Ylide oxonium from carbenes

The oxonium ylide 390 is generated by the interaction of carbene with the unshared electron pair of the oxygen atom of ether 389, and subsequent sigmatropic rearrangement affords 391 [126]. The reaction was applied to the diastereoselective construction of 2,8-dioxabicyclo[3.2.1]octane, the core system 394 of zaragozic acid. The Rh-catalysed reaction of diazo ester 392 generates the bicyclic oxonium ylide 393 from the acetal, and its exocyclic 2,3-shift affords 394 [127]. 

Fig. 4.11. Generation and transformations of oxonium ylides from electrophilic carbene complexes.

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]. 

The major reaction pathways for sulfonium ylide formation generated from a metal carbene complex and sulfide are [2,3]-sigmatropic rearrangement and [l,2]-shift, similar to those of the oxonium ylide formation. 

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). 

The Al-0 bond is therefore more nucleophilic than basic. These same workers then studied the decomposition of various methylating agents MeX (X = Oil, I, 0S03Me) over ZSM-5 [29] (Table 5). The fact that hydrocarbons were formed from dimethyl sulfate argues against TMO ion as a key intermediate, since the oxygen in dimethyl sulfate is too weakly nucleophilic to form an oxonium ion. Hunter and Hutchings propose the initial formation of a surface—bound methoxyl, which is deprotonated to a surface-bound ylide. This surface-bound ylide is isoelectronic with surface-bound carbene [13,30],... 

Unsubstituted benzofuran-3-carboxylates are formed as the major products by the reaction of f-butyl (o-methoxyphenyl) (oxo)acetates with TMSC(MgBr)N2 (eq 26). Alkylidene carbenes are first formed from a-ketoesters, which cyclize to oxonium ylides. Elimination of methylene affords benzofurans. 

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