Trimethyloxonium tetrafluoroborate, alkylation

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). 

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... 

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. 

Very few examples of electrophilic attack at sulfur(II) have been reported for these systems, with the exception of oxidation. Alkylation of the benzothiadiazine (88) with trimethyloxonium tetrafluoroborate gave a poor yield of the imino ether (89). The major product (90) resulted from alkylation at sulfur followed by hydrolytic ring opening of the resulting azasulfonium salt (79JCR(S)214). 

Cerium(IV) ammonium nitrate, 67 Methylation (see also Alkylation) Dimethyl sulfate, 239 Trimethyloxonium tetrafluoroborate, 327 Methylenation of carbonyl groups... 

The 3-substituted 1,2,3-triazole 1-oxides 448 were alkylated at the oxygen by trimethyloxonium tetrafluoroborate using liquid sulfur dioxide as the solvent affording hygroscopic 3-substituted l-methoxy-l,2,3-triazolium tetrafluoroborate 507 in high yield (1987ACSA(B)724).The reactivity of these salts has not been reported (Scheme 147). 

The 2-substituted tetrazole 1-oxides 550 were prepared from the corresponding hydroxytetrazoles 549 by methylation with trimethyloxonium tetrafluoroborate in CH2C12 at 20°C (1984BAU142) or by benzoylation of 1-hydroxytetrazole 549 (1913G71) (Scheme 165). Since N-alkylation and... 

Base-sensitive substrates require some special precautions With the very potent alkylating agent methyl triflate, the highly hindered (and expensive) base 2,6-di-fert-butylpyridine can be used as shown in Scheme 4.114 22 205 Alternatively O-methylation using a combination of trimethyloxonium tetrafluoroborate and l,8-bis(dimethylamino)naphthalene in dichloromethane can be used although this is also a very expensive method [Scheme 4 115].206 Evans and co-workers have made a systematic study of the best conditions for 0-methylating hindered multifunctional substrates,207... 

Hi0 chromium carbene may be prepared by addition of a vinyllithiurn to chromtum hexacarbonyi followed by alkylation with a reactive electrophile such as trimethyloxonium tetrafluoroborate (Meerwein s salt. Me..O BF,-. ... 

The first report on the alkylation of thiophene at sulfur appeared in 1964 when Brumlick and co-workers reported that thiophene could be methylated at sulfur using trimethyloxonium tetrafluoroborate or methyl iodide/silver perchlorate (64JA5360). In this work, the position of methylation was con-... 

Alkylating agents commonly used are methyl iodide, dimethyl sulfate, trimethyloxonium tetrafluoroborate, and triethyl-oxonium tetrafluoroborate. When R = = C02Me or CN, the — M... 

Alkyl cyclopropyl ethers, e.g. 5, have been prepared by treating the corresponding acetates with an alkyllithium (methyl- or butyllithium) and then with a methylating agent like ioodome-thane or trimethyloxonium tetrafluoroborate. ... 

Trimethyloxonium tetrafluoroborate (0.5 equiv) was added a third time and the mixture stirred for 1 h at 25°C. After filtration of excess alkylating agent, the CH2CI2 was evaporated leaving a light-brown precipitate, which was recrystallized from dry EtOH yield 59% white solid mp 121-122°C. 

A-Alkyl- and A,A-dialkylcyclopropanecarboxamides undergo enolization facilitated by an acid, such as perchloric acid and trimethyloxonium tetrafluoroborate. An example is the formation of the morpholinium ion 2. ° ... 

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