Dimethylsulfoxonium Methylide1

[6814-64-8]  · C3H8OS  · Dimethylsulfoxonium Methylide  · (MW 92.16)

(widely used methylene transfer reagent; used for the epoxidation of ketones and aldehydes,1 cyclopropanation of a,b-unsaturated carbonyl compounds1 and also as a methylation reagent2)

Physical Data: pKa of sulfoxonium ion (Me3+S(O)) at 25 °C in DMSO is 18.2;3 mp 9-10 °C, bp 41-43 °C/0.1 mmHg.4

Solubility: sol THF, DMSO, 1,4-dioxane, DMF.

Preparative Method: usually generated in situ by the reaction of trimethylsulfoxonium iodide or chloride with strong base in a appropriate solvent without drying or further purification; not commercially available.

Analysis of Reagent Purity: determined by adding a small quantity of the reagent to water and titrating with standard acid using phenolphthalein as indicator.5

Handling, Storage, and Precautions: reactions using this nucleophilic reagent should be carried out under an inert atmosphere; it is more stable than Dimethylsulfonium Methylide and can be stored in solution under N2 at 0 °C for months without appreciable decomposition.

Preparation.

Dimethylsulfoxonium methylide is a stabilized ylide which is generally prepared and used at rt.1,6,7 Typically, the reagent is obtained by the reaction of finely powdered Trimethylsulfoxonium Iodide (or trimethylsulfoxonium chloride) with a strong base such as Sodium Hydride or n-Butyllithium under nitrogen in a suitable solvent. A variety of anhydrous solvents such as DMSO, THF, DMF, and 1,4-dioxane are reported to work well for this reaction. When NaH is used as the base, after hydrogen evolution has completed, the resulting ylide is ready for further reaction.8

Recently, a less hazardous modification for large-scale production has been reported. This method involves the reaction of Potassium t-Butoxide with trimethylsulfoxonium iodide in DMSO. Use of potassium t-butoxide eliminates the hazards of handling sodium hydride on a large scale, and also eliminates problems with the isolation of products which may be contaminated with the mineral oil from sodium hydride dispersions.9 Preparation of dimethylsulfoxonium methylide can also be achieved by using t-BuOK in t-butanol.10 Alternatively, dimethylsulfoxonium methylide is accessible via a desilylation process.11

Preparations of sulfur ylides under phase-transfer catalyzed conditions have also been reported. One such system involved the use of CH2Cl2, Tetra-n-butylammonium Iodide (TBAI), NaOH, and trimethylsulfoxonium iodide.12 N-Benzyl-N,N,N-trimethylammonium chloride has also been reported as an efficient catalyst for this reaction.13 An alternative system using trimethylsulfoxonium iodide, cetrimide (cetyltrimethylammonium bromide), aqueous NaOH, and 1,1,1-trichloroethane has also been reported.14 Usually, the phase-transfer catalyzed reactions are performed at elevated temperatures from 50 to 75 °C.

Epoxidation.

Dimethylsulfoxonium methylide is commonly used for epoxide preparation by reaction with a variety of compounds which contain an aldehyde or a ketone group (eqs 1-5).1,3,7,8,15-19 Normally very high yields and purities of the desired epoxides are obtained. Use of trimethylsulfoxonium iodide instead of trimethylsulfonium iodide eliminates the need for removal of the noxious byproduct Me2S. The byproduct DMSO generated from dimethylsulfoxonium methylide preparation can be readily removed by using standard organic/aqueous extractions. The reaction involves initial nucleophilic addition (dimethylsulfoxonium methylide) to the carbonyl group and subsequent elimination of DMSO and epoxide formation. Enolization usually does not compete with nucleophilic addition to the carbonyl group.3

Instead of using strong base in an anhydrous media, epoxides have also been prepared under phase-transfer catalysis conditions with a sulfur ylide.12-14,20,21 Activated Barium Hydroxide was reported to catalyze the oxirane formation in interfacial solid-liquid conditions.22,23 These reactions were performed in MeCN in the presence of Ba(OH)2 with a small amount of H2O.

The condensation of 1-tetralone with dimethylsulfoxonium methylide affords the corresponding oxirane in 75% yield.24,25 However, if this parent ketone possesses an aromatic electron-releasing substituent such as a methoxy group, the resulting spiro-oxirane may not be stable and may readily rearrange to give ring-opened compounds.

