Aryldiazonium tetrafluoroborates

Aryldiazonium-tetrafluoroborate werden durch Natriumboranat in Methanol Oder in waBr. Losung unter Reduktion am endstandigcn Stickstoff-Atom zu Aryl-diazen reduziert, das in ein komplizicrtcs Gemisch zer-fallt5 s. 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

The reactions that have been described above have indicated that sulphones interact poorly with electrophiles. However, in 1970, Whiting and coworkers announced the synthesis of aryloxysulphoxonium salts , by the reaction of sulphones with the potent electrophile produced in the thermolysis of aryldiazonium tetrafluoroborates or hexaflu-orophosphates. Fluorobenzenes are by-products of the reaction. In a subsequent paper , Whiting described the reactions of the aryloxysulphoxonium salts, 5, with oxygen and nitrogen nucleophiles. The fundamentals of these are outlined in equations (34) (oxygen nucleophile) and (35) (nitrogen nucleophile). 

Similar mechanisms were proposed for the aromatization of 2,4,6-tri-phenyl-4//-pyran (151c) to 387b with aryldiazonium tetrafluoroborates,356 with 2,6-di-fe/Y-butylphenoxide radical, and tetracyanoquinone dimethide357 on the basis of kinetic and electrochemical experiments. Another free radical chain pathway for the reaction of 151c with trichloromethyl radical and tetrachloromethane was also postulated353 (Eq. 22). 

As noted above, during the past few years the electrocarboylation of several other substrates [6-7, 55-79] involving alkenes [55-58], alkynes [58-60], aldehydes [61], aliphatic ketones [62], imines [63], aryldiazonium tetrafluoroborates [63], derivatives of cinnammic acid [64—66] has been investigated by many groups. 

In 1958, Nesmeyanov and coworkers290 found that decomposition of aryldiazonium tetrafluoroborates with zinc dust in the presence of GeCLt resulted in formation of aryl-trichlorogermanes in <30% yield, isolated as the corresponding arylgermsesquioxanes. 

On the other hand, 3,3-dialkyl-l-aryltriazenes can be easily obtained from the corresponding aromatic amines (by quenching the diazotized arylamine with a secondary amine, such as piperidine). Therefore, this reaction constitutes a mild alternative to the classic Balz-Schiemann thermal decomposition of aryldiazonium tetrafluoroborate salts, a method which in many cases is too drastic and tedious (see Section 1.1.8.5.). 

Thus, benzenediazonium tetrafluoroborate is not the electrophilic aminating reagent of choice, although the diastereoselection in the formation of the new C-N bond is very good. The yields, however, cannot compete with the results obtainable by the use of trisyl azide and di-Ze/7-butyl azodicarboxylate aminations, as shown in the preceding sections. It is possible that some other modified aryldiazonium tetrafluoroborates will prove to be more successful in electrophilic amination reactions. 

The synthesis of A, A -disubstituted 2-amino-5-arylazo-4-phenyl-l,3-selenazoles 62 has been carried out by addition of 2-amino-4-phenyl-l,3-selenazole to an aryldiazonium salt, for example, an aryldiazonium tetrafluoroborate, in DMSO or in acetic acid (Equation 3) <2000JPR169>. 

Surprisingly, isolable azomethine ylide 151 reacts with aryldiazonium tetrafluoroborates, at — 30°C in acetonitrile, to produce fused triazolium salts 231 after the elimination of the formyl moiety from the initial cycloadducts (84TL65). 

The Schiemann procedure for conversion of an amine to an aryl fluoride is improved considerably by use as intermediate of the aryldiazonium hexafluorophos-phate instead of the aryldiazonium tetrafluoroborate. Since these salts are less soluble than the tetrafluoroborates they are obtained in better yield and can be washed thoroughly for removal of impurities. In the preparation of 2-bromofluoro-benzene from o-bromoaniline the amine hydrochloride is diazotized as usual,... 

Aryldiazonium tetrahaloborates, ArN=NBX4, In the Schiemann reaction, an arylamine is diazotized in aqueous hydrochloric acid solution, and the solution or suspension of aryldiazonium chloride is treated with a solution of fluoroboric acid prepared by dissolving boric acid with cooling to 20-25° in commercial 50-60% hydrofluoric acid contained in a reaction flask coated with wax or lined with copper, lead, or silver. The aryldiazonium tetrafluoroborate is collected, dried, and decomposed by pyrolysis. Examples are ... 

Interest continues to be shown in the photodecomposition of diazonium salts,and aryldiazonium tetrafluoroborates have been recommended as potential photoaffinity labelling reagents for proteins.The key step in a recent synthesis of pyrazofurin involves the photodecomposition of the diazopyrazole (158) to give the hydroxypyrazole (159). ° 2-Azahypoxanthine (160) is the major... 

Other methods to prepare aromatic halo- or dihalo-phosphines are occasionally used. Arylbis(dialkylamino)phosphines have been converted into aryldihalophosphines with dry HCl or HBr, e.g. dichloro(2-methoxy- or 2-dimethylaminophenyl)phosphine (equation 29) the o- and m-phenylenebis(dichlorophosphine)s 20 and 21" and the p-phenylenebis(dichlorophosphine) 22. The c -phenylenebis(dibromophosphine) 23 has been prepared similarly Aryldiazonium tetrafluoroborates with PCI3 give chlorophos-phonium salts, which can be reduced to aryldichlorophosphines (equation 30). Primary and secondary phosphines may be chlorinated with phosgene to give chlorophosphines, e.g. hexamethylenebis[chloro(phenyl)phosphine] (equation 31) and 20 ". ... 

