O-Methyldibenzofuranium Tetrafluoroborate1

[36883-49-5]  · C13H11BF4O  · O-Methyldibenzofuranium Tetrafluoroborate  · (MW 270.05)

(methylating agent, more powerful than trimethyloxonium salts1,2)

Solubility: sol dichloromethane; insol benzene.

Preparative Methods: obtained as an unstable off-white solid by heating dry 2-methoxybiphenyl-2-yldiazonium tetrafluoroborate in anhydrous benzene.2,3 The diazonium precursor is conveniently prepared.3 For synthetic applications the reagent is generated in situ by refluxing a solution (or suspension) of the dry diazonium tetrafluoroborate in anhydrous CH2Cl2 in the presence of a suitable substrate.2,3

Handling, Storage, and Precautions: the anhydrous diazonium precursor is unstable; it should be stored in vacuo over phosphoric anhydride at rt for a maximum of 2-3 days. Although information on hazardous properties is lacking, skin and eye contact with solutions of the methylating reagent or with the diazonium precursor should strictly be avoided (see also Triethyloxonium Tetrafluoroborate). Use in a fume hood.

Introduction.

O-Methyldibenzofuranium tetrafluoroborate (1) has not yet been established as a widely employed reagent, although its methylating power has been reported to exceed that of trimethyloxonium salts or methyl fluorosulfonate.2,3 The rare use of (1) may be due to two disadvantages of its use: the multistep synthesis of its precursor and the fact that methyl transfer from (1) to suitable substrates is coupled with the formation of dibenzofuran, which has to be separated from the methylation product. Alkylations with the readily available trimethyloxonium salts do not require extra separation, since only volatile dimethyl ether is formed as a byproduct.

Functional Group Alkylations.

The advantage of the in situ preparation of (1) is clearly demonstrated by the methylation of pyridine: when (1) is generated from its diazonium precursor (eq 1) in the presence of pyridine, N-methylpyridinium tetrafluoroborate is formed in essentially quantitative yield (eq 2).3 Addition of pyridine to the preformed tetrafluoroborate (1) or to the analogous 2,4,6-trinitrobenzenesulfonate (2) gives lower yields (28% or 44%, respectively) of the pyridinium salt.3 The reagent (1) is also capable of methylating halogenated pyridine derivatives (eqs 2 and 3).2,3 When using trialkyloxonium salts the corresponding alkylations occur only to a small extent (<15%).3 In the following sequences the reagent is indicated as MeDIB+BF4- (1), although methylations are performed via the in situ procedure using the diazonium precursor; in all reactions, dibenzofuran is the common byproduct.

Reaction of (1) with N,N-dimethyl-1-naphthylamine results in the formation of the quaternary ammonium salt.2,3 Similar to methylation with trimethyloxonium tetrafluoroborate, benzonitrile reacts with (1) to yield N-methylbenzonitrilium tetrafluoroborate, which may be hydrolyzed to N-methylbenzamide (eq 4).3

O-Methylations via (1) have been reported to occur with tetrahydrofuran, a-pyridone, or dimethyl sulfoxide (eq 5) to give high yields of O-methyltetrahydrofuranium salt,2 2-methoxypyridine,2,3 and methoxydimethylsulfonium tetrafluoroborate,2,3 respectively. Methylation of benzo[b]thiophene via (1) results in the formation of S-methylbenzo[b]thiophenium tetrafluoroborate (eq 6).3

O-Methylation of anisole has not been achieved using either trimethyloxonium tetrafluoroborate or methyl fluorosulfonate.2,3 However, methyl transfer to anisole oxygen has been shown to proceed when O-trideuteriomethyldibenzofuranium tetrafluoroborate is generated in situ from its appropriate diazonium precursor in the presence of anisole. After aqueous workup a mixture of trideuteriomethoxybenzene, unlabeled anisole, and benzofuran is obtained.3

Hydride Acceptor Agent.

Hydride transfer to the methyl group of (1) formed in situ has been reported to occur in the reaction with cycloheptratriene, to give tropylium tetrafluoroborate (eq 7),3 or with triphenylmethane, to yield triphenylcarbenium tetrafluoroborate (21%).3

Related Reagents.

Dimethoxycarbenium Tetrafluoroborate; Dimethyliodonium Hexafluoroantimonate; Methyl Fluoride-Antimony(V) Fluoride; Methyl Trifluoromethanesulfonate; Triethyloxonium Tetrafluoroborate.


1. Perst, H. In Carbonium Ions; Olah, G. A.; Schleyer, P. v. R., Eds.; Wiley: New York, 1976; Vol. 5, p 2012.
2. (a) Copson, A. J.; Heaney, H.; Logun, A. A.; Sharma, R. P. CC 1972, 315. (b) Fieser, L. F.; Fieser, M. FF 1974, 4, 336.
3. Downie, I. M.; Heaney, H.; Kemp, G.; King, D.; Wosley, M. T 1992, 48, 4005.

H. Perst

Philipps-Universität Marburg, Germany



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