Sulphonium cation

The EM refers to the pH-independent reaction which is compared to the attack of water on the dimethyl p-nitrophenyl sulphonium cation Irie and Tanida, 1979... 

Figure 5.1. Variation of half-wave potential (E ) with pH for a-subslituted ketones (a) N-methyl-N-phenacylpiperidinium cation, (b) N-methyl-N-(2-oxopropy )piperidimum cation, (c)diethyl-(phenacyl)-sulphonium cation. Data from refs. (90] and [91].

Sulphonic acid A molecule of the general formula RS020H, where R is an alkyl group. Sulphonium cation A cation of the general formula R3S+. 

Two electrophilic addition pathways for the reactions between alkenes and dialkyl thioxophosphoranesulphenyl bromides (95) have been outlined in a preliminary report. The formation of 00-dialkyl 5-(2-bromoethyl) phosphoro-dithioates (96) is thought to occur via a sulphonium cation, and that of the tetrasulphide (97) to be the result of interaction of dithioate anion with sulphenyl... 

Two-co-ordinate sulphur. Opinions seem divided as to whether nucleophilic attack at two-co-ordinate sulphur,which is an associative process, has a synchronous or an addition-elimination mechanism. For reaction of diaryl sulphides with aromatic hydrocarbons, formation of a sulphonium cation and then of a four-co-ordinate sulphur intermediate has been proposed. The role of the "id orbitals on sulphur in nucleophilic substitutions has been discussed. ... 

From Thiazoles.— Isothiazoles are formed in the photoisomerization of thiazoles. Irradiation of 2,5-diphenylthiazole in benzene affords 3,4-diphenylisothiazole (32%), together with 4,5-diphenylthiazole (8%) and phenanthro[9,10-flf]thiazole (8%). 2,4-Diphenylisothiazole similarly produces minor quantities (3%) of 3,5-diphenylisothiazole. Amongst possible mechanisms of this photorearrangement, the formation of bicyclic intermediates such as (58) or (59), or tricyclic sulphonium cations [(60) or (61)], was considered. ... 

Preliminary observations on the photochemical behaviour of isothiazoles have been briefly reported. The parent base is partially converted into thiazole on irradiation (7% in propylamine, 1% in ether), several unidentified products being formed (15%) side by side. 3-Phenyl- and 3,5-diphenyl-isothiazole are converted into 4-phenyl- (12%) and 2,4-diphenyl-thiazole (48%), respectively, but 5-phenylisothiazole preserves its ring system, merely producing small quantities (2.3%) of the 3-phenyl isomer. The reactions are believed to proceed by way of tricyclic sulphonium cations of the type previously postulated by Kojima and Maeda. It is recalled that the reverse photoisomerization is also on record (see syntheses, above). 

On 100% acrylic materials the quaternary ammonium retarders are used almost exclusively. Other types have been evaluated, however. For example, saturated alkylamines (RNH2 R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as the quaternary types although other factors, such as aqueous solubility at the optimal dyebath pH and resistance to subsequent discoloration, favour the quaternary compounds [34]- On the other hand, bis (hydroxyethyl) coco amine (12.10) had relatively little effect and the amphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethyl-cocoamine oxide (12.12) was quite an effective retarder [34]- Other cationic compounds used [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts, alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives. 

Drijvers and Goethals 52) have reported that excess sulphide functions (monomer and polymer) and diethyl ether have no detectable effect on the dissociation of two sulphonium tetrafluoroborate salts in methylene chloride and nitrobenzene, when present in similar proportions to those in corresponding polymerisation reactions. In contrast to this, however, Jones and Plesch 51) have shown that the dissociation constant of triethyloxonium hexafluorophos-phate in methylene chloride at 0°C increases by a factor of - 2 when small quantities of tetrahydrofuran are added. The latter molecule has a lower dielectric constant than methylene chloride and might therefore be expected to reduce dissociation. These workers have interpreted their results in terms of specific solvation of the cation by ether molecules, with subsequent reduction in the effective charge density of the positive ion and hence in the coulombic force favouring ion pairing, e.g. 

Hydrogen transfer has been induced with macrocyclic receptors bearing 1,4-di-hydropyridyl (DHP) groups. Bound pyridinium substrates are reduced by hydrogen transfer from DHP side chains within the supramolecular species 78 the first-order intracomplex reaction is inhibited and becomes bimolecular on displacement of the bound substrate by complexable cations [5.19]. Reactions with carbonyl or sulphonium substrates have been performed with other DHP containing macrocycles, such as 79 [5.20]. 

