Nitronates mechanisms

The mechanism is presumed to involve a pathway related to those proposed for other base-catalyzed reactions of isocyanoacetates with Michael acceptors. Thus base-induced formation of enolate 9 is followed by Michael addition to the nitroalkene and cyclization of nitronate 10 to furnish 11 after protonation. Loss of nitrous acid and aromatization affords pyrrole ester 12. 

The mechanism for the formation of ( )-169 is explained in terms of an intermolecular nucleophilic dimerization. Nucleophilic addition of C-3 of 19 to the 3 position of the initially generated cation 168 gives an imine-nitrone... 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

Scheme 3.20. Proposed mechanism of asymmetric nitro-aldol reactions catalyzed by LLB, LLB-II, or LLB-LI nitronate...

Prepare the wash solution by adding an excess of solid nitron tetrafluoroborate to 100 mL of water and shaking mechanically for 2 hours. 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

Although these reactions are formulated as ionic reactions via 947 and 949, because of the apparent partial formation of polymers and inhibition of the fluoride-catalyzed reaction of pyridine N-oxide 860 with aUyl 82 or benzyltrimethylsilane 83 by sulfur or galvinoxyl yet not by Tempo, a radical mechanism caimot be excluded [61, 62]. The closely related additions of allyltrimethylsilane 82 (cf. Section 7.3) to nitrones 976 are catalyzed by TMSOTf 20 to give, via 977, either o-unsatu-rated hydroxylamines 978 or isoxazoHdines 979 (cf also the additions of 965 to 962a and 969 in schemes 7.20 and 7.21). 

The mechanism of 1,3-dipolar cycloaddition can be found in Ref. 63 and the references within. The reaction of nitrone with 1,2-disubstituted alkenes creates three contiguous asymmetric centers, in which the geometric relationship of the substituents of alkenes is retained. The synthetic utility of nitrone adducts is mainly due to their conversion into various important compounds. For instance, P-amino alcohols can be obtained from isoxazolidines by reduction with H2-Pd or Raney Ni with retention of configuration at the chiral center (Eq. 8.44). 

Israeli, A., Patt, M., Oron, M., Samuni, A., Kohen, R., and Goldstein, S. 2005. Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxy-lamine derived from cyclic nitrones. Free Radical Biology and Medicine 38 317-324. 

The l,2,4-oxadiazolidin-5-ones 139 undergo retro-l,3-dipolar cycloaddition when heated in vacuum to give the nitrones 140. Treatment in acetonitrile in the presence of base results in attack of the exocyclic a-proton and fission of the N-O bond followed by loss of carbon dioxide and formation of the benzylidenaniline 141 in undisclosed yields via the mechanism shown in Scheme 16 <2006SC997>. 

Study of the kinetics of the oxidation of asymmetric secondary hydroxylamines to nitrones with H2O2, catalyzed by methylrhenium trioxide, has led to the elucidation of the mechanism of the reaction (104). Full transformation of N,N -disubstituted hydroxylamines into nitrones upon treatment with H2O2 occurs on using polymeric heterogeneous catalysts such as polymer-supported methylrhenium trioxide systems (105). 

In order to study the mechanism of reverse Cope elimination reactions in the condensation of pentenal and hexenal with iV-methylhydroxylamine. it seemed reasonable to synthesize unsaturated nitrones (226). 

Formation of nitrones can be achieved in the first stage of a Krohnke type reaction in which p-n trosodi methy 1 an dine reacts with 2-oo-bromoacetylphenoxathiin in alkaline medium (336). The synthesis of a series of cyclic nitrones of structure (182) has been achieved by regioselective, and by an unusual [3 + 2] cycloaddition of a-nitrosostyrenes (181) to 1,3-diazabuta-l,3-dienes (180) (Scheme 2.64) (337a). Theoretical studies of the substitution effect at the imine nitrogen on the competitive [3 + 2] and [4 + 2] mechanisms of cycloaddition of simple acyclic imines with nitrosoalkenes have been reported (337b). 

