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Radical dialkoxy-substituted

It is relevant to note that only cyclic aldo-nitrones tend to react in oxidative alkoxylations to give a,a-dialkoxy-substituted nitroxyl radicals. However, the only exception is methylene nitrone (215), which on oxidative methoxylation gives the a,a,a-trimethoxy-substituted nitroxyl radical (277). This is due to the proton in methylene nitrone (215), which, as in the case of cyclic nitrones, exists in the cis -position to the TV-oxide oxygen (Scheme 2.104) (517). [Pg.217]

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). [Pg.218]

A family of a,a-dialkoxy substituted benzyl radicals (221) has been shown to undergo [3-cleavage both from their ground and excited states. This is in contrast to the stability of benzyl-type radicals toward permanent photochemical... [Pg.286]

Few examples of chelation-controlled allylation reactions involving bicyclic chelates can be found in literature. Nagano and Azuma have shown that the allylation of a dialkoxy-substituted radical adjacent to a dimethyl acetal proceeds, in the presence of MgBr2 OEt2 (2.5 equivalents), with excellent stereocontrol independent of the synjanti stereochemistry of the radical precursor (Scheme 7) [24]. To best explain the high anti ratio obtained, they proposed bicyclic transition state M in which the allylstannane attacks from the side of the pyramidalized radical chelate. Surprisingly, when the same reaction was conducted at a lower temperature (0°C), poor selectivity was observed. No explanation was proposed to account for this decrease. It should be noted that the allylation gave no selectivity in the absence of Lewis acid. [Pg.449]

Some of the most promising alternative candidates were oxygen substituted, for example, for the polymers from monoalkoxy- and 3,4-dialkoxy-substituted thiophenes (see Chapter 3). The oxygen-bearing substituent is able to stabilize free radical and positive charge carrying forms by further delocalization, as depicted in Figures 4.1 and 4.2. [Pg.41]


See other pages where Radical dialkoxy-substituted is mentioned: [Pg.466]    [Pg.210]    [Pg.187]    [Pg.457]    [Pg.216]    [Pg.1697]    [Pg.99]   
See also in sourсe #XX -- [ Pg.449 ]




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