Diaryl sulphoxides

In 1887 Colby and McLaughlin175 found that treatment of benzene with thionyl chloride in the presence of aluminium trichloride produces diphenyl sulphoxide probably via benzenesulphinyl chloride. Later on, some other diaryl sulphoxides were prepared by this procedure176-180 (equation 66 Table 10). Highly reactive aromatic compounds such as naphthyl ethers react with thionyl chloride in the absence of a catalyst181. 

TABLE 10. Diaryl sulphoxides, Ar2SO, from aromatic compounds and thionyl chloride in the presence of A1C13... 

Hydroxy substituted diaryl sulphoxides 126 were prepared by the condensation of m-substituted phenols with arenesulphinyl chlorides in the presence of aluminium trichloride185 (equation 68). 

Chiral alcohols have also been used in an asymmetric synthesis of sulphoxides based on halogenation of sulphides. Johnson and coworkers have found319 that the reaction of benzyl p-tolyl sulphide with JV-chlorobenzotriazole (NCBT) followed by addition of (—) menthol and silver tetrafluoroborate afforded diastereoisomeric menthoxysulphonium salts 267 which, upon recrystallization and hydrolysis, gave benzyl p-tolyl sulphoxide with 87% optical purity (equation 145). More recently, Oae and coworkers reported320 that optically active diaryl sulphoxides (e.e. up to 20%) were formed either by hydrolysis or thermolysis of the corresponding diaryl menthoxysulphonium salts prepared in situ from diaryl sulphides using ( —) menthol and t-butyl hypochlorite. 

The Andersen sulphoxide synthesis is general in scope and a large number of chiral alkyl aryl and diaryl sulphoxides became available from (—)-(S)-276 and other optically active sulphinates343-346 (Table 16). 

However, Gillespie contested Szmant s results158 and proposed that diaryl sulphoxides are initially protonated on the oxygen atom and then undergo a rather rapid sulphonation in the aromatic rings, and concluded that sulphoxides are stronger bases than sulphones. 

They have also demonstrated that a linear energy relationship exists between pKa values of sulphoxides and the hydrogen bonding ability which was determined by IR-stretching shift of the OH band in PhOH in CC14. Oae and coworkers173 determined the pKa values of diaryl sulphoxides and found that the substituent effect does not correlate with the Hammett o values but approximately with the a values. The pKa values thus obtained are summarized in Table 19. 

Although dichloroborane has been successfully used to reduce dialkyl sulphoxides, it was less useful for the reduction of diaryl sulphoxides and recent work26 has found that fully substituted bromoboranes can fill this gap. Three reagents were introduced, viz. bromodimethylborane, 9-bromo-9-borabicyclo[3.3.1]nonane (9-BBN-Br) and tribromo-borane, the reactions being shown in equation (5) ... 

Oxidative chlorination of diaryl sulphoxides with chlorine is generally much more useful than for alkyl-containing substrates. The sulphone is obtained cleanly, with no (or little) halogenation94 The rate of reaction is first-order with respect to both the sulphoxide and chlorine and has an order of minus one with respect to chloride ion. This is consistent with the mechanism given in equation (29). 

In aqueous pyridine solution, most diaryl sulphoxides may be oxidized to the corresponding sulphones with (dichloroiodo)benzene in reasonable yields103. The reaction involves nucleophilic attack by the sulphoxide on the electrophilic chlorine-containing species, yielding an intermediate chlorosulphonium ion which then reacts with water producing the sulphone. If the sulphoxide is optically active, then an optically active sulphone is produced in excellent optical yield when the reaction is carried out in oxygen-18 labelled water104, as indicated in equation (33). 

Photolysis191 and flash vacuum pyrolysis192 of diaryl sulphoxides both lead to the formation of thiosulphonates as shown in equation (73). This reaction presumably occurs by initial S—C bond scission followed by combination of two arylsulphinyl radicals as depicted in equation (74). 

Water insoluble tetrabutylammonium metaperiodate, which can be prepared from sodium metaperiodate and tetrabutylammonium hydrogen sulphate in aqueous solution, was found to be a useful reagent for the selective oxidation of sulphides in organic solvents . The reaction was generally carried out in boiling chloroform and gave dialkyl, alkyl aryl and diaryl sulphoxides in yields which are comparable with those reported for sodium metaperiodate in aqueous methanol solution (Table 4). In the case of diaryl sulphoxides, the yields decrease with prolonged reaction time. 

Electron-donating groups in the para position of the perbenzoic acid tend to decrease the rate of reaction . The reverse seems to be true when these groups are present in a 4,4 -disubstituted diaryl sulphoxide. The effect of ring size on the oxidation of cyclic sulphoxides is apparently very small in dioxane-water solution under either acidic or basic conditions . This suggests that no major hybridisation change occurs for the sulphur atom in going to the transition state. 

The anodic oxidation reaction of sulphoxides was not much studied, and just a few reports are available so far. The conversion into the corresponding sulphones of some phenyl alkyl and diaryl sulphoxides (oxidation potential for 86 + 2.07 V vs. SCE in acetonitrile/NaC104 electrolyte, Pt anode) has been reported. Similarly, diphenyl suiphoxide was long known to be transformed in a quantitative yield into the sulphone (Pt anode, solvent glacial acetic acid). Additional examples of the oxidation of a suiphoxide function attached to aryl groups are available . 

Sulphones and sulphonamides—the S -b O class —are similarly powerful directors , and do not suffer electrophilic attack at sulphur (though they occasionally suffer nucleophilic attack on the aromatic ring) but are less useful because of their more limited synthetic applications -. Aryl f-butylsulphoxides are also powerful ortljo-directors , but less hindered diaryl sulphoxides are susceptible to attack by organolithiums at sulphur. ... 

Reaction of hydrazoic acid with the monoaryl sulphoxides (345) affords benzothiazine oxides (346) and benzothiadiazepine oxides (347), and the same reagent converts the diaryl sulphoxides (348) into the benzothiadiazine oxides (349), which undergo a base-catalysed rearrangement to (350), cf. (228) - (229a). JV-5-Alkylation of the latter occurs on treatment with sodamide and alkyl halides, and reduction of the lactam moiety is brought about by LiAlH4. ... 

New reagents for the sulphoxide to sulphide reduction include cyanuric chloride and fluoride (the former is only suitable for diaryl sulphoxides). 

The basicity-structure relationships applying to a series of diaryl sulphoxides are concluded to be dominated by the inductive effects of the aryl substituents. ... 

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