Dimethyl selenoxide

Dichloro-5,6-dicyan o-l, 4-benzo-quinone. Dimethyl selenoxide. Dimethyl sulfoxide-Oxalyl chloride. 

Furthermore, the first catalytic synthesis of allenes with high enantiomeric purity [15c, 25] was applied recently to the pheromone 12 by Ogasawara and Hayashi [26] (Scheme 18.7). Their palladium-catalyzed SN2 -substitution process of the bromo-diene 16 with dimethyl malonate in the presence of cesium tert-butanolate and catalytic amounts of the chiral ligand (R)-Segphos furnished allene 17 with 77% ee. Subsequent transformation into the desired target molecule 12 via decarboxylation and selenoxide elimination proceeded without appreciable loss of stereochemical purity and again (cf. Scheme 18.5) led to the formation of the allenic pheromone in practically the same enantiomeric ratio as in the natural sample. 

Several cyclobutenes can be synthesized by elimination of selenoxides from the corresponding methyl- or phenylselanyl derivatives 58 or by elimination of dimethyl selenide from the corresponding selenonium salts 60. The results of these conversions are summarized in Table l.23 The cyclobutenes have been used as precursors for substituted butadienes.23... 

The first synthesis of (R)-4,5-dihydro-37/-dinaphtho[2,l-f l, 2 -i ]selenepin oxide 110 has been achieved from (R)-(+)-l,l -bi-2-naphthol, which in turn was obtained by resolution of raol,l -bi-2-naphthol. Palladium-catalyzed alkoxy carbonylation of the alcohol 108 gave a dimethyl ester which was then reduced by LiAlfLi, and the resultant diol converted to key intermediate chloride 109. Cyclization with sodium selenide gave a novel enantiomerically pure selenide, which upon oxidation yielded the desired selenoxide 110 <2000SC2975>. 

J.l Oxidation with Dimethyl Sulfoxide 4.422 Oxidation with Selenoxides... 

Use of dimethyl selenoxide or di(4-anisyl) selenoxide in place of DMSO is claimed to offer some advantages in terms of mildness for polysubstituted or base sensitive benzyl halides. Problems of cost, accessibility and toxicity are not addressed. 

The oxidation of selenoxides to selenones is slow requiring drastic conditions to be used. Diaryl, aryl methyl and dimethyl selenones are prepared by the oxidation of the ctxresponding selenoxides with prolonged exposure to KMn04 or ozone. The direct oxidation of selenides to selenones by FWO with ruthenium(II) complex catalyst, and C u(Mn04)2 has also been described. Aiyl trifluo-... 

The isolable and thermally stable selenoxides are, therefore, rather limited. Stable examples are as follows those derived from selenides which have no hydrogen atoms on the -carbon, such as dimethyl sel-enide, aryl methyl selenides, diaryl selenides and benzyl phenyl selenides, those with an intramolecular hydrogen bonding, such as (36) and (37), and those leading to an unfavorable double bond such as (38). Vinylic selenoxides (39) and (40) are also generally isolable. 

Moreover, tram-33 is also obtained in 77% yield on reaction of acetoxymethy] methyl selenide with cyclohcxene in the presence of hydrogen peroxide 7 or on heating cyclohexene with an excess of dimethyl selenoxide in a mixture of acetic acid and chloroform 8. 

Although the first alkynylselenonium salt, ethylmethylphenylethynyl sele-nonium picrate,was prepared as early as 1965 [10], it is only very recently that the first reactions of selenonium salts have been reported [11]. Among these salts, acyclic dimethyl(phenylethynyl)selenonium tetrafluoroborate (9) was prepared by methylation of methyl phenylethynyl selenide (10) with Meer-wein s reagent. The acyclic derivative 11 and the cyclic analogue 12 were synthesized by treatment of trimethyl(phenylethynyl)silane (13) and triflu-oromethanesulfonic anhydride with diphenyl selenoxide (14) and dibenzose-lenophene 5-oxide (15), respectively (Scheme 2). 

The first application of dimethyl selenoxide (79) as an oxidant to convert selected organophosphorus compounds 80 to their phosphoryl analogues 81 was reported as early as 1978 [35] (Eq. 15). 

The reaction of 5-(trimethylsilyl)cyclopentadiene (82) with dimethyl selenoxide (79) was reported [40] to afford the selenoniocyclopentadiene 83 in almost quantitative yield (Eq. 16). The formation of the selenonium salts 7,11, and 12 in the reaction of the appropriate selenoxide with triflic anhydride has been discussed earlier (see Sect. 2). 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

Acetoxymethyl methyl selenide, CH3SeCH20C0CH3. Mol. wt. 151.06, b.p. 51-52°/10 mm. The selenide is prepared by the Pummerer reaction of dimethyl selenoxide (6, 224) with acetic acid (29.2% yield). 

Dimethyl selenoxide, CHaSeCHa. Mol. wt. 125.03, m.p. 94°. The substance is prepared by oxidation of (CHajaSeBra with AgaO in CHaOH. ... 

Oxidations. Dimethyl selenoxide is more effective than dimethyl sulfoxide (4,194 6, 227) for oxidation of trivalent phosphorus and thiocarbonyl compounds. It oxidizes acyclic P(III) compounds to oxides (with nearly complete inversion of configuration) cyclic P(III) compounds are oxidized with retention of configuration. The same stereochemical result obtains in oxidations with DMSO. 

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