Addition at Sulfur

In reactions not possible with the second-row-element-containing pyrrole and furan, thiophene sulfur can add electrophilic species. Thiophenium salts though not formed efficiently from thiophene itself, are produced in high yields with polyalkyl-substituted thiophenes. The sulfur in such salts is probably tetrahedral, i.e. the sulfur is sp hybridised (CAUTION Methyl fluorosulfonate is highly toxic). 

Even thiophene itself will react with carbenes, at sulfur, to produce isolable thiophenium ylides, and in these, the sulfur is definitely tetrahedral. The rearrangement of thiophenium bis(ethoxycarbonyl) methylide to the 2-substituted thiophene provides a rationalisation for the reaction of thiophene with ethyl diazoacetate, which produces what appears to be the product of carbene addition to the 2,3-double bond perhaps this proceeds via initial attack at sulfur followed by S C-2 rearrangement, then collapse to the cyclopropane. Acid catalyses conversion of the cyclopropanated compound into a thiophene-3-acetic ester. ° 2,5-Dichlorothiophenium bis(methoxycarbonyl)methylide has been used as an efficient source of the carbene simply heating it in an alkene results in the transfer of (Me02C)2C to the alkene.  

Uncontrolled -oxidation of a thiophene leads to 5, 5 -dioxides that from thiophene itself has been isolated, but above -40 °C it dimerises giving eventual products depending on concentration, but with substituted thiophenes the dioxides can be isolated. Peracids or dimethyldioxirane can be used, but do not succeed if there are electron-withdrawing substituents. A solution of fluorine in water (hypofluorous 

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N,N-addition can be understood in terms of the size and spatial distribution of the HOMO (2A2, 19) and LUMO (4Bj, 20). Both orbitals are primarily sulfur-based, so that efficient overlap in the transition state is best achieved via addition at sulfur as opposed to nitrogen. 

Sulfonation can be conducted with naphthalene—92 wt % H2SO4 in a 1 1.1 mole ratio with staged acid addition at 160°C over 2.5 h to give a 93% yield of the desired product (20). Continuous mono sulfonation of naphthalene with 96 wt % sulfuric acid in a cascade reactor at ca 160°C gives... 

Manufacture. Phosphoms sulfochloride is manufactured by the direct addition of sulfur to phosphoms trichloride (50—52). At about 180°C, the reaction proceeds smoothly. Phosphoms trichloride vapor is passed through an excess of sulfur that is either molten or dissolved in an inert solvent. 

At room temperature, the bisulfite pH inflection poiat occurs at pH 4.5 and the monosulfite at pH 9. Analogous equations can be written for magnesium, calcium, and ammonia. The starting raw materials, ia addition to sulfur, are sodium hydroxide, magnesium oxide, calcium carbonate, or ammonia, depending on the base used. The four commercial bases used ia the sulfite process are compared ia Table 4. 

Sodium sulfite undergoes addition of sulfur to form sodium thiosulfate. At acidic pH, the chemistry of sodium sulfite is that of bisulfite, metabisulfite, and sulfur dioxide. 

In the reaction of ethylene with sulfuric acid, several side reactions can lead to yield losses. These involve oxidation, hydrolysis—dehydration, and polymerization, especially at sulfuric acid concentrations >98 wt % the sulfur thoxide can oxidize by cycHc addition processes (99). 

The principal vanadium-bearing ores are generally cmshed, ground, screened, and mixed with a sodium salt, eg, NaCl or Na2C02- This mixture is roasted at ca 850°C and the oxides are converted to water-soluble sodium metavanadate, NaVO. The vanadium is extracted by leaching with water and precipitates at pH 2—3 as sodium hexavanadate, Na V O, a red cake, by the addition of sulfuric acid. This is then fused at 700°C to yield a dense black product which is sold as technical-grade vanadium pentoxide. This product contains a minimum of 86 wt % V20 and a maximum of 6—10 wt % Na20. 

For solvent extraction of pentavalent vanadium as a decavanadate anion, the leach solution is acidified to ca pH 3 by addition of sulfuric acid. Vanadium is extracted in about four countercurrent mixer—settler stages by a 3—5 wt % solution of a tertiary alkyl amine in kerosene. The organic solvent is stripped by a soda-ash or ammonium hydroxide solution, and addition of ammoniacal salts to the rich vanadium strip Hquor yields ammonium metavanadate. A small part of the metavanadate is marketed in that form and some is decomposed at a carefully controlled low temperature to make air-dried or fine granular pentoxide, but most is converted to fused pentoxide by thermal decomposition at ca 450°C, melting at 900°C, then chilling and flaking. 

For solvent extraction of a tetravalent vanadium oxyvanadium cation, the leach solution is acidified to ca pH 1.6—2.0 by addition of sulfuric acid, and the redox potential is adjusted to —250 mV by heating and reaction with iron powder. Vanadium is extracted from the blue solution in ca six countercurrent mixer—settler stages by a kerosene solution of 5—6 wt % di-2-ethyIhexyl phosphoric acid (EHPA) and 3 wt % tributyl phosphate (TBP). The organic solvent is stripped by a 15 wt % sulfuric acid solution. The rich strip Hquor containing ca 50—65 g V20 /L is oxidized batchwise initially at pH 0.3 by addition of sodium chlorate then it is heated to 70°C and agitated during the addition of NH to raise the pH to 0.6. Vanadium pentoxide of 98—99% grade precipitates, is removed by filtration, and then is fused and flaked. 

