Tetrafluoroboric acid / salts

The levels of selectivity achieved in these reactions are amongst the highest reported for non-enzymatic acylative KR, and the scope of the method has been reviewed by Vedejs [40], as has its application in PKR [43]. The PBO catalysts 2a-c are prepared by a multi-step enantioselective synthesis from lactate esters [42, 44] and are air-sensitive hence, the reactions are generally run in de-oxygenated solvents. However, the air-stable tetrafluoroboric acid salts of these catalysts can also be employed with in-situ deprotonation by EtsN these conditions give results comparable with those obtained using the original protocol [45]. (For experimental details see Chapter 14.17.1). 

H- and 3//-Azepines are generally unstable in aqueous acid solution and the few examples of simple azepinium salts, namely perchlorates,77 bromides,105 picrates35201 and a solitary iodide,105 have been prepared under nonaqueous conditions. The fractional crystallization of oxalate salts has been used for the separation of mixtures of 4- and 6-substituted 3f/-azepines,66 and 3,6-di-tm-butyl- and 2,5-di-tert-butyl-3//-azepine, on treatment with tetrafluoroboric acid in acetonitrile, are converted quantitatively into their crystalline tetrafluoroboratc salts.70... 

Hydroxy-4//-selenopyrans 113 have been reported to be air-sensitive intermediates in the transformations of 100c to salts 73a and 73 (R = Me) by successive action of lithium aluminium hydride or methyllithium and then tetrafluoroboric acid (90AG450). 4//-Selenopyranone 100c was reduced to 4//-selenopyran 101c with DIBAL-H (88MI1). 

These salts can be made easily since tetrafluoroboric acid (HBF4) and hexa-fluorophosphoric acid (HPF6) are commercially available. However, the main advantage of the diazonium salts with the anions of these acids is their stability, which is significantly higher than that of probably all other diazonium salts. 4-Nitrobenzenediazonium tetrafluoroborate is nowadays even a commercial product. Preparative diazotization methods with these two acids can be found in Organic Syntheses (tetrafluoroborate Starkey, 1943 hexafluorophosphate Rutherford and Redmont, 1973). 

Transformation of bromocriptine free base 2 into water soluble salt -mesylate, is the only way to obtain a suitable therapeutical form. Crystallization of mesylate using alcohol as a solvent in the presence of excess of strong acid, e.g. methanesulphonic acid can induce formation of 12 -0-alkyl-derivative 2. Until now this derivatisation of ergot molecule has been practically unknown. In continuation we developed the preparative method for obtaining these compounds, (using tetrafluoroboric acid as a catalyst) (ref. 20). 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

Aminoimidazole (25 R = H), which was generated in situ either by reduction of 4(5)-nitroimidazole (27 R = H) (Section III,A,1) or by hydrolysis of f-butyl-imidazole-4-carbamate (Section III,A,2) was diazotized in situ in tetrafluoroboric acid to give 4-fluoroimidazole (73JA4619, 73JOC3647). Diazotization was also successfully performed on the dihydrochloride salt of 4(5)-aminoimidazole (25 R = H) (87JOC5538). 

As shown in Scheme 199, the 5-aminopyrimidine stmcture may be also incorporated into a more complex bicyclic system. Thus, diazotization of 3-amino-4-oxo-4//-pyrimido[ 1,23 lpyndazincs 1198 followed by treatment with 50% aqueous tetrafluoroboric acid results in precipitation of salts 1199. When heated with alcohols, nucleophilic attack on the carbonyl group opens the pyrimidine ring. The obtained species 1200 assume conformation 1201 that is more suitable for bond formation between the opposite charged nitrogen atoms. Alkyl l-(pyridazin-3-yl)-l//-l,2,3-triazole-4-carboxylates 1202 are obtained in 31-66% yield <2002ARK(viii)143>. 

