Tetrafluoroborate counterion

Various diazonium salts have been prepared using this approach55 and thermoanalytically characterized.56 Both the structure of the diazonium moiety and the counterion clearly influence the stability of the diazonium moiety. The thermally stable diazonium ion 71 (Z = Cl, Y = CH20) [ty2 (25°) > 100 days] is also capable of scavenging various nucleophiles (amines, phenols, and anilines).57 This resin with a tetrafluoroborate counterion (resin 66) is called T2 diazonium resin and it is now commercially available from Novabiochem. 

This rearrangement must occur by heterolytic cleavage of the carbon-chlorine bond in 19 to yield the intermediate cation 6+, which is then attacked by the nucleophilic chloride at one of the partially positively charged cyclopropyl groups. With the less nudeophilic tetrafluoroborate counterion, generated by treating 19 with silver tetrafluoroborate under sol-... 

Photooxidation of the triphenylmethyl cation (as its tetrafluoroborate salt) in the presence of an aromatic donor was similarly found to afford bis(triphenyl-methyl) peroxide [31]. The mechanism was proposed to proceed through initial electron transfer from the aromatic donor to the singlet-excited triphenylmethyl cation to give the triphenylmethyl radical. Reaction of the radical with triplet oxygen, and subsequent coupling with another triphenylmethyl radical gave the observed peroxide. It was noted that electron transfer from the tetrafluoroborate counterion to the excited state cation could not be completely excluded, because the cation was slowly photooxygenated in the absence of an electron donor. 

Finally, diphenylamine has been electrochemically oxidized in acetonitrile to produce conducting polymer films [225]. The room temperature conductivity of pressed pellets of the polymer was 10 S cm . No deterioration in conductivity was observed between 25 and 200 C. One tetrafluoroborate counterion appeared to be present for every four monomer units in the oxidized polymer. 

Figure 3 Hoskins and Robson s first report of a 3D coordination polymer based on diamondlike net. The tetrafluoroborate counterions were highly disordered and not localized in the X-ray crystal structure. ...

Figure 5 [Cu(pyrimidine)2]BF4 framework ° , hydrogen atoms, and tetrafluoroborate counterions omitted for clarity. Cu (green), C (gray), H (white), and N (blue).

Tourillon and Gamier first observed true electronic conductivity in polythiophenes in 1982. Thiophene was electropolymerized on platinum electrodes in acetonitrile with perchlorate or tetrafluoroborate counterions. A remarkable conductivity of 10-100 S/cm was obfained. By this experiment, a new era of polyfhiophene chemistry had started, and within a few years polythiophene(s) belonged to the most prominent classes of polymers with electric conductance. Polythiophenes obviously had an enormous potential for technical applications. 

Solubility sol DMF (0.5 mol L ). With the tetrafluoroborate counterion the corresponding salt (TBTU) is slightly more soluble (0.6 mol L ). 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

Oxadiazolo[3,4- ][l,2,3>4]tetrazine 4,6-di-A -oxide 35 is obtained as a yellow crystalline compound (m.p. 110-112 °C (decomp.)) by reaction of the amine 44 with excess nitronium tetrafluoroborate in acetonitrile at —20°C (Scheme 1). Compound 35 is very sensitive to shock and, although it can be stored for long periods at 0 °C, it should be handled with care. The authors propose the mechanism shown in Scheme 1 to account for the formation of compound 35. When the counterion is tetrafluoroborate, the intermediate 46 cyclizes to the bicyclic cation 47, which liberates... 

This substance is best represented as the diazonium zwitterion 26 since its chemistry is analogous to diazonium ion chemistry rather than to diazoalkane chemistry. Because 26 is a diazonium compound without a counterion, it allows one to study pure diazonium ion chemistry. The diazonium nature of 26 is confirmed by its N- and C-NMR spectra compared to those of p-nitrophenyldiazonium tetrafluoroborate (27) and diazocyclopentadiene (28) (78JA4974). Chemical shifts of the carbon attached to the central nitrogen are in 26,5112.2 from Mc4Si in 27,6 121.8 and in 28,6 72.2. The central nitrogen shift is 6 146.0 from HNO3 for 26,152.2 for 27, and 106.2 for 28. The terminal nitrogen is d 59.4 for 26, 57.1 for Tl, and — 8.8 for 28. 