A portion (950 ml) of the rich eluate was adjusted to pH 6.0 by the addition of sulfuric acid. Ultrawet K (7.0 g) in 70 ml water was added slowly to the neutralized eluate to precipitate kanamycin B dodecylbenzenesulfonate which was collected by filtration after adding filter-aid (Dicalite). The cake was washed with water and extracted with 100 ml methanol. After filtering and washing with methanol, sulfuric acid was added to the filtrate until no more kanamycin B sulfate precipitated. After addition of an equal volume of acetone to provide more complete precipitation, the kanamycin B sulfate was collected by filtration, washed with methanol and dried in vacuo at 50°C. 

While the chemistry of alkyl and allylic sulfoxide anions is similar to that of phosphine oxides, phosphinates and sulfone stabilized anions (Sections 1.5.2.2.1 -2), the situation is further complicated by the additional stereogenic center at sulfur. Therefore in all cases, asymmetric induction may arise from the stereocenter at sulfur. 

The addition of the anion of the racemic 2-methyl-2-propenyl sulfoxides, rac-2-methyl-3-(phenyl-sulfinylpl-propene and /w-3-(rerr-butylsulfinyl)-2-methyl-l-propene to 2-cyclopentenone gives mixtures of (E)- and (Z )-y-l, 4-addition products which are a mixture of diastereomers at sulfur2. The (T )-products usually predominate, with the relative proportions of the (Z)-product increasing as the reaction temperature is increased. No asymmetric induction originating from the stereocenter at sulfur was observed when the sulfoxide substituent was phenyl however, there was a marginal improvement in the case of the (Zi)-product when the sulfoxide substituent was ferf-butyl. 

The addition of the anion of the 1,3-dimethyl-2-butenyl sulfoxides to 2-cyclopentenone was examined2. The anion of rar-2-methyl-4-(phenylsulfinyl)-2-pentene gave a 50 50 mixture of ( )- and (Z)-y-1,4-adducts which differed in the relative configuration of the new stereocenter regarding the stereocenter at sulfur. That is, for either the (Z)- or the ( )-product there is complete asymmetric induction from the stereocenter at sulfur, but in the opposite direction. When the rm-butyl analog, ruc-4-(/wt-butylsulfinyl)-2-methyl-2-pcntcne, was reacted, it gave exclusively the ( )-adduct, likewise as a single diastereomer. 

Lithiated racemic (E)- and (Z)-l-(/er/-butylsulfinyl)-2-methyl-2-butenes undergo addition to give predominantly anti-(E)-y- 1,4-addition products2. The sole difference between the major and minor adducts from the (Z)-sulfoxide with those from the (Z)-sulfoxide is in the relative configuration at sulfur. 

In this chapter the addition of carbon nucleophiles to simple a,j8-unsaturated sulfoxides, a-sulfinyl-a,/ -unsaturated ketones and a-sulfmyl-a,/ -unsaturated lactones will be discussed separately, in most cases the asymmetric induction arises from the chirality at sulfur. 

The configuration at sulfur is mainly responsible for the sense of asymmetric induction. It is of interest to note that butylcopper and lithium dibutylcuprate undergo addition reactions in the opposite stereochemical sense. The adducts can be converted to chiral /i-disubstituted carboxylic acids1. 

In the presence of aqueous sodium hydroxide, 2-phenylthiirane dioxide gives styrene and the sulfinate 119. These results have been interpreted111 in terms of initial nucleophilic attack of hydroxide ion at the carbon atom in the 3-position of the three-membered ring in addition to sulfur dioxide elimination (see equation 48). 

More recently, the addition of cyanide ion, generated from TMS cyanide and cesium fluoride, to a-aziridino N-siflfinyl imines, being chiral either at the a position or at sulfur, has been examined [87] (Scheme 28). The configuration of the newly formed stereocenter was determined only by the chiral (S)-sulfinyl group. In fact, the R configuration (diastereomeric excess, de, 98%) was obtained from either the Q -(ii)-imine 186 or the a-(S)-imine 188, giving 187 and 189, respectively. Acyclic 2,3-diaminonitriles can be obtained... 

The promotor effect of SO2 increases with the amount added to the reaction medium (Fig.3). An effect of the addition of sulfur dioxide has also been observed on the oxidation of decane with an increase of the activation energy expected for such a poisoning. This addition leads to a noticeable decrease of the rate of oxidation at low temperature, where Cu sulfate is stable, but the effect becomes negligible at about 600 K. At this temperature, the conversion of decane estimated by the evolution of the peak e/m = 57, characteristic of the hydrocarbon, is close to 100% with CufTi02 catalysts in presence or not of SO2 (Figure 4). With Cu/Zr02 SO2 inhibits decane oxidation below 640 K. At 640 K a conversion of about 60% is observed in both the presence or absence of additive and an acceleration of oxidation is noticed at higher temperatures. 

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