Sulphonium bases

The sulphides unite with alkyl iodides to form salts of the sulphonium bases —... 

Cahours independently discovered the sulphonium bases,but priority belongs to Adolf von Oefele, who found in Kolbe s laboratory that ethyl iodide combines with ethyl sulphide to form a compound similar to an ammonium salt, from which silver oxide formed the corresponding hydroxide. Von Oefele called the compound a sulfin (sulphine) and regarded the radical QH5I... 

The basic function of the iodine in the iodonium bases, which correspond exactly to the ammonium, sulphonium, and oxonium bases, is most interesting. A molecule of diphenyliodonium iodide has the same atoms as two molecules of iodobenzene and decomposes on heating with liberation of heat (contrast other dissociations such as those of N204, NH4C1, PC15) into two moles of C8H8I. Test with a small sample in a tube. 

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. 

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... 

Sulphonium methylide formation requires the presence of a base, and explains the reaction 21b) leading to 5a-cholest-3-one. When no strong base is present, the oxysulphonium ion... 

A lot of progress in this area is due to the work of Trost, who introduced diphenylsulphonium cyclopropylide and phenylthiocyclopropyl lithium as extremely versatile C3-building blocks. The first reagent is easily available from the corresponding sulphonium salt by deprotonation with suitable bases (either under irreversible conditions with dimsyl sodium, or, preferably, in a reversible manner by employing potassium hydroxide in DMSO). The ylide adds to a,) -unsaturated carbonyl compounds forming... 

Intramolecular photoinduced electron transfer (PET) may lead to the generation of acid (i.e., protons) from an anthracene-based sulphonium salt. An amphiphilic anthracene-based photoacid generator (An-PAG) has been synthesized and was reported to induce acid-catalyzed hydrolytic degradation of certain lipids upon UV exposure (Shum et al., 2001). At present, the application of this technique is limited by the short wavelength required to trigger the response. 

Sulphonium methanides, e.g. (167), which are prepared by the reaction of thiachroman-4-ones with dimethyl diazomalonate, rearranged in the presence of base to give 1-benzothiepins, e.g. (169). This reaction parallels the recent conversion of sulphonyUmines into 1,2-benzothiazepines (e.g., see ref. 104). 

Like these, the sulphonium salts are converted into bases when shaken in aqueous solution with silver oxide —... 

The compound is called diphenyliodonium hydroxide—a name which indicates its relation to ammonium, sulphonium, and phosphonium compounds. Diphenyliodonium hydroxide, like other strong bases, forms well characterized salts with acids, which resemble in solubility the analogous salts of thallium. The compound is of special interest as in it iodine, which under other circumstances is an acid-forming element, plays the part of a base-forming element. 

The magnitude of the effect for the elimination reactions indicates a high degree of bond formation between the base and the proton in the transition state. Proton transfer is extensive in the ammonium compound and the proton is just more than half transferred to the base for the sulphonium salt. These... 

For alpha alkyl substitution, the rate of elimination from ammonium and sulphonium salts increases sharply along the series ethyl < isopropyl < r-butyl when ethoxide in ethanol is the base - . Product studies reflect a similar accelerating effect as isopropyl and r-butyl groups are more easily eliminated from R R fCHslzi OH than ethyl . These alpha substituents aid C -X heterolysis and shift the transition state away from the carbanion-like extreme towards one possessing more double-bond character. 

To introduce chirality at C(3), the key intermediate chosen was the protected hydroxyketone 27 (Scheme 7) [33] which was first reacted with dimethyl sulphonium methylide to give the crystalline allylic epoxide 28 (characterized by m.p. and [aj [33] and X-ray analysis [24]). This was readily transformed into f/ )-3-hydroxy-p-cyclocitral (29) via the intermediate aldehydes 30 and 31. Base-catalysed condensation of 29 with acetone gave f/ j-3-hydroxy-p-io-none (32) which was converted into the corresponding crystalline acetate 33 (characterized by m.p., [ajp and CD [34]). Vinylation of 33 furnished a mixture of epimeric vinyl carbinols 34 which were treated with triphenylphosphine and hydrochloric acid. The Ci5-phosphonium chloride 35 thus obtained was identified by m.p., [aj HPLC, IR and H-NMR [33-35]. Contrary to expectations, no racemization at C(3) or dehydration to 3,4-didehydro compounds occurred under the conditions described. 

Synthesis of Monosaccharide Glycosides.- A very useful review has appeared on newer methods for glycoside formation, particularly those based on 0-alkylation and on the use of glycosyl trichloro-acetimidates, sulphonium salts and fluorides. A shorter paper has... 

Steric Effects.—The consequences upon chemical reaction of non-bonded interactions between enantiomeric pairs of molecules have been discussed an antipodal interaction effect was observed in a reductive camphor dimerization and in a camphor reduction. The full paper on the correlation of the rates of chromic acid oxidation of secondary alcohols to ketones with the strain change in going from the alcohol to the carbonyl product has now appeared. It is concluded that the properties of the product are reflected in the transition state for the oxidation. High yields of hindered carbonyls are available from the corresponding alcohols by reaction with DMSO and trifluoroacetic anhydride (TFAA) indeed, the more hindered the alcohol, the higher the yield of carbonyl compound reported Since the DMSO-TFAA reaction occurs instantaneously at low temperatures (<—50°C), it is possible to oxidize alcohols that form stable sulphonium salts only at low temperature. Thus, ( )-isoborneol reacts at room temperature to give camphene, the product of solvolysis of the sulphonium salt the oxidation product, ( + )-camphor, was obtained by the addition of base at low temperature. 

The Stevens rearrangement of a sulphonium salt involves treatment of the salt with base and leads to migration of a group from sulphur to an adjacent carbon atom (equation 91). Analogous Stevens rearrangement of ammonium salts and the related Wittig rearrangement of ethers are well known. 

A variety of substituted sulphonium ylides have been used in the oxiran-forming reaction with carbonyl compounds. Diphenylsulphonium-allylide (6), generated from the salt with t-butyl-lithium, afforded the expected oxiran in 80% yield upon reaction with cyclohexanone. Use of n-butyl-lithium as the generating base resulted in lower yields and contamination by the oxiran formed from a butylide, the latter probably... 

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