Trimethyloxonium tetrafluoroborate

For further information, see The Sigma-Aldrich Library of Chemical Safety Data, 2nded. Leng, R. E Ed. Sigma-Aldrich Milwaukee, 1987 Voi. 2, p 3479. 

Physical Data mp 179.6-180.0 °C (sealed tube, with dec). Solubility sol nitrobenzene, nitromethane, CHCI3, acetone (hot), SO2 (liq) slightly sol CH2CI2 insol common organic solvents. Form Supplied in white crystalline solid commercially available  

Purification highly pure oxonium salt is obtained from dimethoxycarbenium tetrafluoroborate and dimethyl ether. The resulting solid is vacuum dried at 50 °C/1 mmHg for 30 min. 

Functional Group Methylations. The powerful alkylating property of trimethyloxonium tetrafluoroborate allows methylation of sensitive or weakly nucleophilic functional groups. Smooth alkylation of a variety of anions or uncharged molecules has been reported. Examples include carboxylic acids, ketones, lactones, nitriles, O-acetals, S -acetals, sulfoxides,  

The reactions of thioacetals with Me30 BF4 constitute a high-yielding method for deprotection of thioacetals to give ketones (eq 3).  

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The principal electrophiles to attack ring sulfur are either oxidants or alkylating reagents. Thiophene sulfoxide and sulfone formation is discussed in Section 3.02.2.6. Alkylating agents capable of forming thiophenium salts include trimethyloxonium tetrafluoroborate (MeaO BF ) and alkyl fluorosulfonates (ROSO2F). The salts e.g. 87) are conveniently isolated as hexafluorophosphates (88). 

Cleavage of the chiral auxiliary is effected in a three-step procedure commencing with quatemization of the nitrogen with methyl fluorosulfonate, methyl trlfluoromethanesulfonate, or trimethyloxonium tetrafluoroborate. Reduction of the corresponding iminium salt 19 with NaBH4 and acidic hydrolysis of the resulting product affords substituted aldehyde 5 without epimerization of either stereocenter. 

A solution of the benzoxazepinone 2 (4.2 g, 26 mmol) in CH2C12 (200 mL) was stirred under N2 and trimethyloxonium tetrafluoroborate (4.0 g, 27 mmol) was added. The mixture was stirred for 24 h and then neutralized with coned aq K2C03. The filtered solution was evaporated and the residue distilled in a Kugelrohr apparatus to give the product yield 4.2 g (90%) pale-yellow oil. 

The thiazepinone 6 reacts with trimethyloxonium tetrafluoroborate in diuhloromethane, followed by aqueous sodium hydrogen carbonate, to give 2,7-di- c-y7-butyl-5-methoxy-l,4-thiazepine (7) in 65 % yield. No further details were reported.48... 

A stirred solution of the benzothiazepinone 2 (6.0 g, 33.9 mmol) in CH2C12 (200 mL) was treated with trimethyloxonium tetrafluoroborate (5.4 g, 36.5 mmol) and the mixture was stirred for 22 h. An excess of coned aq K2C03 was added, the mixture was filtered and the organic layer was separated and dried. Evaporation, followed by chromatography (silica gel, CH2C12) gave 6 as a yellow oil yield 5.8 g (89%). 

Dimethylquinoxaline and trimethyloxonium tetrafluoroborate gave 1,4,6,7-tetramethylquinoxalinediium bis(tetrafluoroborate) (214) (excess synthon. 

The diquaternary salt 21 of 1,2,4-thiadiazole 1 is obtained when trimethyloxonium tetrafluoroborate is used as the methylating agent (Equation 4) <1972JOC2259>. 

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. 

Methyllithium (40 mL of a 0.5 M solution in diethyl ether, 20 mmol) is added dropwise over 2 h to a well-stirred suspension of chromium hexaearbonyl (4.4 g, 20 mmol) in diethyl ether (600 mL) at room temperature. A yellow, clear solution results. The solvent is evaporated under reduced pressure without heating, and the residue is dissolved in degassed (Nj) water (1()0 mL) and pentane (200 inL). While stirring vigorously trimethyloxonium tetrafluoroborate (3.1 g, 21 mmol) is added in small portions. The aqueous phase should have become slightly acidic... 

Methylation of pyrido[l,2-6]cinnolinium-ll-olate (17, R = H) with the soft dimethyl sulfate gave only the A -methyl derivative 43 (74JHC125), whereas the harder trimethyloxonium tetrafluoroborate yielded the 11-methoxy derivative 77 (92CB929). Methylation of pyrido[2,l-a]phthal-azinium-7-olate (20) with methyl iodide afforded a 1 9 mixture of the... 

Dimethyl ether, with boron trifluoride diethyl etherate and epichloro-hydrin to give trimethyloxonium tetrafluoroborate, 51,142... 

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... 

Methylation of 5-amino-1,2,4-thiadiazoles (17) (R = H, Ph) leads to N-4 derivatives (18) (Equation (3)) <84CHEC-I(6)463>. The diquaternary salt (19> of 1,2,4-thiadiazole (1) is obtained when trimethyloxonium tetrafluoroborate is used as the methylating agent (Equation (4)) <72JOC2259>. 

The 1,3,4-thiazaphospholes are colorless, yellow, or red crystalline solids <85AG125> the 2-phenyl derivative is a pale yellow liquid. They are not oxidized by atmospheric oxygen or sulfur <86PS(28)7i>. Dimethyl sulfate or trimethyloxonium tetrafluoroborate methylates the nitrogen atom <85AG125,... 

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. 

The block copolymer of ethylene oxide and 3,3-dimethylthietane shows useful properties of complexing halogen and heavy metal salts (79MI51402). Thietanes can be polymerized with methylmagnesium iodide as well as with a variety of electrophiles such as methyl sulfate, trimethyloxonium tetrafluoroborate, triethylaluminum, boron trifluoride and phosphorus trifluoride (67IC1461, 67MI51400). Thietane (210) has been patented as a stabilizer for poly(vinyl chloride) (73USP3767615). 

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