Potassium reagents arenes

The introduction of fluorine into activated arenes is achieved by the combination thallium(III) trifluoroacetate/potassium fluoride/ boron trifluoride. Thallium(III) is hereby an oxidative fluo-rinating reagent.910 Representative compounds prepared in this way are listed below. 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

The (dichloroiodo)arene (40 mmol) and finely ground yellow mercuric oxide (10.8 g, 50 mmol) were shaken in a polyethylene bottle with dichloromethane (100 ml). Hydrofluoric acid (48%, 10 ml) was added and the bottle shaken vigorously for about 1 min. The colour of the solution turned from bright yellow to nearly colourless. Then the solvent phase was carefully decanted. The residue was shaken with dichloromethane (50 ml) which was combined with the original solution. The reagent formed was used in this solution for fluorination. In order to find its titre, a portion (1 ml) was analysed by titration of the iodine liberated by reaction with aqueous potassium iodide. The yield was calculated to be in the range 60-90%. 

Potassium permanganate impregnated on alumina oxidizes arenes and alkylarenes to ketones under microwave activation [69c]. Potassium perman-ganate-triethylamine is a convenient reagent for the oxidation of benzylic methyl, methylene and methine groups [70a], Aqueous solutions of permanganate oxidize methane at 40-100 °C to produce carbon dioxide as the sole product [70b], Oxidation of hydrocarbons containing weak C-H bonds (9,10-dihydro-... 

This reaction is the most simple method for synthesis of these structures beside several recent methods based on cross-coupling reactions with different organometallic reagents. The starting materials are very inexpensive and easily available, whereas an excess of arene can be regenerated by distillation of the filtrate after the separation of potassium tetrafluoroborate. This can be also reused as a precipitation reagent for preparation of the parent aryldiazonium tetrafluoroborate from the diazonium chloride... 

Recently it was pointed out [6] that potassium and sodium bromate oxidize arenes in good yields in a 3 2 dioxane-water solution by using cerium ammonium nitrate (CAN) as catalyst. Toluene derivatives are oxidized to ca. 1 1 mixture of benzaldehydes and benzoic acids, while ethylbenzenes are converted to acetophenones. According to the proposed mechanistic scheme, water either influences the polarity of the reaction medium or acts as the reagent, reacting with the intermediate benzylic carbonium ion to give a hydroxy derivative, which is oxidized from either Ce or BrOa ions. 

Heterosubstituted Arenes. As a rule of thumb, the super-basic n-BuLi/f-BuOK reagent having coordinatively saturated lithium preferentially deprotonates such positions where charge excess is most efficiently stabilized. This may result in a different regiochemical behavior with respect to simple organolithium reagents exploiting the so-called optional site-selective metalation. Some representative examples of n-butyllithium/potassium ferf-butoxide metalations of heterosubstituted arenes are shown in Table 5. 

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