Sulphenylation electrophilic

In contrast to the early theoretical work of Rank and coworkers , C-NMR investigations had revealed that the metallated carbon atom in the a-sulphinyl carbanion is nearly planar . A four-centre chelate structure 315 has been proposed for a-lithiosulphoxides, and it is believed to be responsible for the planar configuration of the anionic carbon atom and for the greater stability of o(-sulphinyl carbanions in comparison with a-sulphenyl carbanions This chelation favours one of the two diastereoisomeric carbanions and for this reason a-sulphinyl carbanions react with electrophiles in a highly stereoselective manner (see below). 

Addition reactions of sulphenyl halides (in particular chlorides) to acetylene derivatives have been extensively explored and recently reviewed (Modena and Scorrano, 1968). Although free radical processes may be involved under specific conditions, the addition of both arene-and alkanesulphenyl halides normally occurs by an ionic mechanism, the sulphenyl halide sulphur being the electrophilic centre. 

Electrophilic addition of sulphenyl halides to alkenes occurs, by all the evidence, via cyclic thiiranium ions (Mueller, 1969) and a comparison of the rates of addition to the double and triple bond would be quite interesting. Unfortunately, direct kinetic data for strictly comparable and typical cases are not available. Phenylacetylene has been reported (Kharasch and Yannios, 1964) to react 102 times slower than styrene (in acetic acid at 25°) with 2,4-dinitrobenzenesulphenyl chloride. On the other hand, Thaler (1969), by means of competitive experiments carried out in dilute paraffin solutions at — 70°, estimated that methane-sulphenyl chloride adds to mono- (and di-)alkylacetylenes 50-100 times more slowly than to the corresponding alkenes (cis) (but only ca. twice slower than to trans dialkylethylenes). The paucity of information does not allow generalizations and further work in this area seems desirable also with respect to the much larger rate differences observed in those bromine additions to triple and double bonds which also occur via bridged species. 

Ionic additions which transform cyclopropenes into cyclopropanes are well known. The electrophilic addition of sulphenyl halides proceeds by a two-step mechanism to give trans-disubstituted products but with little regioselectivity. Cyclic sulphonium ion intermediates are probably involved as illustrated for 1-methylcyclopropene. Electrophilic... 

Sulphenylation of allyl phosphine oxide anions gives only 7-addition, in contrast to the predominantly a attack with other electrophiles (Sect. 2.1). This is because the product of a attack can isomerise by a [1,3] shift22. The product of the reaction is a mixture of allyl isomers 156 and 157. These both give the anion 158 on treat-... 

New reactions of various phosphazenes with a wide range of electrophilic species continue to be reported. These include those of alkoxy-derivatives with dialkyl sulphates, e.g. to give (56), or with sulphenyl chlorides, e.g. the reaction that produces (57) (see Scheme 4). With pyridine, compound (56) can be converted into two isomeric products (58) and (59). 

Electrophilic Addition.— The use of electrophilic fluorinating agents has been reviewed. A review on organoselenium chemistry includes data on the addition of PhSeBr to steroidal olefins. An improved procedure for chloro-sulphenylation was exemplified by treatment of 5a-cholest-2-ene with thio-phenol and iV-chlorosuccinimide. The resultant adduct (35) was readily converted into the unsaturated aryl sulphone (36). Bromine fluoride is conveniently... 

The further additions of electrophilic reagents to activated carbon-carbon double bonds in phosphonic derivatives include those of sulphenyl chlorides (equations 24 and 25). 

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... 

By far the most evidence for electrophilic sulphur is found in the sulphenyl halides (RSCl, RSBr) Although these compounds may in theory react either as sources of RS or X (X = halogen), none of the observed reactions of the sulphenyl halides indicate the latter mode of heterolysis. Kharasch et have presented good evidence for the existence of the 2,4-dinitrobenzene-sulphenium ion (Ar ) in strongly acidic media evidence has also been presented of a strong solvent effect upon the rate of reaction of 2,4-dinitroben-zenesulphenyl chloride and cyclohexene - and of a definite substituent effect in the reaction of this sulphenyl chloride with some substituted styrenes in acetic acid . Such observations are entirely consistent with an electrophilic heterolytic ad tion mechanism involving attack by the sulphenyl chloride in the sense iC -Cl. 

There is an example of electrophilic substitution occurring in a side-chain as well as in the azulene ring. Sulphenylation of guaiazulene with trifluoromethylsulphonyl chloride leads to substitution in the 4-methyl and 7-isopropyl groups as well as at the 3-position [138]. 

PBD has some blocky character. An explanation of this effect is that the (f-orbitals of the sulphur atom in a modified PBD unit overlap the re-orbitals of an adjacent double bond and thus activate it towards electrophilic attack of another sulphenyl chloride molecule (Scheme 5). This theory predicts that... 

Olah, continuing his studies on electrophilic reactions at sigma bonds, has described the chlorination (carbon-hydrogen fission) and chlorolysis (carbon-carbon fission) of alkanes. Successive reaction of a thiol with chlorocarbonyl sulphenyl chloride and triphenylphosphine effects conversion into the corresponding alkyl chloride (Scheme 136). 

Dinitrobenzenesulphenyl and o-nitrobenzhydryl esters are carboxyl protective groups which can be removed by photolysis under mild and completely neutral conditions and give the carboxylic acid in high yield (85—98%) [47, 124]. The major by-product in the decomposition of the 2,4-dinitrobenzene-sulphenyl ester (45) is the sulphide (47) formed by electrophilic attack of the cation (46) on the solvent. o-Nitrosobenzo-phenone (48) is formed as the by-product in the decomposition of the nitrobenzhydryl ester. 

Easily prepared thiocarbonyl derivatives of alcohols, such as the 5-methyl dithio-carbonates (64 Y == SMe), can be treated with soft electrophiles A-X to improve the leaving ability of the alcohol oxygen in the presence of a nucleophile and thus provide a simple mild method of hydroxy group replacement (Scheme 27). For instance, use of CH3I as the electrophile produces alkyl iodides, and sulphenyl halides PhSX can be utilized to synthesize chlorides, bromides, or iodides. Similar reactions have been described for some selenocarbonyl alcohol derivatives. ... 

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