The chlorophosphonium complexes 39, produced (Scheme 8) during the synthesis of cyclic esters of arylphosphonic acids from aryldiazonium tetrafluoroborates and cyclic chlorophosphites, are decomposed by H2S to give the analogous esters of arylthiophos-phonic acids ... 

S-MC of diazonium salt 32 proceeds with ligandless Pd at room temperature in dioxane in the absence of a base [49]. Pd(OAc>2 and carbene hgand XVI-2 are effective for the coupling of aryldiazonium tetrafluoroborates with aryl, alkenyl, and alkyl boronates at room temperature [50]. 

This reaction has been modified using aryldiazonium tetrafluoroborate under phase transfer conditions to enhance the stability of diazonium salt. ... 

Fluoroboric acid (tetrafluoroboric acid, fluoboric acid) [16872-11-0] M 87.8, b 130°(dec), pK -4.9. Crystallise fluoroboric acid several times from conductivity water. It can be stored in a glass vessel at room temperature. It is available conunercially as 48% aqueous solution. It is most usefiil for preparing tetrafluoroborate salts which are generally insoluble. For example, addition of the acid to aryldiazonium salt solutions precipitates the more stable aryldiazonium tetrafluoroborate salts which can be washed with H2O to remove impirrities, followed by EtOH and Et20, and stored for short periods of time before further use. It is a catalyst for preparing acetals. [Mathers et al. J Am Chem Soc 37 1516 1915, Wamser Christian JAm Chem... 

Today, the GBH reactions are performed by using aryldiazonium tetrafluoroborate salts. An aromatic amine is diazotized at about -10 to 0 C in the presence of at least 3 equivalents of concentrated hydrochloric acid by adding the solution of sodium nitrite [61]. To the aryldiazonium chloride, 50% aqueous solution of tetrafluoroboric acid is added with additional cooling to precipitate the corresponding aryldiazonium... 

The modem methods for performing the GBH reaction are based on the thermal decomposition of certain aryltriazenes in the presence of trifluoroacetic acid [66,67], or on the phase transfer catalysed reaction of aryldiazonium tetrafluoroborate in the presence of potassium acetate [61], both methods in an excess of arene as the reactant and reaction solvent. The first method requires previous preparation of the aiyltriazene (VII) by diazotation of an aromatic amine in hydrochloric acid followed by addition of piperidine [66], other dialkylamine or 5-aminotetrazole [67]. For example, from the diazotized aniline solution 43, by addition of an equimolar amount of piperidine, the yellow-orange 1-phenyl-3,3-(pentanediyl)triazene (48) readily crystallize in 81% overall yield [66], Scheme 17. 

This reaction is the most simple method for synthesis of these structures beside several recent methods based on cross-coupling reactions with different organometallic reagents. The starting materials are very inexpensive and easily available, whereas an excess of arene can be regenerated by distillation of the filtrate after the separation of potassium tetrafluoroborate. This can be also reused as a precipitation reagent for preparation of the parent aryldiazonium tetrafluoroborate from the diazonium chloride... 

To a concentrated hydrochloric acid (37%, 250 ml, 3 mol) precooled to -5 C, a liquid arylamine (1 mol) was added dropwise, or in several portions (for solids) during 15 min. The mixture was stirred at -5 °C for an additional 45 min (solid amines has to be grounded to fine powder). A solution of sodium nitrite (69.00 g, 1 mol, 1 eq.) in distilled water (250 ml) was added dropwise at -10 to -5 C during 1 h. The reaction mixture was stirred at -5 °C for an additional 1 (-2) h, until the TLC analysis shows the disappearance of the starting amine. To the obtained clear yellow solution of aryldiazonium chloride, 48% aqueous solution of tetrafluoroboric acid (150 ml, 100.80 g, 1.15 mol, 1.15 cq.) was added dropwise at -5 °C during 15 minutes. The mixture was stirred at -10 °C for 30 minutes, and the crystals were collected by filtration, washed with precooled (+5 °C) water (3x20 ml), and dried at 30 C in high vacuum for 20 h. The pure aryldiazonium tetrafluoroborates were obtained in 60-90% yields. 

The Stille reactions with aryldiazonium tetrafluoroborates are performed under the ligandless conditions in the presence of simple palladium-complexes, Pd(OAc)2 or Pd(dba)2. For example, phenyltri-n-butylstannane (184) was reacted with 4-tolyl diazonium tetrafluoroborate (185) to afford 4-methylbiphenyl (139) in 59% yield [107], Scheme 26. 

Aryl bromides or iodides react with di- or monochloro(vinyl)silanes or triethoxy(vinyl)silane in the presence of Et3N and palladium catalyst to give /S-arylvinylsilanes in moderate to excellent yields. In contrast to the simple silyl substituent, the presence of the electron-withdrawing group on sihcon is essential to avoid the elimination of the silyl group. No silver ion is necessary in these examples . The palladium-catalyzed reaction of aryldiazonium tetrafluoroborates with vinylsilanes gives a mixture of terminal and internal vinylsilanes together with styrenes. ... 

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