Hydrogen fluoride-pyridine

There is continuous interest in cleavage of epoxides as a general method lor the preparation of fluorohydrins Epoxides are effectively cleaved under mild conditions by a 70% hydrogen fluoride-pyridine complex (Olah s reagent). The... 

Table 1. Cleavage of Glycidic Esters, Glycidic Amides, and Glycidic Nitriles by Hydrogen Fluoride-Pyridine Complex 110,11]...

Various sources of fluoride ion have been investigated, of which highly nucleophilic tetraalkylammonium fluorides ate the most effective Thuf, fluoro alkyl halides and N (fluoroalkyl)amines are efficiently synthesized by treatment of the corresponding trifluoromethanesulfonic esters with tetrabutylammonium fluoride trihydrate in aprotic solvents [5fl] (equation 34) The displacement reactions proceed quantitatively at room temperature within seconds, but tail with hydrogen fluoride-pyridine and give reasonable yields only with hydrogen fluo ride-alkylamine reagents... 

Cyclopropyl methanols when treated with a combination of hydrogen fluoride, pyridine, potassium hydrogen fluoride, and diisopropylamine undergo fluonnation and rearrangement to give excellent yields of homoallylic fluorides Chlorobenzene substituted cyclopropyl methanols at low temperatures leads to ring expansion to give... 

A variety of media have been used for the Wallach fluorination reaction anhydrous hydrogen fluoride alone or with cosolvents such as methylene chloride, benzene, or tetrahydrofuran and hydrogen fluoride-pyridine alone or with co solvents such as benzene, glyme, or acetic acid [42,43, 46 50] Solutions of cesium fluoride, tetraethylammonium fluoride, or tetrabutylammonium fluoride in strong acids such as methanesulfonic acid or trifiuoroacetic acid with numerous cosolvents have also been studied [48, 49]... 

There seem to have been only two investigations on dediazoniations in a protic solvent, where the observed products indicate that, in addition to DN + AN solvolysis, an aryne is likely to be present as a metastable intermediate. Broxton and Bunnett (1979) have found that 3-nitroanisole is formed in the dediazoniation of 2-nitroben-zenediazonium ions in methanol in the presence of methoxide ions. This has to be interpreted as a product arising from 3-nitro-l,2-benzyne as an intermediate. The occurrence of the aryne mechanism in poly (hydrogen fluoride)-pyridine mixtures, as discovered by Olah and Welch (1975), is mentioned in Section 8.2. 

The noteworthy advantages of the allyl ester are (a) it is readily introduced into amino acids (b) after isomerization (to 1-propenyl) by a palladium(O) catalyst it may be removed under weakly acidic or basic and neural conditions (32), even if sulfur-containing amino acids are present (34) (c) it shows orthogonal stability to the tert-butyloxycarbonyl and 9-fluorenylmethoxy-carbonyl groups (10) and (d) it is not affected by the hydrogen fluoride-pyridine complex (35). 

Preparation of Tertiary and Secondary-Alkyl Fluorides from Alcohols with Hydrogen Fluoride-Pyridine Reagent... 

Hydrofluorjnation of Alkenes with Hydrogen Fluoride-Pyridine... 

The only systems which have achieved wide practical acceptance are hydrogen fluoride/ pyridine and triethylamine tris(hydrogen fluoride) combinations (Table 3). 

The two most widely accepted hydrogen fluoride systems, besides pure hydrogen fluoride, are 70% hydrogen fluoride/pyridine (Olah s reagent)10 11-31-33 and tricthylaminetris(hydrogen fluoride).9 32 33-316 Tetrahydrofuran. various other amines, amides, carbamic acids and esters. 

Hydrogen Fluoride/Pyridine [Pyridinium Poly(hydrogen fluoride)], (70% Hydrogen Fluoride by Weight) 31... 

Branched alkenes and cycloalkenes react more readily. The hydrofluorinations may be carried out advantageously with 70 % hydrogen fluoride/pyridine (Olah s reagent) using tetrahydro-furan as a solvent,10,31 but a homogeneous system forms in this case. Therefore, the workup procedure was carried out by quenching the reaction mixture with ice water and neutralization with sodium hydrogen carbonate hydrofluorinations of various alkenes in this way are described in ref 40. 

Products of monoaddition may be isolated with tetrabutylammonium bis(hydrogen fluoride) if electrophilic alkynes (one or two electron-withdrawing substituents) are used, e.g. formation of 3. In contrast to these results, 70% hydrogen fluoride/pyridine does not add hydrogen fluoride to dimethyl acetylenedicarboxylatc.77... 

Aliphatic and aromatic isocyanates react with 70 % hydrogen fluoride/pyridine at room temperature to give the corresponding carbamyl fluorides in good yields.31... 

Fluorination by cleavage of a C-C bond has been achieved in the case of small carbocyclic rings. Thus, cyclopropane is transformed to 1-fluoropropane (1) by 70% hydrogen fluoride/ pyridine at room temperature (75 % yield).31-6162 See also Houben-Weyl, Vol. 5/3, pp 114-115. 

Fluoro-l-haloalka-l,3-dienes are prepared in a similar manner by treatment of substituted 2,2-(dihalocyclopropyl)methanols with a hydrogen fluoride/pyridine/diisopropylamine/ sodium fluoride system to give compound 2, followed by dehydrohalogenation with potassium /< r/-butoxide.80... 

Moreover, interesting ring-formation reactions to give 7 have also been reported for substituted mclhoxyallenyl(cyclopropyl)methanols using 70% hydrogen fluoride/pyridine in combination with sodium fluoride or potassium hydrogen fluoride.82... 

Alkyl carbamates and sodium nitrite react at room temperature in 70% hydrogen fluoride/ pyridine to give the corresponding fluoroformates in 31 -78% yield.132... 

Diazoalkanes, diazoketones, and diazoacetates can be fluorinated at 0 C with the loss of nitrogen by treatment with 70 % hydrogen fluoride/pyridine (a,a-hydrofluorinations) in 30-95 % yield.31 133 Furthermore, diazo ketones and diazoacetates when N-halosuccinimide is additionally introduced into the hydrogen fluoride medium.133... 

An interesting regioselective deaminative fluorination of a purinc-2.6-diamine derivative 12 has been reported. The nucleoside is selectively fluorinated in the 2-position of the purine moiety by treatment with tert-butyl nitrite in 60% hydrogen fluoride/ pyridine at — 20 C for 10 minutes.149 The hydrogen fluoride concentration of the reagent should not be higher than 65% (suitable 45-65%). 

The ring opening of l,8,8-trimethyl-3-azatricyclo[3.2.1.02,4]octane isomers by 70% hydrogen fluoride/pyridine has also been studied. A series of y-fluoroamines and nonfluorinated amines, isomers of the starting aziridines, was obtained but no /Mluoroamincs were isolated.165... 

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