Tetrafluoroboric acid, reaction

Schiemann reaction (Section 22.17) Preparation of an aryl fluoride by heating the diazonium fluoroborate formed by addition of tetrafluoroboric acid (HBF4) to a diazonium ion. 

Reaction of [Ir( -Cp )Cl2(/A-Cl)2]2 and pyrazole in the presence of potassium hydroxide leads to complex 105 characterized by the dynamic hydrogen bond between three pyrazole nuclei (86AGE1114). 3,5-Dimethylpyrazole in identical conditicHis produces 106 where two pyrazole nuclei share a proton and one is unprotonated. Addition of tetrafluoroboric acid to 106 yields 107, where the second proton is bonded to the nonchelated ligand. Addition of the third proton causes formation of [Ir( -Cp )(Hpz )3](BF4)2. 

Reaction of iminophosphorane 1006 with ethoxycarbonyl isocyanate gave (91T6747) regioselectively thiadiazolotriazine derivatives 1007, whereas treatment of 1006 with aromatic isocyanates in ethanol or in the presence of tetrafluoroboric acid afforded 1008 and 1009, respectively. Similarly, 1006 with methyl isocyanate gave 1010 (91T6747) (Scheme 187). 

JOC3570). Diazotization of l-methylimidazole-5-carbohydrazide (103) followed by treatment with r-butanol gave the carbamate (104), which was dissolved in cold tetrafluoroboric acid when evolution of gas had ceased, the solution was treated with sodium nitrite and irradiated. However, unlike the 4-aminoimidazoles (Section III,A,2), this reaction produced none of the desired 5-fluoro-l-methylimidazole (105) (Scheme 10). Although 5-amino-1-methylimidazole (96 R = Me, R2 = H) was almost certainly formed, ultraviolet spectral analysis showed only traces of a diazo-nium chromophore after addition of nitrite, indicating compound (96 R1 = Me, R2 = H) to be extremely unstable under the acidic reaction conditions (78JOC3570). 

An interesting hydroiodination reaction occurs when a mixture of cyclohexene and triethylsilane in dichloromethane is treated with a mixture of bis(pyridine) iodonium tetrafluoroborate and tetrafluoroboric acid in diethyl ether (Eq. 125). A 50% yield of iodocyclohexane is produced after one hour at 20°.268... 

Some 4,5-dihydro[l,2,4]triazolo[3,4-r-]benzo[l,2,4]triazines 57 easily reacted with aromatic aldehydes to result in the formation of synthetically valuable azomethine imines 58 <2005EJO3553>. The transformation took place at room temperature in the presence of tetrafluoroboric acid in 10 min in high yields. The product 58 was conveniently prepared and stored in the form of tetrafluoroborate salt, and was subjected to further reactions (e.g., 1,3-dipolar cycoadditions see Section 11.19.5.4.) by in situ liberation of the free base prior to transformation. 

PEPTIDE-BOND FORMATION FROM O-BENZOTRIAZOL-1 -YL-N,N,N, N -TETRAMETH YLU RON IUM HEXAFLUOROPHOSPHATE- AND TETRAFLUOROBORATE-MEDIATED REACTIONS OF N-ALKOXYCARBONYLAMINO ACIDS... 

FIGURE 2.18 Peptide-bond formation from O-benzotriazol-l-yl-AW.Af. Af -tctramcthyluro-nium hexafluorophosphate- and tetrafluoroborate-mediated reactions of N-alkoxycarbony-lamino acids.52 54 The peptide can originate by aminolysis of either of two precursors the acyloxycarbocation and the benzotriazolyl ester. 

Olsen and co-workers used a solution of nitronium tetrafluoroborate in acetonitrile for the V-nitration of acetamides and urethanes at —30°C. The following nitramides were obtained by this method V-nitroacetamide (13 %), V-nitro-2-chloroacetamide (55 %), V-nitro-n-butylacetamide (40 %), V-nitrobenzamide (53 %), ethyl V-nitro-n-butylcarbamate (91 %) and V-nitrosuccinimide (43 %). The low yield of V-nitroacetamide, a primary nitramide, is attributed to competing hydrolysis due to the release of tetrafluoroboric acid as the reaction progresses. The scope of the reaction is improved by moving to more basic solvents like ethyl acetate, 1,4-dioxane and trimethyl phosphate. ... 

Dediazoniation itself is not dramatically dependent on the hydrogen fluoride/pyridine ratio, as shown from the dediazoniation of benzenediazonium tetrafluoroborate (previously isolated) in hydrogen fluoride/pyridine mixtures provided that the hydrogen fluoride/pyridine ratio is larger than 6, fluorobenzene is always obtained in quantitative yield (for hydrogen fluoride/ pyridine ratios below 6, byproducts result from the phenyl cation and free pyridine).10 These observations have resulted in new, very efficient syntheses of 4-fluorophenols 2 involving dediazoniation, in hydrogen fluoride/pyridine mixtures, of crystalline 4-hydroxy benzenediazonium chloride and tetrafluoroborate 3 (formed under anhydrous conditions)89,96 as well as quinone-diazide (formed from aminophenol in tetrafluoroboric acid).93,97 Such methods are more efficient than the Balz-Schiemann reaction under standard conditions. 

The basic Balz-Schiemann reaction involves the preparation of an insoluble arenediazonium tetrafluoroborate 1, either by direct diazotization of arylamines in aqueous tetra-fluoroboric acid, or by addition of tetrafluoroboric acid or alkaline tetrafluoroborates to an aqueous solution of another arenediazonium salt. After filtration and air-drying, this solid diazonium tetrafluoroborate is decomposed either neat or suspended in an inert solvent. 

As previously discussed, the main problems in fluorodediazoniation result either from the formation of Lewis acids during the decomposition step (in the diazonium tetrafluoroborate method) or from the presence (or formation) of water in the reaction medium (during in situ fluorodediazoniation in aqueous tetrafluoroboric acid or anhydrous hydrogen fluoride). To circumvent these problems, stable covalent 3,3-dialkyl-l-aryltriazenes have been decomposed with anhydrous hydrogen fluoride. In such a technique, a diazonium fluoride is generated in an anhydrous medium from a soluble and nonionic precursor, without formation of water or Lewis acid (Scheme 7). 

Stable a-protonated pyrrolium salts have been obtained by treating di- and tri-t-butylpyrroles with tetrafluoroboric acid (81LA789) and stable a-protonated thiophenium salts result from the reaction of... 

Ring cleavage occurs when ( y4-dispiro[2.0.2.4]deca-7,9-diene)Fe(CO)3 is treated with tetrafluoroboric acid, to give after exchange with PF6" a stable / -complex cation (equation 146) which, upon further reaction with NaN(SiMe3)2 in MeCN/THF, afforded the... 

The formation of an intermediate difluorodisilene 25 (equation 8) was proposed by Jutzi and coworkers34 in the reaction of decamethylsilicocene with tetrafluoroboric acid. The disilene which was characterized by the 29Si NMR spectrum, then formed the isolable cyclotetrasilane by [2 + 2] cycloaddition. 

The reaction of 82 with protic substrates HX can follow different pathways, as depicted in Scheme 23. In most of the cases studied so far, simple oxidative addition takes place leading to compounds of the type (Me5C5)2Si(H)X190. The reaction of 82 with tetrafluoroboric acid leads to elimination of pentamethylcyclopentadiene and to the formation of a cyclotetrasilane derivative, via several intermediates with divalent silicon from which the dimeric pentamethylcyclopentadienyl(fluoro)silylene was proved by Si NMR spectroscopy191,192. In contrast, an HF-oxidation product is formed in the reaction of 82... 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

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