Methyl cations triethylamine

The reaction between free methyl cations and triethylamine in the gas phase has been studied radiochemically. An experimental study of the energy dependence of the rate coefficients of the reaction of methyl cation with N2O and NH3 in the gas phase in the presence of helium and argon buffers shows that reaction occurs at nearly collisional rates at near thermal energies N2O gives 94% HCO with some CH3O, ... 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

Photolysis of DMDAF in benzene containing methyl alcohol gives the ether expected from the reaction of the singlet carbene. Monitoring this reaction by laser spectroscopy reveals that the detected transient reacts with the alcohol with a bimolecular rate constant very near the diffusion limits. In contrast, the transient reacts with triethylamine at least 100 times more slowly than it does with alcohol (Table 7). This behavior is inconsistent with identification of the transient as the cation or radical and points to its assignment as the singlet carbene. 

Finally, in contrast to the reactions reported above, 49 reacts131 with dimethyl(methyl-thio)sulfonium fluoroborate (DMTSF) and triethylamine tris(hydrofluoride) in dichlorome-thane to give only 5% of the 1,2-addition product 122. The main products, present in 73 27 ratio, are the exo-exo and endo-exo adducts 123 and 124, formed by exclusive exo attack of the electrophile on the double bond, followed by transannular -participation in the intermediate bridged cation and final addition of fluoride to the nortricyclic cation from both the exo or endo side (equation 120). 

Dimethyl and methyl phenyl 4,4-dimethyl-2,6-dioxocyclohexylidene tellurium compounds transferred a methyl group to triethylamine, triphenylphosphane, tris[di-methylamino]phosphane, and triphenylarsane. The methyl onium cations were isolated as lelraphenylborates. The alkylidene organo telluronium intermediate was identified by 31P-NMR spectroscopy in the reaction of the ylide with tris[dimethylamino]phosphanc . 

A mechanism suggested for Swern-Moffatt oxidation with TFAA is shown in Scheme 8.6. In the first step, DMSO reacts with TFAA to form cationic reactive species I, which is known to be stable only below —At higher temperatures, rearrangement of I takes place to give species II. The reaction of II with an alcohol IQ upon treatment with a base leads to formation of a major by-product, trifluoroacetic acid (TFA) ester VII. Therefore, the first step should be carried out below —50 °C. In the second step, reactive species I is allowed to react with an alcohol HI at or below —50°C to obtain intermediate IV. IV may also undergo the Pummerer rearrangement to give a methyl thiomethyl (MTM) ether VI upon treatment with a base. In the third step, IV is treated with a base (usually triethylamine) to obtain the desired carbonyl compound V and dimethyl sulfide. 

The silyl triflate 8 (Scheme 3) shows an ambireactive behavior of the cation. Reaction with a methylmagnesium chloride or water leads to the methyl-substituted product and to the siloxane, respectively, as expected for an electrophilic silicon center. An oxonium ion reactivity is observed in the reaction with neutral Lewis bases such as triethylamine and trimethylphosphine. 

Although substitution at the 2-position of the imidazolium cation was considered to prevent the side reaction in the MBH reaction, Handy and Okello have found that even the 2-methyl substituted imidazolium cation was not completely inert. They found that the 2-methyl group underwent slow proton exchange even in the presence of a weak base such as triethylamine (Scheme 1.113). The acidic nature of this methyl group was further verified by analyzing the products obtained from attempted methylation of the imidazolium salt 302. When 302 was treated with excess NaH and CH3I, none of the expected product 304 was detected, instead product 303 was obtained (Scheme 1.114). ° ... 

This, however, is not what happens. The pAT of this proton is simply too high (well above 30, depending on and R ) for abstraction by triethylamine. Instead, the amine abstracts a proton from one of the methyl groups of the dimethylsulfonium part of the molecule, where the pA is about 16-17, to create a sulfur ylide. This is reasonable a sulfonium cation should stabilize an adjacent negative charge quite effectively ... 

The cycloaddition precursor 227 was prepared by alkylation and decarboxylation of enantiomericaUy pure (3-ketoester 225, which led to ketone 226. Chlorination of 226 was accomplished by quenching the corresponding lithium enolate with triflic chloride to afford a-chloroketone 227. Without purification, a-chloroketone 227 was treated with triethylamine in a solution of 2,2,2-trifluoroethanol and ethyl ether (1 1 mixture). This gave cycloadduct 230 as a 25 1 mixture of isomers in 74% yield from 227 after treatment of 229 with tosic acid. The exquisite stereoselectivity can be rationalized from diene endo attack on the face opposite of the methyl-bearing stereocenter of the cyclic oxyaUylic cation 228. Cycloadduct 230 was then subsequently converted into (+)-dactylol 224 over several steps. 

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