Denitration radical reaction

Photoreduction of aromatic and aliphatic nitro compounds gives hydroxylamines or amines, which is well reviewed.125 The radical reaction of primary nitro compounds with tin hydride does not give the denitrated product (see Chapter 7), but give the corresponding oximes (Eq. 

The Knoevnagel reaction of 6-methoxytetralc nediamine followed by the nitro-aldol reach allylic nitro compounds Subsequent group Base catalyzed cyclizadon gives the I by radical denitration fsee Section 7 2 ... 

Enarnioselective nitro-dlclol reacdon Csee Secdon 3.3 or Michael reaction Csee Secdon 4.4 followed by radical denitration is useful as an altemadve indirect method of enandose-lecdve 1,3- or 1,4-addidon of alkyl anions fsee Eq. 7.70 and Eq. 7.71 . ... 

Gem-nitro imidazolyl alkanes undergo Sjy l reactions with the anion of various nitroalkanes, as shown in Eq. 5.36.54 The nitro group is replaced by hydrogen in 80-90% yield on treatment with Bu3SnH (see Chapter 7, which discusses radical denitration). 

Extension to carbocyclization of butadiene telomerization using nitromethane as a trapping reagent is reported (Eq. 5.48).72 Palladium-catalyzed carbo-annulation of 1,3-dienes by aryl halides is also reported (Eq. 5.49).73 The nitro group is removed by radical denitration (see Section 7.2), or the nitroalkyl group is transformed into the carbonyl group via the Nef reaction (see Section 6.1). 

Enantioselective nitro-aldol reaction (see Section 3.3) or Michael reaction (see Section 4.4) followed by radical denitration is useful as an alternative indirect method of enantioselective 1,2- or 1,4-addition of alkyl anions (see Eq. 7.7087 and Eq. 7.7188). 

Dauzonne has reported a simple synthesis of flavanones by radical denitration and dehalo-genation of 3-chloro-2,3-dihydro-3-nitro-2-aryl-4T/- I-benzopyran-4-ones,92 which are readily prepared by the reaction of salicylaldehydes with l-chloro-l-nitro-2-arylethenes (Eq. 7.73).93... 

Sequential Michael additions are versatile methods for the construction of cyclic compounds. Although a variety of these reactions have been developed, the use of alcohols as nucleophiles for the Michael addition to nitroalkenes has been little studied. Recently, Ikeda and coworkers have reported an elegant synthesis of octahydrobenzo[b]furans via the sequential Michael addition of 1-nitro-cyclohexene with methyl 4-hydroxy-2-butynoate in the presence of t-BuOK followed by radical denitration (Eq. 7.74).94... 

The Diels-Alder reaction followed by radical denitration provides a useful strategy for construction of six-membered compounds, in which the nitro group accelerates the reaction and also controls the regio-chemistry of the addition (Eq. 7.75).95... 

The difficulty in controlling the regiochemistry during radical-denitration of allylic nitro compounds is well known. The migration of the double bond is a serious problem, as shown in Eq. 7.96. This problem is overcome by a hydride transfer reaction in the presence of a palladium catalyst (Eq. 7.97).140... 

The regiochemical control of Pd-catalyzed hydride transfer reaction is much more effective than that of the radical denitration, as shown in Eq. 7.98. The base-catalyzed reaction of nitroolefins with aldehydes followed by denitration provides a new synthetic method of homoallyl alcohols (Eq. 7.99).140 Exomethylene compounds are obtained by denitration of cyclic allylic nitro compounds with Pd(0), HC02H and Et3N (Eq. 7.100).140b... 

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

In a recent application of this strategy, nitrocyclohexane 143 (prepared from nitrosugar 142 by intramolecular Henry reaction) was subjected to a radical denitration by HSnBu3, after protection of the hydroxyl groups to avoid side reactions. Inositol 146 was selectively obtained in good yield, once the hydroxyl protecting groups were removed (Scheme 45).101... 

The authors hold the opinion that the thiophenolyc moiety adds to the olefin bond and an electron adds to the nitro group. Hence, the anion-radical [R R C(SPh)CH(R )N02] controls the reaction. The final product is formed as a result of the cleavage of the latter anion-radical with the expnlsion of the nitrite ion and the phenylthiyl radical. The radical normally transforms into diphe-nyldisulfide. The yields of the denitrated olefines are high and reach 80-95%. 

Modified Giese reaction.6 The radical obtained by denitration of tertiary nitro compounds undergoes inter- and intramolecular reaction with activated double bonds. 

The denitration of tertiary nitroalkanes by BujSnH has proven to be an efficient synthetic methodology [27]. An example is illustrated in Eq. (8) [28]. The reaction proceeds via an intermediate nitroxide radical produced by addition of the tin radical to the oxygen atom of the nitro group followed by cleavage of the carbon-nitrogen bond. 

Chloro-3-nitro-2-phenylchroman-4-ones, available from various benzaldehydes, are readily both dechlorinated and denitrated to flavanones on reaction with Bu3SnH in a radical process initiated by AIBN <97SYN1305>. In the presence of boric acid and piperidine, silica gel effects the conversion of 2, 6 -dihydroxyacetophenones and benzaldehydes into 5-hydroxyflavanones <97H(45)71>. 

The scope of this reaction can be further increased in a number of ways. For example, addition of an electron deficient alkene to the anomeric radical in the denitration process allows the generation of more highly functionalised sugar mimetics (Scheme 11.4) [28]. 

Danishefsky also investigated the development of a trans Diels-Alder reaction in seeming violation of the inherent stereoselectivity of the reaction mechanism. The key to this process was the use of 1-nitrocyclohexene (80) as the dienophile. After a standard intermolecular Diels-Alder reaction with diene 79, cis cycloadduct 81 could be transformed preferentially into trans-fused product 82 upon radical denitration and enol ether hydrolysis. ... 

Scheme 5.1 Proposed degradation pathway of TNT and TNB by modified Fenton reaction by Hess [39]. Initial TNT- and TNB-hydroperoxyl radical formation led to step denitration and hydroxylation of the aromatic ring. The oxidation of phloroglucinol (1,3,5-trihydroxybenzene)...

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