Fluoroborate salt

Diazonium salt chemistry provides the principal synthetic method for the prepara tion of aryl fluorides through a process known as the Schiemann reaction In this pro cedure the aryl diazonium ion is isolated as its fluoroborate salt which then yields the desired aryl fluoride on being heated... 

A standard way to form the aryl diazonium fluoroborate salt is to add fluoroboric acid (HBF4) or a fluoroborate salt to the diazotization medium... 

Diazotization of an arylamine followed by treatment with fluoroboric acid gives an aryl diazonium fluoroborate salt Heating this salt converts it to an aryl fluoride... 

Manufacture. Fluoroborate salts are prepared commercially by several different combinations of boric acid and 70% hydrofluoric acid with oxides, hydroxides, carbonates, bicarbonates, fluorides, and bifluorides. Fluoroborate salts are substantially less corrosive than fluoroboric acid but the possible presence of HF or free fluorides cannot be overlooked. Glass vessels and equipment should not be used. 

The reaction between phthalimidine (30) and triethyloxonium fluoroborate is reported to give the fluoroborate salt of 1-ethoxyiso-indolenine, from which the free base (35) can be liberated.The iso-indolenine (35) apparentlj shows no detectable tautomerism with the isoindole form (see Section IV, A). 

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

While the above examples demonstrate that product control to a significant extent is possible in oxythallation by careful choice of substrate or reaction conditions, the synthetic utility of oxythallation has been illustrated most convincingly by the results obtained with highly ionic thallium(III) salts, especially the nitrate (hereafter abbreviated TTN). Unlike the sulfate, perchlorate, or fluoroborate salts (165), TTN can easily be obtained as the stable, crystalline trihydrate which is soluble in alcohols, carboxylic acids, ethers such as dimethoxyethane (glyme), and dilute mineral acids. Oxidations by TTN can therefore be carried out under a wide variety of experimental conditions. 

The resulting alkoxydlazenium fluorosulfonates are stable, highly hygroscopic, crystalline solids, which are freely soluble in organic solvents, such as dichloromethane (in contrast, the fluoroborate salts are relatively insoluble). 

The reaction between zwitterion 74 and aryldiazonium salt at room temperature provides the fluoroborate salt 75. Upon treatment with a base the salt 75 undergoes a valence bond isomerization to yield triazolo-pyridazine 76 (Scheme 2) <2000CC1785>. 

In 50 the strain resulting from tridentate coordination of 48 is alleviated by the >NH bridge between the thiazole and one of the pyridine rings, creating one six-membered and one five-membered chelate ring. This is evident from the structure of the high spin fluoroborate salt [Fe 502](BF4)2.3H2O in... 

Scheme 3.21 (Zwier et al. 2001). The initial neutral compound shown in Scheme 3.21 is very sensitive to air as a solid and is stable only for a few hours. The fluoroborate salt of the l,6-diazabicyclo[4.4.4] tetradecane cation-radical can be isolated as a dark red solid. The solid is indefinitely stable and in the absence of a base is stable for months in organic or even aqueous solutions(Alder and Sessions 1979). Three-electron N—N bound cation-radicals are known in a wide range of structures. Most examples are, however, unstable outside the glassy or solid state (for a review, see Alder 1983). The tricyclic cation-radical portrayed in Scheme 3.21 is structurally protected from the (N.. N)+ bond being cleaved. This protection provides it a long solution lifetime (Nelsen et al. 1980).

In 1976, the synthesis of oxaziridininm salt 66 was reported by Lusinchi and coworkers (Fig. 23) [128-130]. Salt 66 was obtained by either methylation of the corresponding oxaziridine with FSO Me or oxidation of the corresponding iminium salt with peracid. Subsequently, Hanquet and coworkers prepared oxaziridininm salt 67 by methylation of the corresponding oxaziridine with Meerwein s salt (MejO BF ") or oxidation of the A -methyl isoquinoUnium fluoroborate salt with peracid [131, 132],... 

