Trimethyloxonium cation

Trialkyl cations of oxygen, sulfur, and selenenium (and less commonly tellurium) are onium ions by virtue of their having one extra valence, although these three-coordinate ions invite comparison with carbenium ions. These ions (e.g., 28, 29, and 30) are not planar (as opposed to the analogous carbenium ions), but rather are pyramidal. These species have a well-characterized solution acid chemistry concentrated sulfuric acid is required for observation of the trimethyloxonium cation, but the sulfonium and selenonium ions require less strongly acidic solutions. 

The trimethyloxonium cation was proposed to be an intermediate in one of the proposed mechanisms of carbon-carbon bond formation in MTG chemistry on HZSM-5 (Scheme 2) (142). Chang and co-workers demonstrated that this cation could be ion exchanged into HZSM-5 from cold aqueous (CH3)30+BF4 (143). Munson and Haw (144) demonstrated that... 

Apply the same analysis to trimethyloxonium ion, MesO" a cationic electrophile. Examine the LUMO. What are the best sites for nucleophile-LUMO overlap How would electron transfer affect CO bond lengths ... 

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. 

However, the methyl cation (in itself a very energetic, unprobable species in the condensed state) is not expected to attack a carbon-hydrogen bond in dimethyl ether (or methanol) in preference to the oxygen atom. The more probable attack on oxygen would lead to the trimethyloxonium ion, which was observed experimentally447,463 On the action of a basic site the trimethyloxonium ion can then be deprotonated to form dimethyloxonium methylide ... 

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... 

The much higher stability of onium ions, compart to that of the mqority of carbocations, reflrets the fact that, formally, onium ions can be considared to be equivalent to carbocatkrns bonded to (and therefore stabilized by) nucleophilic ligands such as amines, ethers or sulfides. For instance, trimethyloxonium ion lb can be treated as a methylium cation CH la attached to methyl ether and using... 

Impure 3,7-phenanthroline, obtained by the irradiation of trans- - l-pyridyl)-2-(3-pyridyl)ethylene, was converted into the 3,7-dimethyl diquaternary salt with trimethyloxonium fluoroborate. This salt is reduced to the corresponding radical cation at a potential (E of —0.44 volt in aqueous solution. - ... 

The proposal of Lee and Wu was criticized by 01 ah et al [21] who were unable to induce the insertion of singlet methylene (generated by CH2N2 photolysis) into trimethyloxonium tetrafluoroborate. The alkylation of CH2N2 by alkyl cations or their precursors, on the other hand, is known. Accordingly, Olah and co-workers proposed an alternate interpretation where a labilized "incipient methyl cation" on the acid site reacted with CH2M2 to form ethylene. 

The possibility of using IV-alkylnitrilium salts as acyl cation equivalents has been investigated by allowing nitriles to react with methyl triflate and a nucleophilic aromatic derivative such as 1,3-dimeth-oxybenzene. A/-Methylnitrilium salts have also been made by warming nitriles with trimethyloxonium fluoroborate. Subsequent reactions with pyrroles and indoles gave the expected iminium salts in excellent yields, as shown in equation (61). ... 

Hydride abstraction from unsymmetrically substituted (cyclohexadiene) Fe(CO)3 derivatives may lead to mixtures of salts. The l-OMe(IV), for example, yields V (R = OMe) with some VI the former is hydrolyzed by water to the neutral separable complex 7 from which the salt can be regenerated by reagents such as trimethyloxonium fluoroborate. The 2-OMe (diene) yields VI with a little of the 3-OMe cation . The transition state of the reaction may resemble the cationic product. The l-C02Me (diene) also reacts in the V position d through initial attachment of the trityl to the carbonyl oxygen. Considerable experimental information is available on the course of abstractions ., ... 

Exclusive alkylation of normally unfavored positions in (po/y-N)-heterocyclics can be achieved by using acyl (acetyl, benzoyl, carbethoxy) as N-protective groups. - E Trimethyloxonium fiuoroborate in nitromethane dropped rapidly under anhydrous conditions into a cooled soln. of l-acetyl-1,2,4-triazole in the same solvent, methanol added to hydrolyze the acyl cation to the protonated salt, and the aq. soln. of the latter neutralized with Na-carbonate -> 4-methyl-l,2,4-triazole. Y 88%. F. e., also alkylation with carboxonium salts, s. R. A. Olofson and R. V. Kendall, J. Org. Chem. 55, 2246 (1970). 

Oxidative addition of trimethyloxonium tetrafluoroborate to a Pt(0) complex containing dimethylfumarate and phen ligands 108 leads to the corresponding cationic methylplatinum(ii) complex 109, and substitution of the coordinated olefin by dimethyl acetylenedicarboxylate 110 leads to insertion of the coordinated alkyne into the Pt-Me bond in MeCN, giving the complex having an alkenyl group and the solvent as ligands 111 (Scheme 16). 

The reactions of various iron carbonyl complexes, such as Fe(GO)4(NMe3), with allene compounds under photo-lytic conditions, yield chelated 77 -allyliron complexes. Two brief reviews discussing the chemistry and application to organic synthesis of these (7r-allyl)tricarbonyl iron lactone complexes have appeared recently. Reaction of the iron lactone complexes with trimethyloxonium tetrafluoroborate yields the carbene complex 23 in good yields. Treatment of the cationic carbene complex with triphenylphosphine results in substitution at the terminal end of the allyl ligand of the trimethylenemethane complex 24. 

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