Dimethyl ether protonated

Whereas the C2—C4 alcohols are not carboxylated under the usual Koch-Haaf conditions, carboxylation can be achieved in the HF-SbF5 superacid system under extremely mild conditions.400 Moreover, Olah and co-workers401 have shown that even methyl alcohol and dimethyl ether can be carboxylated with the superacidic HF-BF3 system to form methyl acetate and acetic acid. In the carboxylation of methyl alcohol the quantity of acetic acid increased at the expense of methyl acetate with increase in reaction time and temperature. The quantity of the byproduct dimethyl ether, in turn, decreased. Dimethyl ether gave the desired products in about 90% yield at 250°C (90% conversion, catalyst/substrate ratio =1 1, 6h). On the basis of experimental observations, first methyl alcohol is dehydrated to dimethyl ether. Protonated dimethyl ether then reacts with CO to yield methyl acetate [Eq. (5.154)]. The most probable pathway suggested to explain the formation of acetic acid involves the intermediate formation of acetic anhydride through acid-catalyzed ester cleavage without the intervention of CO followed by cleavage with HF [Eq. (5.155)]. 

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

Wender and coworkers conclude that cobalt-catalyzed benzyl alcohol homologation involves the intermediate formation of car-bonium ions (8). However, since the methyl cation (CH3+) is unstable and difficult to form (9), it is more likely that methanol homologation to ethanol proceeds via nucleophilic attack on a protonated methyl alcohol molecule. Protonated dimethyl ether and methyl acetate forms have been invoked also by Braca (10), along with the subsequent formation of methyl-ruthenium moieties, to describe ruthenium catalyzed homologation to ethyl acetate. 

McMahon and Kebarle (1986) studied (MeF)2H as a model for (HF)2H . They thought this to be reasonable because the hydrogen bond of a proton bound to two methanols or dimethyl ethers, e.g. [MejO - H OMe2], gives cations with very similar energies to that of the hydrated oxonium ion [H2O H OHj] (Grimsrud and Kebarle, 1973 Meot-Ner, 1984). 

Another common solvent that contains the oxygen atom easily available for coordination with metal cations is THE. The ability of anion-radicals to remove a proton from the position 2 of THE is sometimes a problem. Dimethyl ether is more stable as a solvent its oxygen atom is also exposed and can coordinate with a metal cation with no steric hindrance from the framing alkyl groups. An added advantage of dimethyl ether is that, because of its low boiling point (-22°C), it can be readily removed after reductive metallation and replaced by the desired solvent. The use of aromatic anion-radicals in dimethyl ether (instead of THE) is well documented (Cohen et al. 2001, references therein). 

Methanol homologation - The strong acid hydride HRu(00)3X3, present in the catalytic ruthenium iodide solutions for the methanol homologation, is able to directly protonate the substrate and produce the methyl and successively the acetyl intermediates for the homologation to ethanol (eq. 2). It also catalyzes etherification to dimethyl ether (eq. 3). 

The polarization potential provides the energy due to electronic reorganization of the molecule as a result of its interaction with a point positive charge. The sum of the electrostatic and polarization potentials provides a better account of the energy of interaction of a point positive charge than available from the electrostatic potential alone. It properly orders the proton affinities of trimethylamine, dimethyl ether and fluoromethane. 

The infancy of these first-principles methods as applied to periodic zeolite lattices means that further detailed work is necessary, particularly in the area of verification of the ability of the pseudopotential to reproduce dynamic as well as static structural properties. However, the results found with these methods demonstrate that the debate concerning the modeling of the activation of methanol within a zeolite is far from concluded. The proton transfer to methanol as a reaction in its own right is, however, of relatively little interest. It does not govern the pathway or energetics of reactions such as dehydration to give dimethyl ether (DME). These are governed instead by the individual transition states that lead to the products, as we discuss in the next section. 

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 ... 

Other proton donors (HBr, HC1, H2S, H2Se, HCN, CH3SH) can have a similar promoting effect as water (56, 61, 70, 71). For alkylation of benzene with propylene the activity of HNaX, CaX, and NaX increased on addition of C3H7C1 (72), and CCU had a similar effect on the activity of NaY, CdY, and BaY (73), but it was not understood why the activity of CaY for the same reaction decreased on addition of the same products. The activity of NaY for alkylation increases considerably by introducing dimethyl ether and n-butyraldehyde. 

Olah et al.603 have observed the formation of cation 309 (protonated fluorometha-nol) upon treatment of formaldehyde in HF-SbF5 [Eq. (3.81)]. When Minkwitz et al.605 attempted to isolate salts of the ion, however, the hydroxymethyl(methylidene) oxonium ion 310 was obtained [Eq. (3.81)]. Crystal structure analysis of the hexafluoroarsenate salt shows that cations and anions are connected by short H -F distances, forming a three-dimensional network. The bond lengths of the C-0=C fragment (1.226 and 1.470 A) are longer than those in formaldehyde (1.208 A) and dimethyl ether (1.410 A). The C—O—C bond angle is 121.2°. 

Figure 4.3. H NMR spectrum of protonated dimethyl ether in HS03F-SbF5- SC)2 solution.

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