Trimethyloxonium fluoroborate 1258

The previously-reported alkylation reactions of aminocyclophosphazenes by trimethyloxonium fluoroborate, Me30BF4, have been extended. - With the dimethylamino-derivatives, N3P3Cl8-n(NMe2) (n = 1—4, or 6), a series of monomethylated derivatives, e.g. (44), was obtained, in which... 

When heated with trimethyloxonium fluoroborate in 1,1-dichloroethane, dilactone 842 experiences cleavage of both lactone rings to give a mixture of diene diesters, catalytic reduction of which produced 853 Reductive methylation of 853 proceeded with installation of the methyl groups on the exterior face for obvious steric reasons. Acyloin condensation followed by ferric cloride oxidation furnished a-diketone 854 which proved to be highly responsive to photoexcitation. However, irradiation of 854 did not provide 855 as expected. Rather, a most unusual reaction pathway was followed to deliver diol 856. 

Piperazine-2,5-diones react with trimethyloxonium fluoroborate (Meer-wein s reagent) or triethyloxonium fluoroborate to generate the corresponding lactim ethers. 

The 1,3,5-benzothiadiazepine system (561) has recently been prepared for the first time via methylation of the sulfimide (560 R = CH2Ph) with trimethyloxonium fluoroborate. The rearrangement did not take place however when R = Me or when alkyl halides were used as alkylating agents. The product (561) rearranged readily in acid to give a benzothiazo-line (79JCR(S)214>. 

The first diquaternary salt (67) of 1,7-phenanthroline was prepared using trimethyloxonium fluoroborate in nitromethane.15 The salt is immediately demethylated in the 1-position in dimethyl sulfoxide, trifluoroacetic acid, or water to give 68. If ethylene chloride is used as quaternizing solvent, the monoquaternary salt (68) is obtained. 

Phenanthroline is preferentially quaternized at the 9-position and gives a 1,9-dimethyl diquaternary salt with trimethyloxonium fluoroborate in nitromethane. This diquaternary salt also is readily demethylated in the 1-position.15... 

Phenanthroline reacts with trimethyloxonium fluoroborate in ethylene chloride solution to give the 2,7-dimethyl diquaternary salt.15,307 From polarography experiments it was shown to be reduced by a one-electron transfer to the corresponding radical cation. The reduction potential is lower than that of diquaternary salts of 1,10-phen-anthroline, an E0 value of —0.33 volt being recorded in aqueous solution. 

Crude 2,8-phenanthroline, prepared from irradiating trans-1-(3-pyridyl)-2-(4-pyridyl)ethylene, has been directly converted into the 2,8-dimethyl diquaternary salt with trimethyloxonium fluoroborate. This salt too is reduced by a one-electron transfer to the corresponding radical cation. The reduction potential (E0) is —0.47 volt in aqueous solution.15,307... 

Crude 3,8-phenanthroline was likewise converted into the 3,8-dimethyl diquaternary salt with trimethyloxonium fluoroborate. The salt... 

Dimethyl ether is converted to trimethyloxonium fluoroborate by the combination of boron trifluoride and methyl fluoride ... 

Scheme 2 Rapoport s Synthesis a Et2S04, tris(2-hydroxypropyl)amine, acetone, A, 1.5 h, 88% b Na, Ethyl cyanoacetate, EtOH, 93% c Pt02, H2 (33-49 psi), 12N HCl/EtOH, 7h d toluene, reflux, 1 h, 100% for steps c and d e trimethyloxonium fluoroborate, CH2CI2, rt, 43 h f NaBH4, EtOH, 5-10 °C - rt, 24 h, 61% for steps e and f g 10% Pd/C, CH20 aq,...

Figure 9. Modifications of carboxyl groups (a) esterification or amida-tion via water-soluble carbodiimides (b) trimethyloxonium fluoroborate ...

This reaction is an equilibrium and it may be necessary in making sulfonium salts from less reactive sulfides (sterically hindered ones for example) to use more powerful alkylating agents with non-nucleophilic counterions, for example, Me30+ BF4, trimethyloxonium fluoroborate (also known as Meerwein s salt). The sulfur atom captures a methyl group from 0+, but the reverse does not happen and the BF4 anion is not a nucleophile. 

The quaternization of 1-substituted imidazoles is usually easy unless steric factors intervene, or strongly electron-attracting groups are present, for example, 1-acylimidazoles can only be alkylated at N(3) with powerful alkylating agents such as methyl fluorosulfonate or trialkyloxonium fluoroborates. Trimethyloxonium fluoroborate does not methylate 1-dimethylaminosulfonylimidazole. Regiospecific synthesis of 3-substituted L-histidines can be achieved by alkylation of Ar-/-butoxycarbonyl-l-phenacyl-L-histidinc methyl ester at N(3), followed by reductive removal of the phenacyl group (Scheme 15). 

Cyclodimerization of -amino acids gives 2,5-dioxopiperazines 438. Treatment of 2,5-dioxopiperazines 438 with triethyl- or trimethyloxonium fluoroborate followed by oxidation with DDQ, chloranil, or iodine results in the formation of pyrazines 439 (Scheme 204) <1972J(P1)2494>. 

Aldehyde synthesis 2,4-Dimethylthiazole (1) when treated with n-butyllithium in dry THF and then with benzyl chloride yields 2-(2-phenylethyl)-4-methylthiazole (2) in 94% yield. This product is then alkylated with trimethyloxonium fluoroborate and reduced with sodium borohydride to give 2-(2-phenylethyl)-3,4-dimcthylthiazolidine (3) in high yield. The final step is hydrolysis using mercuric chloride as catalyst 3-phenyl-propanal (4) is obtained in 60% yield. 

The acyl phosphate (3) also N-methylates pyridine to form the salt (7). Treatment of (7) with trimethyloxonium fluoroborate regenerates (3). Treatment of (7) with alcohols or phenols gives the salt (8), which is rapidly hydrolyzed to the phosphate ester (9). 

Methylation of l - azabicycl(i. U(I ltex-i-enes. The 1,3-di-azabicyclo[3.I.(Jlhex-ene-3 (1) is selectively methylated by trimethyloxonium fluoroborate to form 2,2,3-trimethyl-4-phenyl-6-/i-nitrophenyl-l-aza-3-azoniabieyclo[3.I.0]hexene-3 teirafluoro-borate (2) in 93% yield. Reaction of (1) with /n-chloroperbenzoic acid (1, I35-I39 ... 

As mentioned earlier (Section IV,B,l,a) N-acylimidazoles, although resistant to quaternization by virtue of the electron-withdrawing nature of the acyl group, can be methylated by reagents such as methyl fluorosulfo-nate and trimethyloxonium fluoroborate. ... 

Oxonium ions have three bonds to a positively charged oxygen atom. All three bonds can be a bonds as in H30 or Meerwein s salt, trimethyloxonium fluoroborate, a stable (though reactive) compound described in Chapter 21, or one bond can be art bond as in the acetal intermediate. The term oxonium ion describes either of these structures. They are like alkylated ethers or 0-alkylated carbonyl compounds. 

Preparation.1 The reaction of lithium N-methylbcnzenesulfonimidoylmethide with benzaldehyde gives (2).2 This is converted by dehydration (TsOH) and methylation with trimethyloxonium fluoroborate into the reagent (1). 

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