Free radicals methoxy groups

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 reaction products are the free amine, hydrogen, CaBr2 and a brown powder. Here too, a compound with free radicals is indicated by the ESR spectrum. Since the methylamine group is nearly the same size as the methoxy group, the composition is analogous to the methoxy derivatives. The radical concentration has the same value. 

When other free radical processes are available, the mixture of products becomes more complicated. For instance, 0-(methoxymethyl)phenylsilacyclobutane, when irradiated, is proposed to undergo a competing loss of ethylene to yield silene and migration of the methoxy group from the benzylic carbon to silicon, leading to the mixture of products (Scheme 6) <2003JA8096>. 

Thus, the principle of the formation of carbocations, clearly shown by Scheme (51), requires judicious choice of both components the source of free radicals should not only be able to dissociate sily - either thermally or photo-chemically - but also exhibit a chemical structure which stabilizes the resultant carbenium ion (e.g. the methoxy groups and an aromatic ring as in Scheme (50). 

A second, closely related experiment was conducted with the Barton ester 6 [4]. Here, the results were cleaner owing to the stronger C-H bonds, which effectively eliminated the deprotonation pathway and the formation of any allyl radicals. Moreover, as expected, because of the electron-donating effect of the para-methoxy group the diffusively free radical cation 8 could now be observed even in pure acetonitrile (Scheme 2). However, the telling feature of this series of experiments was... 

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]. 

One of the limitations of anionic polymerization with respect to preparation of block copolymers is the rather limited range of monomers that can be polymerized anionically to form polymers with well-defined stmctures. One solution to this problem is to utilize anionic polymerization to form a well-defined polymer that is functionalized with an end group that can be used to initiate polymerization via another polymerization method, for example, controlled free-radical polymerization. One such functional group is the aminoxy group which can be used to initiate nitroxide-mediated radical polymerization (NMP). °° PSLi has been reacted with 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (MTEMPO), a stable nitroxide free radical, in THF at -78 °C as shown in eqn [30]. The mechanism of this functionalization was presumed to occur... 

Commercial vinylsilanes usually have the vinyl group directly attached to the silicon atom. Common hydrolyzable groups are methoxy, ethoxy, or 2-methox-yethoxy. The vinyl functionality is used in polymers that are cross-linked by a free-radical process (peroxide cure), but it is, however, not sufficiently reactive for all systems, and methacryloxy functionality is sometimes preferred as shown in Table 4.2 [35]. Vinylsilanes because of their overall cost/performance advantages have become the industry standard for EPR and EPDM and wire and cable applications. 

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