Reaction at Sulphur

Reaction at Sulphur.—By the use of S-tracer it was shown that diben-zothiophen does not exchange sulphur on heating. Methods for the synthesis of 5-aryl- or 5-alkyl-dibenzothiophenium salts have been worked out. The reaction of S-aryldibenzothiophenium fluoroborates with aryl-lithium was shown to proceed according to two routes, leading to biaryl and 

Murata, T. Tatsuoka, and Y. Sugihara, Tetrahedron Letters, 1S174, 199. 

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S to N transfer of the nitroso group has also been postulated on a number of other occasions53,71 to explain enhanced reaction rates. Detailed kinetic studies on the nitrosa-tion of thioproline72 (12) and thiomorpholine73 (13) reveal that two pathways can exist, which depend on the reaction conditions, (a) a direct reaction at nitrogen and (b) a direct reaction at sulphur followed by S to N migration of the nitroso group. 

In other reactions, cyanide functions rather simply as a nucleophile. The organic examples are numerous. Reaction at sulphur to form thiocyanate is a convenient inorganic example this occurs with thiosulphate tetrathionate and pentathionate, and with sulphur (Sg and Sg) itself The reactions which... 

A theoretical analysis has been made of che dual reactivity of salts of phosphorus(V) thio acids. In alkylation reactions, the variation in products brought about by changes in alkylating agent can be illustrated by reference to the diethyl phosphorothioate anion for this, triethoxonium fluoroborate yields almost exclusively the S-ethyl derivative, alkyl tosylates in HMPT react to yield both S and 0-alkyl derivatives, and chlorotrimethylsilane reacts only at oxygen. S-alkylation occurs almost exclusively in non-polar or weakly polar solvents, but also in ethanol. In HMPT the S 0 ratio was shown to depend on the substituents attached to phosphorus. It was suggested that 0-alkylation occurs when the free ion is available, whereas reaction at sulphur occurs when the oxygen end of the 0-P-S triad is preferentially solvated and so shielded. 

Reaction at Sulphur Thiophen Dioxides. - The stable S-ylide (167) was formed, in 40% yield, in the reaction of 2-isobutenylthiophen with dimethyl diazomalonate in the presence of rhodium acetate.The transient thiophen... 

Reaction at Sulphur Thiophen Dioxides.— Treatment of thiophenhydroximine-dicarboxylic acids (115) with Raney nickel gave 2-amino-octanedioic, 2-aminodecanedioic, and other aliphatic 2-amino-dicarboxylic acids. ... 

Reaction at Sulphur.— The kinetics of the oxidation of benzo[/>]thiophen and its 3-substituted derivatives by perbenzoic acid in dichloromethane at 30 °C have been studied. The rate constants obtained for the first step, the oxidation to the sulphoxide, have been correlated with values, whereas the rate constants for the second step, of oxidation to the sulphone, have been correlated with values. A number of benzo[ft]thiophen and dibenzothiophen derivatives and their sulphoxides have been S- and O-alkylated, respectively, with silver perchlorate and alkyl halides. 5-Methoxydibenzothiophenium perchlorate reacted with amines to give 5-aminosulphonium salts, and with certain carbanions to give zwitterionic compounds such as (394). 

Reaction at Sulphur.—The oxidation of benzo[6]thiophens to sulphoxides by t-butyl hypochlorite has been studied. In the presence of a ruthenium catalyst, dibenzothiophen was oxidized by oxygen to the sulphone. The reactions of benzo[ >]thiophen 1,1-dioxides with various reagents such as amines, amino-alcohols,3-aminopropylsilanes, and aromatic sulphonyl chlorides have been studied. Four detailed papers on the reaction of dibenzophenonium salts with aryl-lithiums have appeared in which the mechanism of ligand exchange was elucidated. " ... 

Large quantities are now manufactured by the reaction between sulphur vapour and methane at a temperature of 900-1000 K in the presence of a clay catalyst ... 