Dimethylsulfoxonium methylide has also been employed in nucleoside preparations.26 The reagent has been used to introduce the spiro epoxy group at the C-2 position of 2-keto-b-L-nucleosides. In this case, better yields of oxiranes are obtained with the use of trimethylsulfoxonium chloride instead of trimethylsulfoxonium iodide. Variation of the base used has a major effect on the course of the reaction. Only 2-L-galacto-spiro-epoxynucleoside is obtained if the dimethylsulfoxonium methylide is prepared from BuLi (eq 6).27

In some examples, selective epoxidations were observed in the reaction of dimethylsulfoxonium methylide with dicarbonyl compounds. Selective condensation (83% yield) of dimethylsulfoxonium methylide with the six-membered-ring carbonyl group of trans-1,6-dimethylbicyclo[4.3.0]nonane-2,7-dione was observed, while the cyclopentanone remained unreacted (eq 7).28

The further transformation of the product epoxides to other functionalities by treatment with nucleophiles or Lewis acids has been widely reported. Epoxidation of quadricyclanone and adamantanone with dimethylsulfoxonium methylide provided spiro-oxirane products in good yields.29,30 In the case of adamantanone, the product oxirane was further converted to the rearranged acid by reaction with Boron Trifluoride Etherate followed by oxidation with Jones' reagent (eq 8). Similarly, reaction of dimethylsulfoxonium methylide with methylheptenone followed by treatment of BF3.OEt2 provided the rearranged 2-isopropyl-5-methylcyclopentanone (eq 9).17

Reactions of hydroxybenzopyranones, hydroxyphenylalkanones, and 2-(o-hydroxyphenyl)alkyl ketones with dimethylsulfoxonium methylide provide an efficient route to oxygen heterocycles (eq 10).31-33 The mechanism involves an intramolecular ring opening of the oxirane intermediate.

Stereoselective Epoxide Synthesis.

Dimethylsulfoxonium methylide has been reported to show higher stereoselectivity in reactions with aldehydes and ketones than its sulfonium analog. Reaction of 4-t-butylcyclohexanone with dimethylsulfoxonium methylide gave exclusively the cis-oxirane, while reaction with Dimethylsulfonium Methylide gave products with a ratio of 17:83 of the trans and cis isomers (eq 11).1,34 Presumably the sulfoxonium ylide provided the kinetically controlled product, while the smaller sulfonium ylide gave a mixture of the kinetically and thermodynamically favored products. Similarly, epoxidation of 4-protoadamantanone with dimethylsulfoxonium methylide gave almost exclusively the exo isomer (exo:endo = 15:1), while the dimethylsulfonium methylide provided a mixture of the exo and endo isomers in a 3:2 ratio (eq 12).35

Several examples of conversion of chiral ketones and aldehydes to epoxides with high diastereomeric purity have been reported.36-38 Stereoselective reaction of the nucleoside 2-keto-3,5-O-(tetraisopropyldisiloxane-1,3-diyl)uridine with dimethylsulfoxonium methylide at 0 °C afforded only one isomer of the product epoxide in 63% yield.39 Reaction of the heterocyclic carbonyl compounds with dimethylsulfoxonium methylide also provided the corresponding oxiranes, such as 1-oxa-6-heterospiro[2,5]octanes, in good yields and isomeric purities.40,41 One example showed that dimethylsulfoxonium methylide reacted chemoselectively with the five-membered ketone ring in the presence of the six-membered carbonyl function to give the monoepoxide in good yield (eq 13).

In some cases, solvent appears to affect the stereoselectivity of the oxirane formation. Reaction of 3a,18-dihydroxy-17-noraphidicolan-16-one with dimethylsulfoxonium methylide in THF gives a 1:1 mixture of epoxides. When THF is replaced by DMSO as the solvent, the isomeric ratio is improved to 1:3. In the mixed solvent DMSO/DMI system, the ratio is further improved to 1:4.42

Generation of New Sulfur Ylides.

Treatment of cyclic b-chloro-substituted enones and chloropyrimidines with dimethylsulfoxonium methylide results in the formation of new sulfoxonium ylides.43-47 Dimethylsulfoxonium methylide also reacts with acid anhydride,48 acid chloride,49 isocyanate,50 and other activated carbonyl groups51,52 to generate new sulfur ylides for further reactions. This method has been used to prepare acylated stable ylides53 which can be converted to other products or further reacted with a second mole of the a,b-unsaturated carbonyl compounds (eqs 14-16).

