Trimethyloxonium ion

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 ... 

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 ... 

The oxonium ylide mechanism requires a bifunctional acid-base catalyst. The validity of the oxonium ylide mechanism on zeolites was questioned459,461,464 because zeolites do not necessarily possess sufficiently strong basic sites to abstract a proton from the trimethyloxonium ion to form an ylide. It should, however, be pointed out, as emphasized by Olah,447,465 that over solid acid-base catalysts (including zeolites) the initial coordination of an electron-deficient (i.e., Lewis acidic) site of the catalysts allows formation of a catalyst-coordinated dimethyl ether complex. It then can act as an oxonium ion forming the catalyst-coordinated oxonium ylide complex (10) with the participation of surface bound CH30 ions ... 

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

The intermediacy of trimethyloxonium ion has been proposed in the first step of the acid-base-catalyzed conversion of methyl alcohol into gasoline range hydrocarbons over W03-A1203.117 The crucial step is the base-catalyzed subsequent deprotonation of the trimethyloxonium ion to the very reactive surface-bound... 

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... 

Studies of the distribution of the positive charge (electron deficiency) in oxonium ions (calculations using the CND02 method) showed that the positive charge is located not on the oxygen atom but mainly on the adjacent carbon and hydrogen atoms. The calculated charge distribution in the trimethyloxonium ion may be illustrated as foUows ... 

The predominant adsorbed species is expected to be the trimethyloxonium ion. This ion is also fairly large, and its formation moves the compensating positive charge "uncomfortably" far from the negative lattice position it is supposed to compensate. Therefore... 

Unlabeled trimethyloxonium ion with BF4 , PF6 , and SbCI6 as counter ion gave similar product distributions. cNot determined. 

Substituent effects on the reactions of arenesulphonates with trimethyloxonium ion (equation 13) in acetonitrile70 (and with methyl triflate71) show p = —1.1, e.g. p-methoxy-benzenesulphonate anion reacts about 15-fold faster than p-nitrobenzenesulphonate... 

It is not necessary to invoke a free Meerwein-type trimethyloxonium ion in the heterogeneous catalytic reaction. Lewis-type coordination complexes of dimethyl ether with the acidic catalyst sites having oxonium ion character can be involved, giving subsequently via deprotonation surface-bound oxonium ylides followed by methylation and elimination of ethylene (Scheme 20) . ... 

In one of the few alkylations known for oxysulphonium ylides, the phenacylide (93) underwent Oalkylation with trimethyloxonium ion to give a mixture of stereoisomers. Schmidbaur and Kapp °° have published... 

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