Nitroso acetals reactions with nucleophiles

Reactions of Cyclic Conjugated Ene Nitroso Acetals with Nucleophiles The reaction of six-membered cyclic ene nitroso acetals (475) with nucleophiles has a high potential and will probably provide the basis for a promising procedure for the synthesis of polyfunctional compounds from very simple precursors, the more so that the configurations of stereocenters in the starting nitronate (475) can be retained in particular transformations (Scheme 3.253). Unfortunately, the available data (264) are insufficient to elucidate the complete mechanism of this process. 

A recently developed general procedure for the synthesis of cyclic nitroso acetals is based on the reaction of cyclic nitronates with C-nucleophiles under conditions of electrophilic catalysis (Scheme 3.153 for more details, see Section 3.5.2.3). 

In many cases, the yields of these products are high. However, the use of /V-silylated triazoles as nucleophiles or the use of cyclic nitroso acetals (475) substituted at the C-3 atom leads to a noticeable decrease in the yield of the oximes. Therefore, steric hindrance in nitroso acetals and a decrease in nucleophilicity of A-centered nucleophiles result in an increase in the contribution of side reactions. It should be emphasized that C -nucleophiles, such as anions of nitro compounds, are not involved in coupling reactions with cyclic nitroso acetals (475). However, the products, which formally correspond to the C,C-coupling mechanism, can be prepared by the nucleophilic substitution of chlorine in compound (476 d) by a Sa/2 mechanism (Scheme 3.254, product (483c), the yield was 79%). 

It cannot be ruled out that this reaction occurs by an Sjv2 mechanism, which has been proposed many times earlier for the interpretating the reactions of nucleophiles with BENA (see, e.g., Scheme 3.236). The removal of the Me3Si group from nitroso acetals is facilitated by the contribution of the interaction. 

The process shown in Scheme 3.265 substantially complicates the reaction of cyclic nitroso acetals with nucleophiles at the p-C atom of the enamine fragment. 

Mercuric acetate and mercuric trifluoroacetate are the usual reagents.66 The synthetic utility of the mercuration reaction derives from subsequent transformations of the arylmercury compounds. As indicated in Section 7.3.3, arylmercury compounds are only weakly nucleophilic, but the carbon-mercury bond is reactive toward various electrophiles. The nitroso group can be introduced by reaction with nitrosyl chloride67 or nitrosonium tetrafluoroborate68 as the electrophile. Arylmercury compounds are also useful in certain palladium-catalyzed reactions, as discussed in Section 8.2. 

The reactivity of the ethenediazonium salt 9.100 towards the nucleophiles mentioned shows that it has the properties of the corresponding carbocation, since it can ethylate the nucleophile and is prone to attack at the C()ff)-atom of the original ethene-l-diazonium ion. The thermal decomposition pattern is typical of that for an oxonium salt. Reactions with amines are similar to those of ketene acetals. No product that could be explained in terms of an azo coupling reaction, e.g., with 2-naphthol, could be observed. The electrophilicity of the diazonio group is, therefore, low. N-Azo coupling products with azide ions have been postulated with good arguments, however, by Kirmse and Schnurr (1977) with certain short-lived ethene diazonium intermediates produced from nitroso oxazolidones. 

The molecular mechanism of the domino reaction of nitroalkenes with silyl enol ethers to give nitroso-acetal adducts has been investigated using computational procedures at the PM3 semiempirical level. The reaction has been found to comprise three consecutive steps the first and rate-determining step is the nucleophilic attack of the... 

The vicinal position of nucleophilic amino and strongly electrophilic nitroso groups makes A -aryl-2-nitrnsoanilines 413 (Wrobel and Kwast 2007, 2010) very interesting starting materials for domino reactions with properly eqmpped dipolar parmers, leading to a variety of heterocyclic systems. The reactions of these compounds with comparatively acidic sulfones (Scheme 2.75, route a) or acetates and phosphonoacetates (Scheme 2.75, route b) appeared to be efficient ways of the synthesis of benzimidazoles 414 (Wrobel et al. 2011) and quinoxalin-2(lff)-ones 415 (Wrobel et al. 2013). 

The mechanism for the Demjanov reaction is a well accepted mechanism. The formation of the diazonium occurs as outlined. Reaction of the amine 1 with activated NOX 12 (X is thought to be ONO) delivers the A-nitroso compound 13, which undergoes rearrangement to provide hydroxyl-diazo material 14. Protonation of 14 and loss of water provides the diazonium, which suffers loss of N2 to provide cation 16. Carbocation 16 then undergoes rearrangement to provide 3, which traps a nucleophile (typically water sometimes nucleophilic co-solvents or acid counterions like acetate) and provides the alcohol 4. 

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