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low-nucleophilicity counterions, particularly tetrafluorobo-rates (B I, ), enabled Meerwein in the 1940s to prepare a series of oxonium and carboxonium ion salts, such as R30+BF4 and HC(OR)2+BF4, respectively.13 These Meerwein salts are effective alkylating agents, and they transfer alkyl cations in SN2-type reactions. However, simple alkyl cation salts (R 1 BF4 ) were not obtained in Meerwein s studies. The first acetyl tetrafluoroborate—that is, acetylium tetrafluor-oborate—was obtained by Seel14 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature [Eq. (3.1)]. 

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)]. 

With these anthracene-linked dimeric cinchona-PTCs, the Najera group investigated the counterion effect in asymmetric alkylation of 1 by exchanging the classical chloride or bromide anion with tetrafluoroborate (BF4 ) or hexafluorophosphate (PF6-) anions (Scheme 4.10) [17]. They anticipated that both tetrafluoroborate and hexafluorophosphate could form less-tight ionic pairs than chloride or bromide, thus allowing a more easy and rapid complexation of the chiral ammonium cation with the enolate of 1, and therefore driving to a higher enantioselectivity. However, when... 

The catalytic influence of 18-crown-6 on the production of fluorobenzene from diphenyliodonium tetrafluoroborate and KF in dichloroethane was documented some years ago [123]. More recently, diaryliodonium salts have been used for direct syntheses of [18F]-fluoroarenes [124,125]. After an initial study in which various counterions were surveyed, this was finally accomplished by the treatment of diaryliodonium triflates and trifluoroacetates with 18F K+-APE 2.2.2 (i.e., the aminopolyether 4,7,13,16,21,24,27-hexaoxa-l,10-diazabicyclo-[8.8.8]hexacosane) or Cs 18F in acetonitrile (Scheme 44). An added feature of these studies is rather extensive confirmation that nucleophiles are preferentially directed to the more deactivated ring of unsymmetrical diaryliodonium ions, unless one of the rings possesses ortho-substituents [125]. 

Another way to activate l-(l-iodoalkyl)-l-silacyclobutanes toward ring expansion is to use silver acetate in acetic acid. In this case, the reaction is believed to proceed via formation of a carbocation a to the silicon. The acetate counterion acts as a nucleophile, attacking the activated SCB with C-Si bond migration (Scheme 35) <1991TL6383>. Silver tetrafluoroborate in dichloromethane induces ring enlargement as well, but shows much lower efficiency (30% yield upon treatment with MeLi) <1994BCJ1694>. 

Diazotization procedures. Widely used for the production of aromatic fluorine is the Balz-Schiemann reaction. The approach involves diazotization of the aniline and isolation of the insoluble tetrafluoroborate salt, followed by decomposition under heating conditions (Fig. 32). Initially introduced in 1927 [137,138], it did not achieve commercial utility until the mid-1980s. A modification of the Balz-Schie-mann reaction involves replacing the tetrafluoroborate with other counterions such as a fluorine anion [139],... 

Interestingly, these authors53 and later others54 showed the importance of the silver counterion in such rearrangements. While silver tetrafluoroborate or silver nitrate gave the [3,3]-sigmatropic shift product mainly or exclusively, silver trifluoroacetate yielded the dienyl acetate through isomerization from the allenic ester (Scheme 3.33). 

As discussed in Chapter 2, interception of cation 2.31 with a nucleophilic counterion such as acetate produces the 2,5-addition product 2.32. Tetrafluoroborate is a non-nucleophilic counterion and hence the only pathway available to 2.31 is loss of a proton to give nitrofuran 2.33 directly. 

The effect of benzo substitution on thiinium tetrafluoroborate is shown in Figure 17. In a study of thiinium salts with different counterions (BF/ BPI14, I-, TfO") and solvents (CD3CN, DMSO-r/6) it was evident that these changes have the least effect on the C(2) chemical shift . 

Analogously, fragmentation to form the aromatic tropylium ion (pKR- = +4.76) [33] is observed when 43 (R—H) is treated with perchloric acid [117]. To demonstrate the counterion effect discussed above, we combined tropylium tetrafluoroborate (44BF4 ) with isobutene in methanol and isolated 43 (R = CH3) in 45% yield [118],... 

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