The latter on treatment with thallium trifluoroacetate, under carefully defined conditions followed by aqueous work-up, led directly to narwedine (291) [74] in 51% yield. The remarkable transformation is believed to occur via the cationic intermediate 292, formed by an oxidative process, undergoing a 1,2 C-C shift to generate the carbenium ion 293. Aromatisation involving the cleavage of C-Pd bond, followed by hydrolysis of the resulting sulphonium salt 294 liberated the phenol, which smoothly underwent Michael addition to the proximate enone system, furnishing ( )-narwedine (291) in 44% overall yield. 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

In general, polymerizations with these salts are much more efficient than those with the unsubstituted salt. Considering that the structure of the propagating species and the rate of the polymerization (kp) are expected to be the same in all three cases, the enhanced activity may be attributed to the stabilization of the benzyl cation by the substituents. Similar effects were observed with the benzylic sulphonium salts. However, more detailed studies [30] of polymerization and hydrolytic properties of various p-substituted... 

Ammonium Phosphonium Sulphonium Figure 2.1 Examples of organic cations used to provide ionic liquids... 

These are much slower than to the preceding group of monomers, evidently because of the lower reactivity of oxonium, sulphonium, ammonium, phos-phonium and siloxonium, ions. Moreover, monomers with these heteroatoms are strongly basic, and therefore cations are preferentially solvated by the monomers. This reduces the probability of other kinds of transfer to solvent, impurities, etc. Many heterocycles, e. g. A-substituted aziridines, thiethanes [62], tetrahydrofuran [63], under suitable conditions polymerize by a living mechanism, i. e. without transfer. In situations where transfer does occur, it is assumed to proceed by the mechanism disscussed previously, for example by transfer to the counter-ion. With regard to transfer intensity, vinyl ethers can be ordered between the hydrocarbon monomers and the heterocycles. The mechanism of transfer in their polymerization has yet to be studied. 

Cationic polymerization of propylene sulfide provides a good example of a ocess in which, at the first, non-stationary stage, the originally formed active species (strained tertiary sulphonium fons holding a monomer molecule) are converted into the dormant species (non-strained sulfonium fons) which are at equilibrium with the strained ones ... 

The hemideuteriated isotopomeric cations 25 and 26, obtained in equimolar amounts by addition of methylbis(methylthio)sulphonium hexachloroantimonate 27 to deuteriated olefin, E-t-butyl-t-butyl-D -ethylene 28 (equation 38), have been converted to the non-separable hemideuteriated thietanium ions 29 and 30, characterized spectroscopically. These rearrangements have been followed at 25 °C (over periods of 1-16 weeks) by... 

Cationic polymerization is the conversion of low molecular weight monomeric molecules into high molecular weight polymeric ones, via a mechanisn involving stepwise growth of a carbonium (RjC ), carbox-onium (RO=CR2), oxonium (RsO" ), sulphonium (R3S ) or immonium (R2N=CR2) ion. Both vinyl and cyclic monomers are susceptible to polymerization by such intermediates, e.g. 

The cationic polymerization of thietane is generally accepted to proceed in the same manner as that of the oxetanes. The most detailed study was carried out by Goethals et al. [42] on thietane and methyl thietanes in CH2CI2 with Et3 0 BF4 as initiator. The initiation reaction was immediate and quantitative [68]. For these monomers propagation also occurred by nucleophilic attack of the monomer on the a-carbon of the cyclic sulphonium ion. For example, the propagation reaction for... 

The hemideuteriated isotopomeric cations 25 and 26, obtained in equimolar amounts by addition of methylbis(methylthio)sulphonium hexachloroantimonate to... 

In an effort to mimic the proposed cyclization of the germaci tdienyl cation (450) to the cadinane skeleton, Ito et a/.215 have examined the fate of the four hedycaryol phenyl sulphides (451)—(454) on reaction with methyl iodide (cf. Vol. 10, p. 51). In all four cases the products derived (455)—(462) were eudesmane derivatives and this finding has been explained in terms of Hi-induced cycliz-ations, the HI being generated from the initially formed sulphonium salts. De-... 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

These are much slower than to the preceding group of monomers, evidently because of the lower reactivity of oxonium, sulphonium, ammonium, phos-phonium and siloxonium, ions. Moreover, monomers with these heteroatoms are strongly basic, and therefore cations are preferentially solvated by the monomers. This reduces the probability of other kinds of transfer to solvent, impurities, etc. Many heterocycles, e. g. A-substituted aziridines, thiethanes... 

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