Sulphonium salts cyclic

Few 1 -benzothiophene-S-oxides 218 were obtained in moderate yields by treatment of 1-arylacetylenes 219 with sulfur dioxide and benzene in the presence of antimony pentafluoride250 (equation 127). A series of cyclic sulphoxides have been prepared by hydrolysis of the corresponding alkoxy sulphonium salts 220251-254 (equation 128). Syn-sulphoxide 221 was obtained in a low yield (15-20%) in the reaction of the dianion of cyclooctatetraene 222 with thionyl chloride255 (equation 129). 

By contrast, O in (43) is sufficiently electronegative not to donate an electron pair (unlike Oe in ROe and RCO20 above), and hydrolysis of EtOCH2CH2Cl thus proceeds via ordinary SN2 attack by an external nucleophile—which is likely to be very much slower than the internal nucleophilic attack in (42) — (44). That a cyclic sulphonium salt such as (44) is involved is demonstrated by the hydrolysis of the analogue (45), which yields two alcohols (the unexpected one in greater yield) indicating the participation of the unsymmetrical intermediate (46) ... 

Reduction of sulphonium salts polarographic half-wave potentials, Ey. ref. [54], in water cyclic voltammetry peak potentials, Ep ref. [55], in acetonitrile at glassy carbon, scan rate 50 mV s. ... 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

On treatment with nucleophiles, the cyclic sulphonium salt (20) can react in three different ways, sulphur compounds giving ethyl 1-thioglycoside derivatives with the nucleophile bonded to C-6. 

PPV (62a) has been prepared by a number of different methods which were studied in detail by Horhold and Opfermann [390]. It can be synthesized by bifunctional carbonyl olefination of terephthalaldehyde according to Wittig s reaction and from />-xylylene-bis(diethyl phosphonate), as well as by dehydrochlorination of/p-xylylene dichloride with sodium hydride in A jiV-dimethylformamide and with potassium amide in liquid ammonia. The most popular route to PPV used today is the precursor route, first described by Wessling and Zimmermann [391-394] and Kanabe and Okawara [395], starting from the monomers p-xylylene-bis(di-methylsulphonium tetrafluoroborate) [395] or chloride (Scheme 11.17) [391-394]. The latter is polymerized to yield a water soluble sulphonium salt polyelectrolyte (63d) which is then purified by dialysis [396]. The precursor polymer is converted to PPV (63e) by the thermal elimination of dimethyl sulphide and HCl. The method was later developed by Horhold et al. [397], Lenz and co-workers [398,399], Murase et al. [400] and Bradley [401], One of the major improvements within the last years has been the use of tetrahydrothio-phene instead of dimethyl sulphide in the synthesis of the precursor polymer [402]. The use of the cyclic leaving group facilitates the elimination when the precursor polymers is heated at 230-300°C and leads... 

Synthesis and Properties.—The two most general methods for preparing sulphonium ylides continue to be a-deprotonation of a sulphonium salt and the reaction of a sulphide (or disulphide with a carbene. A new development involves the reaction of the thianthrene or phenoxathiin cation radical with a dicarbonyl compound, e.g. ethyl benzoylacetate, to give (4). An infrequently used but useful route to sulphonium ylides involves reaction of sulphides e.g. dithia[3,3]cyclophanes with benzyne. A detailed description of a preparation of the sulphonium salt precursor to Trost s diphenylsulphonium cyclopropylide has appeared. The selectivity of ylide formation in the reaction of cyclic and acyclic sulphides with carbenes has been examined and compared with the much... 

Most of the salts capable of polymerising the vinyl monomers already described are also efficient initiators of cyclic monomer polymerisations, but in addition appropriate salts of more stable species such as oxonium, sulphonium and diazonium ions... 

However a slow re-initiation of polymerisation, giving rise to a second slower stage, occurs on formation of a three membered cyclic sulphonium ion salt by intramolecular nucleophilic attack of the penultimate sulphur in the dormant polymer ... 

As mentioned already when a cyclic tertiary sulphonium ion salt [136] was used as an initiator then almost identical results were obtained. However, it was observed that the value of kp fell slightly as the initial catalyst concentration was increased. In fact careful analysis of the data for initiation by Et3 0 BF4 indicates a similar trend. This variation of fep is an indication that more than one ionic species contributes to propagation, and Drijvers and Goethals [130] have analysed in this way data from polymerizations using ethyltetramethylene sulphonium tetrafluoro-borate. 

A number of intramolecular acylations of vinylsilanes have been reported164,165 (equations 52 and 53). Trost has cyclised the thioketal 30 using dimethyl (methylthio)sulphonium fluoroborate which leads to the rearranged product 31166 (see equation 54). Overmann has used iminium salts to achieve both endocyclic and exo-cyclic ring closures167 - 169 (equations 55 and 56). In this latter case it is proposed that 32... 

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