions. 

The presence of an M - 15 peak (loss of CH3), or an M - 18 peak (loss of H20), or an M — 31 peak (loss of OCH3 from methyl esters), and so on, is taken as confirmation of a molecular ion peak. An M — 1 peak is common, and occasionally an M - 2 peak (loss of H2 by either fragmentation or thermolysis), or even a rare M - 3 peak (from alcohols) is reasonable. Peaks in the range of M - 3 to M — 14, however, indicate that contaminants may be present or that the presumed molecular ion peak is actually a fragment ion peak. Losses of fragments of masses 19-25 are also unlikely (except for loss of F = 19 or HF = 20 from fluorinated compounds). Loss of 16 (O), 17 (OH), or 18 (H20) are likely only if an oxygen atom is in the molecule. 

Apart from fragmentations, the main byproducts isolated are partially fluorinated compounds in which the remaining hydrogen atoms are attached to the most hindered carbon as demonstrated in the case of di-tert-butylmethane (l)11 and in the preparation of dimethyl bexa-fluorobicyelo[1.1.1]pentane-l,3-dicarboxylate(2, R = F). Apart from the perfluorinated products considerable amounts of polyfluoro derivatives are also obtained.12... 

In the syntheses of heterocyclic compounds with perfluoroalkyl groups, the key methods are based on two types of chemical transformations. The first type involves the introduction of the perfluoroalkyl group on the heterocyclic system, namely, substitutional fluorination of fragments already available in the system or direct introduction of the perfluoroalkyl group onto the heterocycle. 

Losses of fragments of masses 19-25 are also unlikely (except for loss of F = 19 of HF = 20 from fluorinated compounds) loss of 16(0), 17(OH), or 18(H20) are likely only if an oxygen atom is in the molecule. 

When we assume the molecular orbital of a fluorinated compound or its parent compound from a fragmentation of the organic moiety and fluorine or proton atoms, newly generated molecular orbitals could be illustrated as shown in Figure 1.1. 

When performing CID fragmentation with fluorinated compounds, unfortunately, there is no common fragmentation pathway that is significant for fluorine. Furthermore, very odd pathways may occur in the presence of fluorine, which will be subject of this section. Due to the unique properties of fluorine, fragmentation pathways of fluorinated molecules may differ largely from their non-fluorinated counterparts. 

ENVIRONMENTAL ANALYSIS PERSISTENT ORGANIC POLLUTANTS TABLE 13.2 Names, Acronyms, MS Fragmentation Data for Fluorinated Compounds ... 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

Fluorination. Direct fluorination of quinoline was accompanied by extensive fragmentation of the heteroring, but trifluoromethyl hypofluorite in trichlorofluoromethane at -70°C converted 5-fluoro-8-hydroxyquino-line into the 5,7-difluoro-8-hydroxy product (72JMC987). Quinoline, itself, was perfluorinated by fluorine and cobalt(III) fluoride (56JCS783), whereas cesium tetrafluorocobaltate at around 350°C converted it into a mixture of saturated polyfluoro compounds (82JFC413). It is much more satisfactory to introduce fluorine by nucleophilic methods. 

Further development in the chemistry of oxazolidinone antibacterials was based mainly on the assumption that the 4-pyridyl moiety of one of Dupont s lead compounds, E-3709, might be amenable to replacement by suitably saturated heterocyclic bioisosteres [48]. This assumption was based on an example in which successful replacement of the piperazine ring system in the quinolone antibacterials, such as ciprofloxacin, with a pyridine fragment, such as seen in Win-57273, results in improvement of both the antibacterial and the pharmacokinetic profiles of the compounds. Similarly, as in the case of ciprofloxacin and Win-57273, it was predicted that the presence of a small but highly electron-withdrawing fluorine atom would be tolerated at the meta position(s) of the central phenyl ring, and would confer enhanced antibacterial activity and/or other desirable properties to the targeted oxazolidinones, as shown in Fig. 3. 

The violent nature ofreactions between fluorine and hydrocarbon compounds has already been noted here, and the direct fluorination of organic polymers is not a exception it is so exothermic that if the reaction is not controlled, it generally leads to fragmentation and charring of the substrate. Moderation of the reaction rate can be effected by ... 

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