Methylation.

Dimethylsulfoxonium methylide is a useful methylation reagent for the conversion of acids,54 oximes,55 N-heterocycles,56 and aromatic hydrocarbons57 to the corresponding methylated products. For example, nitrobenzene and 6-benzyladenine were methylated with dimethylsulfoxonium methylide in 67% and 63% yields (eqs 17 and 18).2

Preparation of Azetidine Derivatives.

N-Arylsulfonyl-2-phenylazetidines can be prepared by reaction of dimethylsulfoxonium methylide with N-arylsulfonyl-2-phenylaziridines in 51-72% yield.58 Stereospecific conversion of N-arylsulfonylaziridines to N-arylsulfonylazetidines has also been reported. Reactions with cis-aziridines give trans-azetidines, and reactions with trans-aziridines produce the cis-azetidines. Presumably an SN2 1,4-elimination mechanism is involved (eq 19).59 Although several 2- and 2,3-substituted N-arylsulfonylazetidines have also been synthesized in good yields from reactions with dimethylsulfoxonium methylide, the fused azetidines could not be prepared by this procedure.60 Dimethylsulfoxonium methylide also converts N-arylsulfonyloxaziridines to azetidines via a methylene transfer reaction. Similar reactions with N-alkyloxaziridines give only the deoxygenated product.61

Preparation of Oxetanes.

Dimethylsulfoxonium methylide is an efficient methylene transfer reagent in reactions with terminal epoxides to provide the corresponding oxetanes (eq 20).10,62,63 The oxetanes can also be prepared from ketones or aldehydes with dimethylsulfoxonium methylide via double methylene transfer reactions.

Cyclopropanation.

Cyclopropanation using the Simmons-Smith conditions (see Iodomethylzinc Iodide) does not work well for a,b-unsaturated carbonyl compounds. This difficulty has been overcome by the use of dimethylsulfoxonium methylide as the methylene transfer reagent. This method has been successfully applied to unsaturated ketones,64-66 esters,67-70 lactones,71 amides,72 nitriles,73 and nitro compounds.74,75 Dimethylsulfoxonium methylide appears to work better than the corresponding sulfonium analog in these reactions. Reaction of the sulfonium methylide with a,b-unsaturated carbonyl compounds gives the kinetically controlled 1,2-addition oxirane product,1,66 while reaction with dimethylsulfoxonium methylide gives the 1,4-addition cyclopropane derivatives in synthetically useful yields (eqs 21-23).64,70

The mechanism of cyclopropanation involves an initial reversible conjugate addition of dimethylsulfoxonium methylide to the a,b-unsaturated carbonyl compound, followed by an irreversible ring closure; if excess dimethylsulfoxonium methylide is used in the reaction, epoxidation of carbonyl group may follow the cyclopropanation (eq 24).76 Usually, the reaction is carried out in DMSO over a wide range of temperatures under an inert atmosphere. Cyclopropanation using TBAI as phase-transfer catalyst in an aqueous system was also reported.12 Reaction of dimethylsulfoxonium methylide with fluorine-containing a,b-unsaturated ketones can result in products from different reaction pathways.38,77,78 Reaction of dimethylsulfoxonium methylide with a-fluoro-substituted enones gives the corresponding fluoro-epoxides only.79

The cyclopropanated carbonyl compounds can readily be further transformed to other functionalities.72,80,81 For example, a substituted vinyl phenyl ketone was converted to the cyclopropane intermediate with dimethylsulfoxonium methylide and the product was further transformed to a 1-tetralone derivative by acid-catalyzed cyclization.80 The cyclopropyl amide which is derived from reaction of an a,b-unsaturated amide with dimethylsulfoxonium methylide can be further converted to a ketone or an acid derivative by reaction with an organometallic nucleophile or an anhydrous base.72

Cyclopropanation with dimethylsulfoxonium methylide in multifunctional compounds can be chemoselective.66,82,83 Dimethylsulfoxonium methylide reacts preferentially with the less sterically hindered a,b-unsaturated carbonyl function to give the corresponding cyclopropane (eq 25).65,84,85 On the other hand, reaction of dimethylsulfoxonium methylide with bifunctional cyclic enediones affords the oxirane product instead of generating the cyclopropyl derivative (eq 26).85

