Fluorination with CsSO4F. Part 21. Effect of the structure of alkene and alcohol on stereochemistry and relative rate of fluoroalkoxylation
✍ Scribed by Stojan Stavber; Tjaša Sotler-Pečan; Marko Zupan
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- French
- Weight
- 817 KB
- Volume
- 50
- Category
- Article
- ISSN
- 0040-4020
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✦ Synopsis
Alnrtrutr 1-Alkoxy-f-fluoro-1-phenyl bensocycloalkanes are efficiently formed by reactions of lpbenylbensocycloalkenes with caesium fluoroxysulphate (CFS) in alcohols (&OH, EtOH, i-PrOH) at room to moderately elevated temperature. The stereochemistry of the reaction depends on the ring sise of the substrate and on the structure of the alcohol, and is sm predominant in the case of fluorination of five-, anti predominant in six-, and slightly to nearly exclusively syn predominant in seven-membered ring beneocycloalkene analogues. The relative rates measured for methoxy-fluorinations of a set of seven phenylsubstituted alkenes with CFS in methanol show a trend to increasing reactivity with decreasing ionisation potential of the named alkenes yielding a linear correlation relationship with a very high correlation factor rz0.945 and slope -1.76, thus indicating that r-bond disruption in the substrate alkene is the rate determinig step of the reaction of the alkenes studied with CFS under the reported reaction parameters. The family of socalled "electrophilic" fluorinating reagents is a very important group of chemicals useful for the selective introduction of a fluorine atom into organic molecules2, which covers the fluorofunctionalisation of a comprehensive spectrum of organic compounds, including those that are bioactive3. Elemental fluorine' , XeF,' , N-p and fluoroxy reagentssB4~' are the main representatives in the group. The high reactivity of elemental fluorine and most of the fluoroxy reagents demand special reaction conditions, equipment and precautions, while caesium fluoroxysulphate (CsSO,F; CFS) is reported to be an easily handled reagent, useful under normal reaction conditions for the selective fluorofunctionalisation or oxidation of a variety of organic molecule8 b* ' . Phenyl-substituted alkenes have many advantages as model substrates for the study of electrophilic addition reaction in general. When fluorination with "electrophilic" fluorinating reagents is the subject of study, the regiospecificity of the reactions and the stability of the reaction product, in addition to the distinctive possibility of modulation of electronic and steric effects by structural variations, make the phenyl-substituted alkenes decisively advantageous in comparison with other alkenes. Although electrophihc addition to a carbon-carbon double bond is one of the most studied types of reaction in organic chemistry*, only a few kinetic evaluations of the fluorination of alkenes as a function of their structure are known' * ** lo. We now report some kinetic and new stereochemicd aspects of the fluorination of alkenes with CFS.