With dimethyl sulfonium salts

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

The ionic species 5, as well as 6, represent the so-called activated dimethyl sulfoxide. Variants using reagents other than oxalyl chloride for the activation of DMSO are known. In the reaction with an alcohol 1, species 5, as well as 6, leads to the formation of a sulfonium salt 7 ... 

The C6 position of a V,./V-dialkyl-3//-azepin-2-amine, e.g. 1, displays remarkable reactivity towards electrophilic substitution, and with dimethyl(methylsulfanyl)sulfonium tetrafluorobor-ate undergoes methylsulfanylation and quaternization to yield 6-(methylsulfanyl)-3//-azepin-ium tetrafluoroborates, e.g. 2, identical to those obtained by quaternization of the free bases with trityl tetrafluoroborate.64 Basification of the salts with potassium carbonate yields the free bases, e.g. 3, which decompose during distillation or on exposure to air. 

Acetylenedicarboxylic acid, dimethyl ester, 50, 25, 36 Acetylenes, reaction with tri-methylsilyl azide, 50, 109 Acetylenic sulfonium salts, furans from, 53, 3 2-Acetylindane-l,3-dione, 52, 4 2-ACETYL-6-METHOXYNAPHTHALENE, 53, 5... 

This method has been used to prepare several different tris(dialkyl-amino)sulfonium difluorotrimethylsilicates, including salts with greater organic solubility such as the tris(diethylamino)sulfonium and tris(pyrrolid1ne)sulfon1um2 difluorotrimethylsilicates. The tris(dimethyl-amino)sulfonium salt, however, is highly crystalline and thus has an advantage 1n ease of preparation and purification over these other salts. 

A. Dimethylprop-2-ynylsulfonium Bromide. A mixture of 6.2 g. (0.1 mole) of dimethyl sulfide (Note 1), 11.9 g. (0.1 mole) of 3-bromopropyne (Note 2), and 10 ml. of acetonitrile (Note 3) is stirred magnetically for 20 hours (Note 4) in a darkened 100-mi. round-bottomed flask (Note 6) fitted with a calcium chloride drying tube. The resulting white, crystalline mass is filtered with suction and washed with three 50-ml. portions of dry ether (Note 6) to give 16.4 g. (90%) of the sulfonium salt, m.p. 105-106°. This material may be used in the next step without purification but, if desired, it may be recrystallized from ethanol-ether (Note 7) with minimal loss to give a product melting at 109-110°. 

The ring-opening reaction on 4-methyl-2-thia-7-aza-tricyclo[5.3.0.01,4]decan-8-one 39 was carried out by treatment with dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) <20010L1781, 2004JOG33>. Methylthiolation of the thietane sulfur produces a transient alkylthiosulfonium salt which easily dissociates to the iV-acyliminium ion 40. Once formed, 40 readily undergoes loss of a proton to give the product 40 (Equation 11). 

Fig. 17.13. Mechanism of the Swern oxidation of alcohols. The actual reagent is an "activated DMSO" (compound B or D), which reacts with an alcohol with formation of A or C, respectively. Dissociation leads to the sulfonium salt E, which is then converted into the sulfonium ylide F after NEt3 addition and raising the temperature from -60 to -45 °C. /3-Elimination via a cyclic transition state generates the carbonyl compound and dimethyl sulfide from F.

Irrespective of whether the initially formed sulfonium ion B or the subsequently formed sulfonium ion D reacts with the alcohol, the alcohol is taken up by such a sulfonium ion with formation of sulfuranes A (first case) or C (second case). Any of these sulfuranes would yield the sulfonium salt E after dissociation. Once this sulfonium salt has formed, five equivalents ofNEtj are added to the reaction mixture, which then is allowed to warm up from -60 to -45°C. Under these conditions, the sulfonium salt E is deprotonated to give the sulfonium ylide F. This ylide undergoes a /3-elimination via a cyclic transition state to form the desired carbonyl compound and dimethyl sulfide as a side-product. 