Chemoselective reaction of a methoxy-substituted dienone with dimethylsulfoxonium methylide results in cyclopropanation of the unsubstituted double bond.83 Reaction of dimethylsulfoxonium methylide with the conjugated dienone 2,3-benzotropone gives a 1.1:1 mixture of the mono- and dicyclopropanated products.86 One conjugated dienone, however, gave only the 1,6-addition product (eq 27).82

Stereoselectivity of Cyclopropanation.

Cyclopropanation of a,b-unsaturated carbonyl compounds with dimethylsulfoxonium methylide can be highly stereoselective. Reaction of methyl 2-(4-oxo-2-cyclohexenyl)acetate with dimethylsulfoxonium methylide gave the corresponding anti- and syn-cyclopropane derivatives in a 85:15 ratio; the favored anti isomer results from the nucleophilic addition of the dimethylsulfoxonium methylide to the less hindered side of the a,b-unsaturated ketone (eq 28).87 This steric effect was also observed in the cyclopropanation of 3b-acetoxy-16a,17a-methylene-5-pregnen-20-one.88 Stereoselective cyclopropanation of a ten-membered enone with dimethylsulfoxonium methylide gives the bicyclohumulenone in 90% yield, with none of the cis isomer observed. Peripheral addition of oxysulfurane proceeds through the lower-energy conformation which leads to the most likely enolate intermediate. Subsequent ring closure gives the trans cyclopropane.89 A recently reported asymmetric cyclopropanation of a chiral vinyl sulfoxide with dimethylsulfoxonium methylide gave high yield and stereoselectivity ((R,S):(S,S) = 5.9:1) (eq 29).90

Cyclopropanation of a,b-unsaturated bicyclic lactams with dimethylsulfoxonium methylide also proceeds with high diastereoselectivity. In these cases, a change of the angular substituent in the bicyclic lactam leads to a complete reversal in endo-exo selectivity (eq 30).91

Reaction with Other Functional Groups.

Reaction of a-halo carbonyl compounds with dimethylsulfoxonium methylide provides the a-cyclopropyl carbonyl derivatives rather than the oxirane.92 Presumably the mechanism involves a double methylene transfer with the dimethylsulfoxonium methylide. Dimethylsulfoxonium methylide has also been reported to react with 1,3-dipolar compounds for the preparation of heterocyclic products (eq 31).93

Methylene transfer onto the triple bond of a,b-alkynyl ketones by reaction with dimethylsulfoxonium methylide has not been observed. Reaction of 1,3-diphenyl-2-propyn-1-one with dimethylsulfoxonium methylide gives only the Michael addition complex 1-methyl-3,5-diphenylthiabenzene 1-oxide.94,95 Reaction of dimethylsulfoxonium methylide with thiobenzophenone gives 1,1-diphenylethylene sulfide in 71% yield.96 Treatment of 4-dimethylamino-1-thia-3-azabutadienes with dimethylsulfoxonium methylide provided thiazol-2-ine derivatives. The mechanism involves regioselective ylide addition to the amidine group, followed by cyclization with elimination of DMSO (eq 32).97

Methylene transfer of dimethylsulfoxonium methylide to imines results in aziridine derivatives.98 A facile procedure for the preparation of 3-amino-2,3-dihydrobenzofuran from dimethylsulfoxonium methylide and o-hydroxybenzylideneaniline has been reported. The mechanism involves a ring opening of the initial aziridine intermediate (eq 33).99,100

Reaction with Organometallics.

Dimethylsulfoxonium methylide has been reported to react with transition metal organometallic compounds as simple terminal ligands, bridging groups, or chelating moieties.101-106 For example, the reaction of two equivalents of dimethylsulfoxonium methylide with the metal-functionalized 1,3-diphospha-2-propanone Fe2(CO)6[(i-Pr)2N-PC(O)P-N(i-Pr)2] affords the 1,5-diphospha-3-pentanone derivative, a product resulting from the insertion of a CH2 group into the P-(CO) bond.107


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John S. Ng & Chin Liu

Searle Research & Development, Skokie, IL, USA



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