Oxonium ylide mechanism

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 ... 

Three issues need to be addressed in connection with oxonium ylide mechanisms. The first question concerns the existence of oxonium ylides. Whereas sulfonium, phosphonium, and ammonium ylides are well known, oxonium ylides have not been isolated. Secondly, if they exist, will they undergo Stevens rearrangement, or decompose via other routes Finally, is the zeolite conjugate base sufficiently basic to abstract a proton from oxonium ions to form an ylide ... 

Fig. 2. Oxonium ylide mechanism for deuteration and carbon-carbon bond formation.

A mechanism that has received a great deal of attention is the oxonium ylide mechanism.Dimethylether is methylated to trimethyloxonium, which is subsequently deprotonated to form surface associated methylene-dimethyloxoniumylide. The next step is either an intramolecular Stevens rearrangement, leading to the formation of methylethyl-ether, or an intermolecular methylation, leading to the formation of ethyl-dimethyloxoniumion. In both cases ethylene is obtained via jS-elimination. 

Oxonium ylide mechanism. Adapted from SpiveyJj, Froment CF, Dehertog WJFI, Marchi AJ. In SpiveyJj,... 

More recently, in a theoretic study, Lesthaeghe et al. claimed that no ethylene can be formed through oxonium ylide mechanism, and their findings correspond to the fact that direct mechanisms fail in explaining the ethylene formation in MTO [122]. They found that ethylene cannot be eliminated from the oxonium intermediates and should be produced via hydrocarbon pool mechanism. 

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

Oxonium ions and oxonium ylides are invoked in several reaction mechanisms (see Sections 14.02.10.3 and 14.02.10.4). 

The most popular mechanisms at present invoke oxonium ylides as intermediates. van den Berg et al. [20] proposed that DME is protonated by a Bronsted site, and the resultant ion suffers nucleophilic attack by a second molecule of DME to form TMO with release of MeOH. The TMO ion is then deprotonated by a basic site to form the dimethyloxonium methylide, which undergoes a Stevens-type rearrangement to give methylethyl-oxonium ion. MeOEt is subsequently formed upon p— elimination. No experimental evidence was offered in support of the scheme. 

The 3+2 cycloreversion of the transient bicyclo[m.3.0]alkan-3-on-2-yl-l-oxonium ylide that was genenrated by the rhodium-catalyzed intramolecular reaction of a tetrahydrofuran substituted diazoketone was found to be stereospecific as illustrated below. The result supports a concerted mechanism <04JOCI331>. 

Proposed mechanisms for C-C bond formation can be organized into the following classifications carbene [70,71], carbocation [72], oxonium ylide [73,74], and free radical [75]. Some of these mechanisms [72-74] invoke a framework-bound methoxy species as a methylating agent or intermediate in the reaction. Dybowski and coworkers reported a C NMR study that supported the formation of methoxy groups in HZSM-5 [76], but the existence of these species is still controversial [77]. Framework-bound alkoxy species clearly do form on other molecular sieve catalysts. For example, Anderson and Klinowski have shown that when methanol is heated to 573 K on the silicoaluminophosphate catalyst SAPO-5, a framework-bound methoxy species forms, which is readily seen in a C MAS spectrum obtained after heating [77]. 

The mechanism of rhodium-catalysed O—H insertion of diazomethane and diazoacetate with water has been computationally investigated. These DPT calculations have unmasked that rhodium oxonium ylide is largely easier to form than the free ylide. The results have further indicated the preference for an ionic stepwise pathway rather than a concerted one. 

Prof. Yu summarized that the metal carbene O-H bond insertion mechanism includes three steps. Step a is that transition metal catalyst decomposes the a-diazo carbonyl compound A and releases nitrogen to form the metal carbene product B Step b is that metal carbene product B and the substrate R2OH form metal-associated oxonium ylide C Step c is that metal-associated oxonium ylide (C or... 

Formation of ethylene and propylene via oxonium methyl ylide mechanism. 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

---