Hexachloroantimonate

Alessandro Dondoni University of Ferrara, Ferrara, Italy 

Pedro Merino University of Zaragoza, Zaragoza, Spain 

Functional Group Methylthiolation. The main feature of methylbis(methylthio)sulfonium hexachloroantimonate (MesSs SbClg) is the easy transfer of a methylsulfenylium ion to a variety of functional groups more nucleophilic than the sulfur of dimethyl disulfide. The low nucleophilicity of the disulfide and of the hexachloroantimonate ion allows the isolation, or the characterization, of positively charged reaction products. 

Solubility sol CH2CI2, SO2 reacts with H2O, protic solvents, acetone, and other enolizable carbonyl derivatives. 

CD2CI2 of the pure compound shows two signals at 5 3.6 and 3.0 with an intensity ratio 1 2. At higher temperature in this solvent, as well as in other solvents (SO2, CDsCN ), a single coalescence line is observed. This behavior is due to trace amounts of dimethyl disulfide impurities.  

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When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

A substituted acyl amino group can be introduced by reaction of pyridazine 1-oxide with A-phenylbenzonitrilium hexachloroantimonate 3-A-benzoylanilinopyridazine is formed (75JOC41). 

For purposes of characterization of enamines the perchlorate salts are preferred, as they crystallize well, and the perchlorate anion has no tendency to add to the iminium cation. Other salts, including hexachlorostannates (13), hexachloroantimonates (13), chlorides, bromides, tetraphenylborates, and nitrates, have also been used. Recently a method for the preparation of iminium salts directly from aldehydes or ketones and the amine perchlorate has been reported (16). 

Tri- butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate, It was shown by Farcasiu that 1,5-diketones can also he dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature. ... 

Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA) differs from the other promoters in that its cation is a radical, and as such produces radical cationic sulfonium ions as glycosylating species from thioglycosides.85 The use of this promoter arose from earlier work on the electrochemical generation of 5-glycosyl radical cations as glycosylating species. 

The dimerization of 1,3-cyclohexadiene gives 30% adduct after 20 h at 30 °C [32]. In the presence of a catalytic amount of tris(p-bromophenyl) aminium hexachloroantimonate (ArsN SbCle Ar = /iBrC6H4) in CH2CI2 at 0°C, the cyclodimerization occurs in 15 min with 70% yield with a greater diastereos-electivity endojexo = 5 1) than that observed under thermal... 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

Benzoxazoles are produced in high yield from a-acylphenol oximes by a Beckmann rearrangement using zeolite catalysts <95SC3315>. The reaction of the o-benzoquinone 40 with aromatic aldehyde oximes produces the benzoxazoles 41 <95ZOR1060>. The fused oxazolium salts 43 (R = Me, Et, Pr , or Ph R2 = Me or Pr ) are formed from tropone and nitrilium hexachloroantimonates 42 <96JPR598>. 

JOC3172, 1996J(P2)2085>. Chemical electron transfer (CET) oxidation of these bicyclic housanes with tris(4-bromophenyl)aminium hexachloroantimonate performed at 0°C in the absence or presence of base (2,6-di-/-butyl-pyridine) leads to the formation of two olefinic products 369 and 370, which were isolable after flash chromatography on silica (Scheme 56) <1996JOC3172>. 

The success of this transformation depends upon the oxidation potential of the ESE group (Eox 1.5 V), which is lower than that of the alkyl silyl ether group (Eax 2.5 V). Recently, Schmittel et al.35 showed (by product studies) that the enol derivatives of sterically hindered ketones (e.g., 2,2-dimesityl-1-phenyletha-none) can indeed be readily oxidized to the corresponding cation radicals, radicals and a-carbonyl cations either chemically with standard one-electron oxidants (such as tris(/>-bromophenyl)aminium hexachloroantimonate or ceric ammonium nitrate) or electrochemically (equation 10). 

