Selenonium salts

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Methylation of relatively acidic (pKa < 13) carbon nucleophiles occurs at neutral pH in aqueous media when substituted methylsul-fonium and -selenonium salt are used as electrophiles (e.g., 6.2).37... 

Table 9 Preparation of selenonium salts 69 from the corresponding chloroselenuranes 68 (Scheme 8)...

Optically active sulfonium and selenonium salts are well known and the stereochemistry of the isomers has been studied.1 3 Optically active cyclic diaryl(alkoxy)-sulfonium salts 14, 15, and 16, stabilized by intramolecular sulfur-oxygen interaction, were synthesized in 2000 by reacting optically active spirosulfuranes with trimethyloxonium tetrafluoroborate.29 The absolute configurations were assigned on the basis of the reaction mechanism. The sulfonium salts were hydrolyzed in KHC03aq. to yield optically active sulfoxides in over 86% ee (Scheme 7). 

Alkylation of Etiolates with Chiral Selenonium Salts, Sulfonium Salts and Mixed Sulfates... 

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 reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

Selenides are also nucleophilic and produce isolable selenonium salts (9) when treated with alkyl halides. They are easily oxidized to selenoxides (10) and further to selenones (11) under more forcing conditions (see Section 4). Reduction of selenides to the corresponding hydrocarbons is most conveniently achieved with nickel boride,or with tri-n-butyl- or triphenyltin hydride under radical conditions. " Other reagents for reductive deselenization include Raney nickel, lithium triethylborohydride, and lithium in ethylamine (Scheme 4). Benzylic selenides undergo radical extrusion reactions under thermal or photolytic conditions to produce... 

The structures of organic selenium compounds are closely related to those of the homologous sulfur compounds. However, in sharp contrast to organosulfur compounds, most of the organoselenium derivatives contain the divalent, dicoordinate selenium atom. Tetravalent, tricoordinate selenium compounds such as selenoxides, selenonium salts and seleninic acids are much less common. Only a few hcxavalent selenium compounds such as selenonic acids and selenones are known. Thus, asymmetric C —Se bond formation is limited to creation of a carbon — selenenyl bond. 

Keywords Asymmetric synthesis. Oxidation, Selenium dioxide, Selenoxides, Selenuranes, Selenonium salts, Selones... 

More recently, it has been reported [7] that similar properties were observed for Se-trifluoromethylated dibenzoselenonium salts. Se-(Trifluoromethyl) dibenzoselenophenium triflate (4) was successfully synthesized in high yield by the direct fluorination of an equimolar mixture of the selenide 5 and triflic acid (TfOH) with diluted molecular fluorine. The same salt was also obtained by cyclization of the selenoxide 6 with triflic anhydride (Tf2 0) - a reaction which probably occurs via the selenonium salt 7 (Scheme 1). 

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 selenonium salts 9 and 11 were found to react with one equivalent of sodium benzenesulfinate in an alcohol giving the products shown together with their ratio in Scheme 3. It is evident from the above data that the reaction of 9 and 11 with sodium benzenesulfinate gave (Z)-alkoxyvinyl sulfone 17a and the reactions at room temperature or at 60 °C afforded (Z)-bis-(phenylsulfonyl) styrene (18) as a by-product. In contrast, the reactions of salts 9 and 11 with benze-nesulfinic acid in isopropanol afforded (Z)-()5-phenylsulfonyl)vinylselenonium salts 19 and 20 in yields of 76 and 72%, respectively (Eq. 3). Neither the sulfone 17c nor the vinylsulfone 18 was isolated from these reactions [11a]. 

Reaction of the selenonium salt 11 with acetylacetone (23) and t-BuOK in THF gave the furan derivative 24 and diphenylselenide (25) in 40 and 62% yield, respectively. However, the reaction of this salt with benzoylacetonitrile (26) and 1,3-indandione (27) afforded the selenurane intermediates 28 and 29 in yields of 88 and 57 %, respectively. These selenuranes gave after refluxing in chloroform the furan derivatives 30 and 31, diphenyl selenide (25) and the structure 32 in the yields shown in Scheme 4. 

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). 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

Among these methods, the metallation reaction is the method of choice for selenides bearing electron-withdrawing groups able to stabilize the carbanionic center. This also applies to selenoxides, selenones and selenonium salts. 

SynAesis of a-meUMotdkyl selenoxides, selenones and selenonium salts... 

Alkyl metals are not suitable for the metallation of tiiese compounds. For example, BuaU reacts at -78 C on the seletuum atom of selenonium salts and produces a novel organolithium in which the carb-aruonic center is more stabilized (Scheme 50, a and b), whereas alkylmagnesium bronudes and chlmides unexpectedly lead to decyl bromide and chloride respectively on reaction with decyl phenyl selenrme (Scheme 50, c). 

Allyldimethyl- and allylmethylphenyl-selenonium salts react under similar conditions with potassium r-butoxide (THF, 20 "C, 20 h) and lead to homoallyl selenides in very high yields, including the particularly strained alkylidenecyclopropane derivatives (Scheme 65, b). These result from the metallation of... 

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