At about the same time, a synthesis of leukotriene-A, also termed SRS-A ( slow reacting substance of anaphylaxis ), which made use of the Wittig olefmation was described77). The ylide of 100 is reacted with ethyl 5-formyl-2,4-pentadienoate 101 to give the ( , , Z, Z)- tetraenoic ester 102. Reduction and mesylation of 102, subsequent conversion into the sulfonium salt, and treatment of the latter with a base yields a sulfonium ylide which is reacted with methyl 4-formylbutanoate 69 to the epoxy-tetraenoic ester 103. After separation of the cis-epoxide by HPLC, 103 was treated with the S-trimethylsilyl derivative of glutathione dimethyl ester N-trifluoroacet-amide. The diastereomeric products thus obtained were separated by means of HPLC and hydrolyzed to 104 77) (Scheme 18). 

Chloroethyl(dimethyl)sulfonium iodide, ClCH2CH,S(CH,)2l (1). The salt, m.p. BBSS , is prepared by reaction of 2-chloroethyl methyl sulfide with CH,I. ... 

The key to successfully using dimethyl sulfoxide as an oxidant for alcohols is to activate the sulfur atom prior to reaction with a nucleophilic alcohol function. This activation involves electrophilic attarlr upon the sulfinyl oxygen by a varied of electrophiles. The initial product formed when an alcohol does attack the activated dimethyl sulfoxide is known to be the sulfonium salt (1 Scheme 1). 

This evidence is based on IR-spectroscopic measurements on the copper-ylide complex prepared by two independent routes. First, sulfonium salt 417 was made by alkylation of dimethyl siflfide 416 with bromide 415. The... 

Reaction with epoxides. Dimethyl sulfoxide and a strong acid (2,4,6-trinitro-bcnzcncsulfonic acid, TNBSA, was used) reacts with epoxides (1) to form alkoxy-sulfonium salts (2) in good yield. Since hydrolysis of alkoxysulfonium salts proceeds exclusively by attack at sulfur, the reaction provides a stereospecific synthesis of 1,2-glycols. Thus the reaction of lis- and rrans-9,10-epoxystearic acids with DMSO-... 

Dibromomethylsulfonium salts [CH3SBr2 ][A ] (313) and dimethyl-iodosulfonium salts [(CH3)2SI ][A ] (312) with A = AsFe , SbCle were synthesized via halogen exchange from the corresponding chloro-sulfonium analogues with HBr or HI in liquid SO2, or by the reactions... 

Treatment of the sulfonium salt (77) with 1.0 mol equiv. of DBU in dimethyl sulfoxide at ambient temperature afforded the 3-amino-2-pyrroline... 

In the first step of the Swern oxidation, DMSO reacts with oxalyl chloride to give an electrophilic chlorosulfonium ion 36, carbon monoxide and carbon dioxide. Ion 36 then reacts with alcohol 37 to give a sulfonium salt 38, which is deprotonated by NEts and then converted to the aldehyde 40 and dimethyl sulfide. 

In the first step of this reaction sequence, the primary alcohol 21 is oxidized to the corresponding aldehyde 38 in a Parikh-Doering oxidation which is related to the Swern oxidation. In general, this type of oxidation is conveniently carried out by addition of a solution of pyridine-SOs complex in DMSO to a mixture of the alcohol, DMSO and NEts. It can be assumed that dimethyl sulfoxide and sulfur trioxide react to form 0-dimethylsulfoxonium sulfate 40, which then further reacts with primary alcohol 39 to give 0-alkyl dimethylsulf-oxonium intermediate 41. Then, sulfonium salt 42 is formed and subsequently deprotonated by NEts to give sulfonium ylide 43. Finally, an intramolecular p-elimination occurs to provide the desired aldehyde 44 and dimethyl sulfide. 

Trost has reported the use of dithioacetals as initiators in vinylsilane-terminated cyclizations (Scheme 11). Reaction of the dimethyl thioketal (26) with dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) led to the formation of the allylic sulfide (28) via a 2,3-sigmatropic rearrangement of the initially formed sulfonium salt (27a), which may have been catalyzed by complexation with excess DMTSF. 

Not only is dimethyl ether a poor nucleophile, it is also a gas and is lost from the reaction mixture. The same principle is used to make sulfides from other sulfides. With that clue, and the position of this reac- Brv tion in the sulfonium salt section, you should be able to work out the mechanism and say why the reaction works. 

Dicyanoketene ethyleneketal reacts with triethylamine in tetrahydrofurune to form the quaternary ammonium inner salt (1) it reacts with dimethyl sulfide to form the sulfonium inner salt (2). The ketal can be converted Into pyrimidines, pyra/oles, or isoxazoles In one step. "... 

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