Alkoxyamines, oxidation

The Manganese(V) catalyzed oxidation of S derivatives in the presence of a nitroxide provides excellent yields of phenylethyl alkoxyamines (Scheme 9.22).175,176 Alkoxyamines can also be prepared from acrylates by... 

The fact that hydroxylamine has been found among the products of transformations of nitroxyl radical during the oxidation of hydrocarbons is the evidence in support of this mechanism. Both mechanisms described earlier are realized in parallel and supplement each other. The result of the competition between them depends primarily on the temperature, because the thermal decomposition of alkoxyamine AmOR requires a fairly high activation energy (see Table 19.11). 

An existing polymer with an appropriate end group can be reacted with an alkoxyamine for instance, alkoxide polymerization of ethylene oxide yields a hydroxyl-terminated polymer that undergoes substitution (in the presence of sodium hydride) with a halogen-containing alkoxyamine. 

Sulfur stabilized nitrenes have been implicated in aziridination, e.g. (4) + (5) — (6) (94CPB27). Oxynitrenes derived from the oxidation of alkoxyamines are known, but are generally less efficient in aziridination. 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

The oxidation of amines, acylhydrazines, and alkoxy amines described in this section involves the formation of nitrenes or other intermediates, depending on the nature of the nitrogen substituent and the oxidant, although lead tetraacetate is commonly employed. For example, a nitrenium ion or an amino lead derivative was proposed as the intermediate in the oxidation of alkoxyamines with lead tetraacetate 2. However, evidence has been provided that the jV-acetoxy species is the intermediate in the aziridination of alkenes with V-aminophthal-imide and /V-aminoquinazolinone, where a mechanism analogous to the Bartlett mechanism for the peracid epoxidation of alkenes should be operating3,4. 

The oxidation of A-alkoxyamines with lead tetraacetate generated aziridines, albeit in low yields (<20%)9 11. Despite the almost complete diastereoselectivity claimed in the preparation of A-methoxyazi ridine 1 from methoxyamine and ( )-2-butene loss of the geometrical purity was observed in the reaction of excess ( )- and (Z)-2-butene with A-(butoxy)aminc in dichloro-methane to give 2n. For this reason a two-step mechanism involving an intermediate N Pb species or nitrenium ion was proposed. However, the completely diastereoselective preparation of both trans- and cis-2 was successively claimed by different authors (GC-MS analysis)49. 

Similar to hydroxylamines, alkoxyamines may be oxidized to the nitroso compounds under loss of the alkyl group [165]. Radical cations or radicals obtained from 7V-alk-oxyamines may undergo intramolecular cyclization to double bonds like in pentenyl-alkoxyamines [26] or in 5-methoxyamino-5-methyl dibenzocycloheptene [166] [Eq. (33)]. 

This reaction is achievable with a large palette of nucleophiles such as nitriles, cyanide, R-metal compounds, malonates, P(OR)3, as well as olefins enriched in electrons. Amides (e.g. dimethylformamide) as well as /V-carbo-alkoxyamines [41] can also be efficiently substituted by nucleophiles (two-electron oxidations). 

In an interesting application of classical organic chemistry, Bergbreiter employed a Meisenheimer rearrangement of allyl TV-oxides as a route to alkoxyamines.89 As shown in Scheme 12, 2,2,6,6-... 

Studer and Janza have developed a method for the generation of alkoxyamidyl radicals starting from the corresponding acylated alkoxyamines using IBX as a SET oxidant [1181]. For example, the stereoselective 5-exo cyclization of the respective N-centered radical generated from alkoxyamide 861 affords isoxazoUdine 862 (Scheme 3.344) [1181]. 

A QD-based sensor for carbon-centered radicals (alkyl or phenyl radicals) was developed by the use of 4-amino-2,2,6,6-tetramethylpiperidine oxide (4-amino-TEM-PO)-functionalized CdSe QDs. The presence of a nitroxide moiety in the functional capping layer leads to quenching of the luminescence of the QDs. The nitroxide units of the capping layer read with the carbon-centered radical to yield the diamagnetic alkoxyamine-modified protecting layer. As the resultant units are unable to quench the luminescence of the QDs, this leads to an intensified luminescence upon interadion with readive carbon-centered radicals. 

In a similar approach, quinolines 40 were prepared starting from alkoxyamine 36 and isonitriles 37 induced by TEMPO using an inter-intramolecular sequence [23] (Eq. 9.7). The precursor 36 via thermal C O homolysis and addition with aryl isonitrile 37 affords the corresponding imidoyl radical 38. Radical 38 can further react in a 5-ero-type cyclization to give the primary radical 39, which can undergo HAS and oxidation to provide the corresponding quinoline 40 ... 

NFSi has been used as a versatile oxidant in reactions that involve a Pd(II)/Pd(IV) catalytic cycle. The oxidative addition of NFSi to Pd(II), which generates a reactive Pd(IV) species is the first step in the functionalization of a Pd-C bond as exam-plified in the arylation of a Pd(II)-alkyl complex (eq 56). Depending on the reaction conditions, a competition between di-amination (see section NFSi as an amination reagent) and car-boamination exists. In the presence of an alcohol as solvent, a palladium-catalyzed alkoxyamination is observed with use of NFSi as oxidant. ... 

---