Crystalline cation-radical salts are generally isolated via three preparative procedures by using a nitrosonium (NO+) salt, antimony pentachloride (SbCIs), or triethyloxonium hexachloroantimonate (Et30+SbCl ) as a mild one-electron oxidant. 

The strongly oxidizing SbCl5 is an effective oxidant for the preparation of cation-radical salts of hexachloroantimonate (SbCl ) from a variety of organic donors, such as para-substituted triarylamines, fully-substituted hydroquinone ethers, tetraarylethylenes, etc.176 For example, the treatment of the hydroquinone ether EA (2 mmol) with SbCl5 (3 mmol) in anhydrous dichloromethane at — 78°C immediately results in an orange-red solution from which the crystalline cation radical salt readily precipitates in quantitative yield upon the slow addition of anhydrous diethyl ether (or hexane)173 (equation 36). 

The ready separation of the hexachloroantimonate salts of various cation radicals is possible owing to their insolubility in diethyl ether (or hexane) under conditions in which the reduced antimony(III) chloride is highly soluble. In the case of EA+ SbClg", the isolated product is quite pure as determined by iodometric titration. However in many other cases, the Lewis acid SbCl5 effects... 

Triethyloxonium hexachloroantimonate is a selective and mild one-electron oxidant for the facile preparation and isolation of crystalline cation-radical salts from a variety of aromatic and olefinic donors.177 Thus, in a general procedure, a slurry of Et30 + SbCl (3 mmol) and dimethoxytriptycene, DMT (2 mmol) is stirred in dichloromethane at 0°C. The heterogeneous mixture immediately takes on a bright yellow-green coloration (lmax = 466 nm), and on continued stirring (for about 1 h) it is transformed to yield a dark solution of DMT+. The highly pure DMT+ SbCl salt is isolated in quantitative yield by precipitation with diethyl ether (equation 38). 

A straightforward synthesis for [l,2,4]triazolo[2,3-c][l,3,5]oxadiazinium salts has been described by Hamed et al. <1995JPR274>. The transformation of azaallenium hexachloroantimonate 98 with benzhydrazone was carried out in dichloroethane under reflux conditions to yield the fused quaternary salt 99. 

Scheme 5.1 Mechanism of thioglycoside activation (a) by thiophiles X1 such as /V-bromosuccinimicle (NBS), 11,12 methyl triflate,13 dimethyl(methylthio)sulfonium triflate (DMTST),14 phenylselenyl triflate (PhSeOTf),17,18 iV-iodosuccinimide/triflic acid (MS/TfOH),19 20 and iodonium di-sym-collidine perchlorate (IDCP)21 (b) by tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA,+)25 and (c) via anomeric sulfoxides.26 The stereochemical outcome of these glycosylations follows the same general trends as with many other glycosyl donor/promoter combinations (m-CPBA = mcta-chloroperbenzoic acid).

As far as the polymerisations of DCA other than DXL by non-protonic initiators are concerned, Kops and Spanggaard [11] favour the ring expansion mechanism for the dimerisation and polymerisation of the cis- and tnms-7,9-dioxabicyclo[4.3.0] nonanes by phosphorus pentafluoride or triethyloxonium hexachloroantimonate, although they have not obtained any definitive evidence for it. 

While ki2 in AN is tenfold higher than in DE, k2i in AN is only three times its value in DE. These findings are in accordance with the relationship between the formation constant of hexachloroantimonate and the donicity of the utilized solvent, as has been stated in Sect. 3.1. The values for the equilibrium constants [SbCle]" obtained from the kinetic measurements are in agreement102 with those found from equilibrium studies4 in the respective solvents (Fig. 17). In solvents of very low donicity the /f[SbCl6 ] values are lower than expected on the basis of the DN-Ai[SbCl6 ]" plot. This may be attributed to the presence of polymeric SbCl5 units, and thus to the involvement of a second equilibrium. 

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