EPR study of electrochemical properties of nitrones and registration of resulting radical cation (RC) or radical anion (RA), such as, in the nitrone transformation into nitroxyl radicals, allows us to get direct answers to the questions concerning mechanisms of nitrone group reactions. The following schemes A-E can be realized depending on conditions as below (Scheme 2.77) ... 

To study mechanisms C—E, it seems reasonable to employ both, electrochemical approaches and EPR-spectroscopy. It is important to be aware of the electrochemical properties of nitrones if used as spin traps for production of spin adducts (SA) is possible not only via homolytic process (C) but also via ionic processes shown in Scheme 2.77. In the case of (B), protonation can protect the... 

Oxidative Alkoxylation of Nitrones to a-Alkoxy Nitrones and a-Alkoxy Substituted Nitroxyl Radicals The first direct experimental evidence of the possibility to carry out radical cation nucleophilic addition to nitrones with the formation of nitroxyl radicals has been cited in Section 2.4. Further, such a reaction route was referred to as inverted spin trapping this route is an alternative to a conventional spin trapping (508-512). Realization of either mechanism depends on the reaction conditions namely, on the strength of both nucleophile and oxidant. The use of strong oxidants in weak nucleophilic media tends to favour the radical cation mechanism... 

In contrast to this, it has been concluded that the formation of spin adducts on ultraviolet irradiation or mild oxidation of indole nitrones, in the presence of a N -heleroaromatic base, proceeds according to the Forrester-Hepburn mechanism (522). 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

Aldol-type reactions of nitrones (303) occur with electron-deficient ketones, such as a-keto esters, a, 3-diketones, and trifluoromethyl ketones. These reactions are catalyzed by secondary amines. The use of chiral cyclic amines A1-A7 leads to a-(2-hydroxyalkyl)nitrones (304) in moderate yields and rather high optical purity (Scheme 2.120) (381). The mechanism of the nitrone-aldol reaction of iV-methyl-C-ethyl nitrone with dimethyl ketomalonate in the absence and presence of L- proline has been studied by using density functional theory (DFT) (544). 

Therefore, nitrones, being potential sources of stable radicals are able to participate directly in the growth stage of the polymer chain. In the case of nitrones the pseudoliving mechanism is realized at lower temperatures (50—60° C) then in the case of nitroxides (> 100° C). 

Dipolar cycloadditions of fullerene C6o to nitrones have been studied. Their mechanism, regiochemistry, and nature of addition have been investigated. All of the reactions lead to the formation of fullerene fused heterocycles. Theoretically, these reactions can lead to four types of additions, such as closed [6,6], open [5,6], closed [5,6], and open [6,6] additions (Scheme 2.317). Energetics and thermodynamic analyses of these reactions show that closed [5,6] and open [6,6]... 

It was demonstrated (83) that the reaction of dinitrostyrenes (28) with aryl diazo compounds RR CN2 afford nitronates (24 g) in good yields. These products contain the nitro group at the C-4 atom in the trans position with respect to the substituent at C-5 (if R =H). Since the reaction mechanism remains unknown, the direct formation of cyclic nitronates (24 g) from pyrazolines A without the intermediate formation of cyclopropanes also cannot be ruled out. 

Potassium nitroacetate 53a reacts with Me3SiCl in aprotic solvents to give SENA (51a) in moderate yield. At the same time, the introduction of yet another electron-withdrawing group (N02 or CC>2Me) stabilizes the anion of salt (53) to an extent that it does not react with Me SiCl by the Sm mechanism without electrophilic assistance. Hence, K or Na salts 53b, C are inert with respect to halosilanes, and silver or mercury salts are required for the preparation of the corresponding nitronates. The latter salts are much safer to use as diox-anate complexes. These complexes react with halosilanes in inert aprotic solvents... 

Another synthetic route to boryl nitronates (71k-m), which is, however, poorly developed (231), is based on the reaction of trialkylborons (exemplified by triethylboron) with a-nitroolefins (72a-c) proceeding by the 1,4-addition mechanism (Scheme 3.70, cf. Scheme 3.56). 

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