Reactions with trimethyloxonium tetrafluoroborate

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). 

Perhaps the most effective synthesis, albeit an expensive one, involves the direct methylation of alkylimidazoles using trimethyloxonium tetrafluoroborate with concomitant formation of the tetrafluoroborate anion as shown in Scheme 4.3 [12]. This is a single step reaction in which no chloride is present at any stage of the synthesis. The only by-product from this reaction is dimethyl ether which is a volatile and inert gas and it is easily removed. 

The vinylketene complex 140.g was shown to react toward electrophiles at the ketene oxygen. Although iodomethane had no effect, reaction with trimethyloxonium tetrafluoroborate caused methylation to produce the cationic complex 143 in fair yield. 

One of the major advantages of oxonium salts is that alkylations can be effected under reaction conditions that are generally much milder than those necessary with the more conventional alkyl halides or sulfonates. Triethyloxonium tetrafluoroborate, for example, has usually been employed at room temperature in dichloromethane or dichloroethane solution. Occasionally chloroform16-22 or no solvent at all4-20 is used. Difficult alkylations can be effected in refluxing dichloroethane.29 30 The less soluble trimethyloxonium tetrafluoroborate has been used as a suspension in dichloromethane or dichloroethane, or as a solution in nitromethane or liquid sulfur dioxide. Reports of alkylations in water23 and trifluoroacetic acid21 have also appeared. Direct fusion with trimethyloxonium tetrafluoroborate has succeeded in cases where other conditions have failed.25-30... 

Product elucidation of trimethyloxonium tetrafluoroborate and methyl iodide reactions with [l,2,3]triazolo[4,5- -pyridazines (Schemes 1-3) using mainly 2-D heteronuclear multiple bond correlation (HMBC). NMR... 

The unsymmetrical 3,6-dialkoxy-2,5-dihydropyrazines are prepared via the jV-carboxyanhy-drides of the particular amino acid (e.g., 7) and subsequent reaction with the methyl ester of glycine or alanine to give the corresponding cyclodipeptides (e.g., 8). These hexahydrodioxopy-razines are converted to the 2,5-dialkoxy-3,6-dihydropyrazines by alkylation using Meerwein salts, such as trimethyloxonium tetrafluoroborate or triethyloxonium tetrafluoroborate n lb. 

Dimethyl(methyltliio)sulfoniuin tetrafluoroborate, CH3SS(CH3)2BF4 (1). Mol. wt. 196.1, m.p. 82-84°. The salt is prepared by reaction of dimethyl disulfide with trimethyloxonium tetrafluoroborate.1... 

The application of this cycloaddition-retro-Mannich-Mannich sequence to a synthesis of the alkaloid mesembrine is outlined in Scheme 2545. Irradiation of 104 led to the formation of ketoimine 105 in 74% yield. Reaction of 105 with trimethyloxonium tetrafluoroborate then gave iminium ion 106, which on refluxing with 4-dimethylamino-pyridine (DMAP) yielded mesembrine in 84% yield. 

On successive treatment with trimethyloxonium tetrafluoroborate, triethylamine and aqueous acid, oxime acetate 83 prepared from cyclohexanone oxime and Ac20 yields a-acetoxy ketone 87 (Scheme 7). A similar result is obtained with other symmetric ketones including 4-heptanone, deoxybenzoin and dibenzyl ketone. 87 can also be formed by the reaction of cyclohexanone and O-acetyl-Af-methylhydroxylamine hydrochloride upon hydrolysis. If the reaction time is prolonged prior to hydrolysis, ketoamide 89 is isolated as the main product. The key step of the reaction is the [l,3]-acetoxy shift of 7V-acetoxyenamine 85 to the a-acetoxyimine 86. The formation of 89 provides support for the intermediacy of acetoxyimine 86 in the reaction sequence. The evidence available suggests that the [l,3]-acetoxy migration proceeds via a Claisen-type rearrangement (Scheme 7)78. 

Chiral epoxides. A new method for resolution of racemic epoxides involves cleavage with sodium thiophenoxide to give racemic -hydroxy sulfides (2a and 2b) these are converted by reaction with 1 into diastereomeric carbamates, which can be resolved by chromatography. Cleavage of either carbamate with trichlorosilane affords optically pure j3-hydroxy sulfides (2) in yields of 80-90%. These are converted into optically active epoxides (3) by trimethyloxonium tetrafluoroborate followed by treatment with base. The epoxides are obtained in optical purities of 85-100%. ... 

The reaction of A -methylthio(dimethyl)amine with a two-fold excess of an alkene in the presence of trimethyloxonium tetrafluoroborate produced, in moderate yield, the l-dimethylamino-2-methylthio adduct 3, after demethylation of the intermediate ammonium salt by excess trimeth-ylamine120. More conveniently, the same product is directly obtained by using boron trifluo-ride-dimethyl ether complex as Lewis acid. 

For the preparation of the required 1 2 [Ru ligand 47] intermediate, a pro-tection-deprotection strategy was employed. This involved reaction of 47 with trimethyloxonium tetrafluoroborate to block one metal co-ordination site in 47 to yield 48. After co-ordination of one ruthenium ion, the remaining site was unblocked by reaction with DABCO in refluxing acetonitrile. Reaction of this 1 2 (metal ligand) product with a second metal ion was then carried out. 

Methylsulfanyl-4-azatricyclo[3.3.2.0 ]deca-3,6,9-triene (17) was prepared from lactam 11 and phosphorus pentasulfide in pyridine which gave the thioamide 15 (68%), which on reaction with trimethyloxonium tetrafluoroborate gave the methylsulfanyl iminoether 16 (71%). Acetone sensitized photoisomerization of 16 produced 3-methylsulfanyl-4-azatricyclo-[3.3.2.0 ]deca-3,6,9-triene (17) in 93% yield when the photolysis of 16 was not carried out under conditions of triplet sensitization, only polymeric dark material was produced. ... 

Methylation of methylselanyl- and phenylselanylcyclopropanes took place on reaction with either trimethyloxonium tetrafluoroborate or a mixture of iodomethane and silver(I) tetra-fluoroborate resulting in Se-C bond formation.Yields close to 80% of the (1-decylcyclo-propyl)dimethyl- or (l-decylcyclopropyl)methylphenylselenonium tetrafluoroborate were obtained. 

Methylation of iminothiazolines (175) followed by reaction with NaHSe gives 2-selenoxo-thiazolines (176) (Equation (27)) <84PJC447>. 3-Alkyl-2-(A-cyanoimino)thiazolidines (177) are methylated with trimethyloxonium tetrafluoroborate the resulting nitrilium salts react with sodium diethyl malonate to afford methylenethiazolidines (178) and urea derivatives (179) along with the iminothiazolidines (180) (Equation (28)) <93H(36)55>. 

Recently methyltriphenylammonium tetrafluoroborate was prepared from triphenylamine and trimethyloxonium fluoroborate. Demethylation was affected by butyllithium in hexane or methylene chloride and with phenyllithium in benzene or ether. No evidence for biphenyl or diphenyl-methylamine was noted and only triphenylamine was characterized. In demethylation reactions with potassium methoxide in methanol-O-d, no exchange of methyl hydrogen for deuterium was observed. Thus, no proton abstraction processes occurred in these decompositions 114>. 

Lactone is reacted with trimethyloxonium tetrafluoroborate (Me30+BF4 ). The produce is next refluxed with sodium iodide in acetone until complete disappearance of the 5-lactone. By taking entropy effects, stereoelectronic effects and the relative ease of SN2 reaction on primary versus secondary v.v tertiary carbons into consideration, comment on the possible product(s) profile. 

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