Arylcarbodiimides also dimerize with alkyl phosphites (68CB174) and in special cases with triethylamine (76CC567). HBF4 is effective in giving the corresponding fluoroborate salts which can be converted to the bis(imino)-l,3-diazetidines with base (Scheme 87) (68AG(E)72, 72LA(762)167>. 

Diazonium Fluoroborates. Dr R.P. Johnson 8e J.P. Oswald of Abbot Laboratories, North Chicago, III urge caution regarding the stability of aromatic diazonium fluoroborate salts. These salts are intermediates in the Schie-mann-Balz reaction for replacing the aromatic primary amino group by fluorine... 

As mentioned earlier, tetraalkylammonium hexafluorophosphate and tetra-fluoroborate salts are replacing the corresponding perchlorates for general use. There does not appear to be any reason to use the perchlorates in view of the potential safety (explosion) hazard that they present [42,71,72]. A number of methods for preparation of hexafluorophosphate and tetrafluoroborate salts have been reported [9-11,57,73]. 

A somewhat unusual alkylation, which may be included here for convenience, is the reaction of diphenylcyclopropenone148 or the salt 98148 with an indolizine to form the cation 99, isolated as its tetra-fluoroborate salt. 

SYNTHESIS A solution of 76.6 g 2,5-dimethoxyaniline in 210 mL H20 containing 205 mL fluoroboric acid was cooled to 0 °C. with an external ice bath. There was then added, slowly, a solution of 35 g sodium nitrite in 70 mL H20. After an additional 0.5 h stirring, the precipitated solids were removed by filtration, washed first w ith cold H20, then with MeOH and finally Et20. Air drying yielded about 100 g of the fluoroborate salt of the aniline as dark purple-brown solids. This salt was pyrolyzed with the cautious application of a flame, with the needed attention paid to both an explosion risk, and the evolution of the very corrosive boron trifluoride. The liquid that accumulated in the receiver was distilled at about 120 °C at 20 mm/ Hg, and was subsequently washed with dilute NaOH to remove dissolved boron trifluoride. The product, 2,5-dimethoxyfluorobenzene, was a fluid, straw-colored oil that weighed 7.0 g. 

These methylene-bridged complexes III are extremely robust, air-stable substances. We have sought without success to effect insertions into the Pd-C bonds. The complexes are unreactive toward carbon monoxide (at 5 atm at 30°C) or sulfur dioxide. Reaction with methyl isocyanide or pyridine results in displacement of the terminal halide ions and produces cations that have been isolated as hexa-fluorophosphate salts [Pd2(dpm)2( -CH2)(CNCH3)2][PF6]2( (CN) = 2217 cm-1) and [Pd2(dpm)2(/Lt-CH2)(py)2][PF6]2. Treatment of III with fluoroboric or trifluoroacetic acid slowly results in the protonation of the methylene group which is converted into a terminal methyl group. The resulting brown complex, which has been isolated as its tetra-fluoroborate salt has been shown by H-l and P-31 NMR spectroscopy and X-ray crystallography to have Structure IV. 

King and Tsang (22) have recently studied the relative rate of bimolecular substitution in dibenzyl ethylsulfonium salt 17 and the cyclic sulfonium fluoroborate salt 75. They found that the reaction of thiocyanate anion with salt 72 which can easily take the arrangement 70 is 8000 times faster than the corresponding reaction with the cyclic sulfonium salt 75, which is tied up in the arrangement 7U... 

Alkyne insertion into the Pd complexes 492 derived from 2-iodobenzyl sulfides provides a regiospecific route to mixtures of 1/7-2-benzothiopyrans and 1/7-2-benzothiopyranium fluoroborate salts. Thus, unsymmetrically substituted phenylalkynes afford the 3-phenyl derivatives (Scheme 180) <1995JOC1005>. 

The first synthesis of arylboronic esters 209-215 via the coupling of bis(pinacolato)diborane(4) with easily prepared aryldiazonium tetra-fluoroborate salts was reported. The palladium-catalyzed borylation reaction proceeds efficiently under mild reaction conditions in the absence of a base to afford various functionalized arylboronic esters in moderate to high yields (Scheme 34).121... 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

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