Dialkyl sulphoxides reactions

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) ... 

The reactions between PCI5 and alkenes or alkynes have already been discussed extensively in connection with the synthesis of a variety of phosphonic acid types (as their acid dichlorides). Successful applications have also used alkenyl alkyl sulphides (to give the dichlorides 381) to an enyne (to give the dichloride 382) and to RSC CCl (to give 383)584,585 variation of the Pummerer reaction consists in the interaction of PCI5 and a dialkyl sulphoxide, during which a trichlorophosphonium salt intermediate is decomposed with SO2 to yield a (2-alkylthioethenyl)phosphonic dichloride (Scheme 34). ... 

Some limitations of the subject surveyed have been necessary in order to keep the size of the chapter within the reasonable bounds. Accordingly, to make it not too long and readable, the discussion of the methods of the sulphoxide synthesis will be divided into three parts. In the first part, all the general methods of the synthesis of sulphoxides will be briefly presented. In the second one, methods for the preparation of optically active sulphoxides will be discussed. The last part will include the synthetic procedures leading to functionalized sulphoxides starting from simple dialkyl or arylalkyl sulphoxides. In this part, however, the synthesis of achiral, racemic and optically active sulphoxides will be treated together. Each section and subsection includes, where possible, some considerations of mechanistic aspects as well as short comments on the scope and limitations of the particular reaction under discussion. 

Olah and coworkers56 found that treatment of dialkyl, arylalkyl and diaryl sulphides with nitronium hexafluorophosphate (or tetrafluoroborate) 32 at —78° in methylene chloride resulted in the formation of sulphoxides in moderate to high yields (Table 3). In the oxidation of diphenyl sulphide which affords diphenyl sulphoxide in 95% yield, small amounts of the ring nitration products (o- and p-nitrophenyl phenyl sulphides) were formed. However, diphenyl sulphone and nitrophenyl phenyl sulphoxide were not detected among the reaction products. 

It was reported earlier that even dialkyl sulphides are efficiently oxidized to sulphoxides without a concomitant C—S bond cleavage by NBS or N-chlorosuccinimide (NCS) when the reaction is performed in anhydrous methanol at low temperature103. iV-Chloro-Nylon-6,6 in methanol-water or dioxane-water104 and iV-bromo- -caprolactam in water or alcohols105 were also used successfully for oxidation of sulphides. 

Photochemical synthesis of sulphoxides was reported for the first time by Foote and Peters111 in 1971. They found that dialkyl sulphides undergo smoothly dye-photosensitized oxidation to give sulphoxides (equation 32). This oxidation reaction has been postulated to proceed through an intermediate adduct 63, which could be a zwitterionic peroxide, a diradical or cyclic peroxide, which then reacts with a second molecule of sulphide to give the sulphoxide (equation 33). 

Rajanikanth and Ravindranath44 have recently published a deoxygenation reaction for sulphoxides that uses metallic lithium in refluxing dimethoxyethane. Dialkyl and alkyl phenyl sulphoxides were reduced cleanly in yields around 70%, even if sterically hindered, but benzyl sulphoxides gave mixtures of products. For example, benzyl phenyl sulphoxide gave frans-stilbene (33%), benzyl phenyl sulphide (20%) and diphenyl disulphide (47%). These products can be rationalized by reaction pathways such as in equation (17) ... 

A high yield synthesis involving formal oxidation of a sulphoxide to a sulphoximine has been reported183 using O-mesitylenesulphonyl hydroxylamine (NH2OMes) (equation 67). The reaction is successful for a wide range of sulphoxides including dialkyl, diaryl and cyclic species. 

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. 

An alternative route to sulphones utilizes the reaction of the appropriate activated halide with sodium dithionite or sodium hydroxymethanesulphinite [6], This procedure is limited to the preparation of symmetrical dialkyl sulphones and, although as a one-step reaction from the alkyl halide it is superior to the two-step oxidative route from the dialkyl sulphides, the overall yields tend to be moderately low (the best yield of 62% for dibenzyl sulphoxide using sodium dithionite is obtained after 20 hours at 120°C). The mechanism proposed for the reaction of sodium hydroxymethanesulphinite is shown in Scheme 4.20. The reaction is promoted by the addition of base and the best yield is obtained using Aliquat in the presence of potassium carbonate. It is noteworthy, however, that a comparable yield can be obtained in the absence of the catalyst. The reaction of phenacyl halides with sodium hydroxy-methane sulphinite leads to reductive dehalogenation [7]. 

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