Sulphonium compounds

As S+, isoelectronic for the outer electrons with nitrogen, sulphur can form three bonds as in the sulphonium compounds... 

The fact that sulphur dioxide possesses a dipole moment and is triangular in shape lends support to the view that the contribution of the tetravalcnt structure is very small. A direct indication of the existence of the positively charged trivalent state lies in the existence of the sulphonium compounds (RgS) Cl which are formed readily from thioethers and alkyl halides and behave in solution as strong electrolytes containing the (RsS)" ion. 

Higher molecular weight amines (primary, secondary and tertiary), higher molecular weight quaternary ammonium compounds, quaternized sulphonium compounds... 

The S-adenosyl homocysteine produced in the transmethylation reactions is generally cleaved to adenosine and homocysteine. The latter can be degraded as previously discussed or be remethylated to methionine and eventually regenerate S-adenosyl methionine. Thus the operation of a methionine cycle provides a route whereby one-carbon metabolites reduced through the tetrahydrofolic acid sequence provide methyl groups for biosynthetic pathways. Certain other sulphonium compounds such as... 

This sulphonium compound, often referred to as active methyl , serves as a methyl donor for biological synthesis. The list of compounds which derive methyl groups by transmethylation from SAM is extensive and... 

The transport of MTA in mammalian cells has been investigated only in isolated and perfused rat liver l. Fig. 10 shows that the thioethers methionine and MTA are incorporated at a much higher rate than Ado-Met, probably because of its charged sulphonium pole. The existence of a high affinity permease specific for thioethers and a low affinity system specific for sulphonium compounds has been postulated. 

Sulphonium Compounds, Sulphonic Acids, and Sulphones.—Although thorough studies of the conjugation properties of the sulphonium group are not yet available, the electronic structure of the sulphonium cyclopen-tadienylide (22) has aroused interest/ ir-Bond orders have been calcu-... 

Gleave JL, Hughes ED, Ingold CK. 54. Mechanism of substimtion at a saturated carbon atom. Part III. Kinetics of the degradations of sulphonium compounds. J Chem Soc. 1935 236-244. 

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. 

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy, phenacyl/ cyano- and carbamoylmethylide (easily prepared by the reaction of dibutyl teUurides with the appropriate substituted methyl hahdes, followed by treatment with a base), undergo Wittig-type olefmation reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium yhdes, which are inert towards carbonyl compounds. 

In sharp contrast to the olefmation reaction employing stabilized telluronium ylides, semi-and non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides). 

For a monograph on sulfur ylides. see Trost Melvin Sulfur Ylides Academic Press New York. 1975. For reviews, sec Fava in Bcrnardi Csizmadia Mangini Organic Sulfur Chemistry Elsevier New York. 1985, pp. 299-354 Belkin Polczhaeva Russ. Chem. Rev. 1981, 50, 481-497 Block, in Stirling The Chemistry of the Sulphonium Group, part 2. Wiley New York. 1981. pp. 680-702 Block Reactions of Organosulfur Compounds Academic Press New York. 1978. pp. 91-127. 

The chirality exhibited by the pyramidal sulphur compounds (e.g. sulphonium salts, sulphoxides and sulphinic esters) should be noted, but cannot be considered within the scope of this book. 

S NMR spectra of thiocarbonyl, thiophosphoryl compounds, and of some sulphonium salts have also been published. Machiguchi et al. have reported the 33S NMR spectrum of tropothione41 2, a thiocarbonyl compound, and of its corresponding S-oxide, 3 and sulphenic acid, 4.42... 

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. 

Sulphonium ylides are in certain cases unstable and they undergo further transformation affording useful final products. In this way allylic sulphides and selenides were used to transfer an alkylthio- or alkylseleno-group onto the a-carbon of / -dicarbonyl compounds in the form of their ylides the sequence of reactions were a transylidation followed by [2,3]-sigmatropic rearrangement. 

The elements yield tetravalent derivatives m special circumstances, iu which they function as the basic elements. In the case of oxygen they are termed oxonium salts sulphur yields sulphonium salts whilst selenium and tellurium offer an even wider range of derivatives. Thus, the following types of compounds are now well known ... 

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