Alkoxy radicals methoxy

For example, with the simplest alkoxy radical, methoxy (CH3O), the following reaction occurs,... 

Fig. 8 Schematic representation of the geometries of the alkoxy radicals, methoxy, ethoxy, vinoxy, and i-propoxy. The highest occupied 7r-orbitals, approximately localized on the 0 atom, are indicated for each molecule.

These reactions become all the more important when the oxygen concentration is low and the concentrations of alkoxy and particularly methoxy radicals increase. Thus, the sudden onset of the final stage of the reaction is explained by the conversion of peroxy radicals in the absence of oxygen to the much more reactive alkoxy radicals (5). 

The ability of the oxygen-centered radical to abstract hydrogen from an adjacent carbon allowed the same team to develop a method for the selective removal of methoxy protecting groups in carbohydrates (Scheme 31) [48]. As exemplified by the substrate 44, treatment with DIB/iodine promotes formation of the alkoxy radical which abstracts a hydrogen from the C-4 methoxy... 

Rate coefficients for the addition of NO to alkoxy radicals have been inferred from the reverse reaction and estimates of the reaction entropies involved. These reactions are fast. Batt et al. (1975) reported values of 2 x 10 n cm3/molecules s for methoxy and about 5 x 10 11 for... 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

Relatively few studies have dealt with the reactions of primary and secondary alkoxy radicals (isopropoxy, methoxy, etc.) with monomers, fhese radicals are conveniently generated from the corresponding hyponitrites (Scheme... 

Boto and Hernandez have reported a short, efficient synthesis of chiral fiiryl carbinols from carbohydrates, such as 615, based on the alkoxy radicals Iragmentation reaction leading to the intermediate product 616 (Scheme 3.241) [648]. The same authors have developed an efficient procedure for the selective removal from carbohydrate substrates of methoxy protecting groups next to hydroxy groups by treatment with the PhI(OAc)2-l2 system [649]. 

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). 

Investigation of replacement of the 5-methoxy group by substituents with different electronic and lipophilic properties and methylation of the indole nitrogen or its replacement by a sulfur atom was evidence for the shift of the 5-methoxy group to the 4-position of the indole nucleus led to the most active radical scavenger but much less effective as a cytoprotectant [135]. 5-alkoxy-2-(N-acylaminoethyl)indole (Fig. 15) appeared as the key feature to confer both antioxidant and cytoprotective activity to the structure. Antioxidant activity seems essential for cytoprotection, but it is not sufficient, and there is no statistically significant correlation between the two types of activity. 

The fate of radicals captured by the surface sites with the formation of the alkoxy groups depends on the number of carbon atoms in the alkane molecule, as well as on the properties of the catalyst surface. According to the data obtained by Aika and Lunsford with the use of IR spectroscopy and TPD [16], in the case of MgO (an oxide with very high E[0 ) the methoxy groups decompose forming CO and H2, but in the case of higher alkoxides the formation of corresponding olefins takes place. 

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]. 

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