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. 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

The solution is cooled to -78°C in a dry ice-acetone bath, and 38.19 g (0.23 mol) of tetrafluoroboric acid-diethyl ether complex (Note 10) is added dropwise by syringe over a 30-min period. The solution is allowed to warm to room temperature. The yellow precipitate is filtered off and collected in a 250-mL Schlenk tube, washed with ether, dried under a stream of nitrogen and finally under reduced pressure (oil pump). The bright yellow salt 1 weighs 43. 2-57. 6 g (60-80%) and may be used without further purification (Note 11). U may be stored indefinitely under nitrogen at 0 C without decomposition. 

If H3BO3 is treated with HE, tetrafluoroboric acid HBF4 is slowly produced in solution. This strong acid has not been isolated from solution, but numerous salts, tetraflouroborates, have been prepared. These salts are similar in solubilities to perchlorates (salts C104 ). 

A very important advance was made in 1927, with the disclosure24 by Balz and Schiemann of a new synthetic route (which has ever since been named after them) from an arylamine to an aryl fluoride. After diazotization in the normal way, the readily available tetrafluoroboric acid, or a tetrafluoroborate salt, was added to precipitate the insoluble diazonium tetrafluoro-borate salt. This was then dried and heated it decomposed (usually at 100-150°C) to give a good yield of the corresponding aryl fluoride. 

Introduction of Fluorine with Boron Trifluoride and Tetra-fluoroboric Acid Complexes and Salts of Tetrafluoroboric Acid... 

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. 

Usually, diazonium chlorides (from sodium nitrite and 37 wt % hydrochloric acid) are used as intermediates. They are treated with an alkaline tetrafluoroborate or tetrafluoroboric acid provided that its use is not prohibited by solubility problems, tetrafluoroboric acid delivers purer diazonium tetrafluoroborates than sodium or ammonium tetrafluoroborate.172 Diazonium hydrogen sulfates are also valuable auxiliary salts since they are insoluble in cold water and are obtained quantitatively and in a purer state than diazonium chlorides.118 Their warm, aqueous solutions deliver pure diazonium tetrafluoroborates when treated with tetrafluoroborate sources.137... 

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... 

PF6, SbF6, C104-, FSO3 counterions. The X-ray crystal structure of nitronium ion is known with hydrosulfate anion.515 The most widely used tetrafluoroborate nitronium salt (N02+BF4 ) is prepared by treating a mixture of nitric acid and anhydrous hydrogen fluoride wit boron trifluoride516 [Eq. (4.145)]. 

Li et al. used a similar rearrangement to synthesize 1,4-oxazepine 252, but instead of antimony pentachloride, tetrafluoroboric acid was employed as the acid. In the solution of bicyclic geminal arylazo isocyanate 250 in diethyl ether, a solution of tetrafluoroboric acid was added to form triazolium tetrafluoroborate salt 251. The rearrangement of 251 occurred quickly under mild conditions with ring expansion to yield the tricyclic 252 in less than 5 min. The isomeric form 253 is not observed (Scheme 45) <2004SC1691>. 

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... 

BMIm]BF4 BMImCl [BMIm]PF6 CCS ecu [Choline] [Pro] 1 -butyl-3-methyl-imidazolium tetrafluoroborate 1 -butyl-3-methyl-imidazolium chloride 1 -butyl-3-methyl-imidazolium hexafluorophosphate CO2 capture and storage/sequestration CO2 capture and utilization (2-hydroxyethyl)-trimethyl-ammonium (iS )-2-pyrrolidine-carboxylic acid salt... 

Azine approach. Reissert compounds such as (222) on treatment with acid form transient cyclic intermediates containing the 3,4-fused oxazole ring. When the acid is tetrafluoroboric acid, the Reissert salt (223) can be isolated (73JA2392). 

Condensation of acetic or benzoic esters with benzene-1,2-dithiol in the presence of tetrafluoroboric acid/ether complex yields 2-methyl- or 2-phenyl-1,3-benzodithiolylium salts, respectively (77S263). 

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