The possibility that the rate of reaction of benzene is affected by the phenomenon of reaction at the encounter rate is a matter of importance, because benzene is the datum relative to which comparisons of reactivity are made. Up to 68 % sulphuric acid the slope of a plot of log [kffi moU s i) against — + log is unity for data relating to 25 °C, and... 

A very interesting approach to optically active sulphoxides, based on a kinetic resolution in a Pummerer-type reaction with optically active a-phenylbutyric acid chloride 269 in the presence of /V,A -dimethyIaniline, was reported by Juge and Kagan332 (equation 149). In contrast to the asymmetric reductions discussed above, this procedure afforded the recovered sulphoxides in optical yields up to 70%. Chiral a, /1-unsaturated sulphoxides 270 were prepared via a kinetic resolution elaborated by Marchese and coworkers333. They found that elimination of HX from racemic /i-halogenosulphoxides 271 in the presence of chiral tertiary amines takes place in an asymmetric way leading to both sulphoxides 270 and 271, which are optically active (optical yields up to 20%) with opposite configurations at sulphur (equation 150). 

Still has also carried out mechanistic experiments9 3 from which he could deduce that the major reduction pathway is by attack of hydride ion at the sulphur atom. This conclusion was deduced from the fact that reduction with sodium borodeuteride-aluminium oxide gave a sulphoxide that had only incorporated about 25% mole equivalent of deuterium on to a methyl carbon atom bound to the sulphur atom. The mechanistic pathway for direct reduction is outlined in equation (38), whereas the pathway whereby deuterium could be incorporated is portrayed in equation (39). These reactions support the proposed mechanism for the hydride reduction of sulphones as outlined in Section III.A.l, namely that attack at sulphur by hydride ions may occur, but will be competitive with proton abstraction in cases when the attack at sulphur is not facilitated. 

In all the desulphonylation reactions discussed in Sections III.B and III.C the sulphur is lost from the starting sulphone and is reduced in the process simultaneously, the former carbon-sulphur bond is either reduced to a C—H bond or is converted into a C=C bond. The reactions described in this section have the common thread that the sulphur atom is lost with reduction at sulphur, but the carbon atom is converted directly into a carbonyl group. Formally, these reactions offer a route from alkyl halides to aldehydes or ketones. 

An excellent review of the preparation and chemistry of sulphoximines (also known as sulphoximides) has been published177. Here, some of the more interesting and important reactions amounting to oxidation at sulphur will be considered. 

Two contrasting conclusions have been reported in the reactions of lithium aluminium hydride in THF with phosphine oxides and phosphine sulphides respectively. The secondary oxide, phenyl-a-phenylethylphos-phine oxide (42), has been found to be racemized very rapidly by lithium aluminium hydride, and this observation casts some doubt on earlier reports of the preparation of optically active secondary oxides by reduction of menthyl phosphinates with this reagent. A similar study of the treatment of (/ )-(+ )-methyl-n-propylphenylphosphine sulphide (43) with lithium aluminium hydride has revealed no racemization. These results have been rationalized on the basis of the preferred site of attack of hydride on the complexed intermediate (44), which, in the case of phosphine oxides (X = O), is at phosphorus, and in the case of the sulphides (X = S), is at sulphur. Such behaviour is comparable to that observed during the reduction of phosphine oxides and sulphides with hexachlorodisilane. ... 

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

Despite the aversion of the sulphonyl group to function as a nucleophile, there are a limited number of literature reports that deseribe sueh a process . The first of these does not lead to a reduetion at sulphur and is thus not direetly relevant, but the report by Braverman and Duar deseribes reaetions in whieh an acetylenie sulphinate ester ean undergo a [2,3] sigmatropie rearrangement to an allenie t-butyl sulphone. Reaetion of this sulphone with the appropriate eleetrophile, for example bromine, gave a y-sultine, formation of whieh requires the partieipation of the sulphonyl group in an intramoleeular cyclization process. The reactions are outlined in equation (40). 

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