Aromatic compounds, addition cycloaddition reactions

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). 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

Although the number of Diels-Alder cycloadditions with open-chain and alicyclic dienes is very large, the number of examples with aromatic heterocyclic compounds is relatively small. The introduction of a vinyl group as a substituent onto a heterocycle increases the number of possibilities of reaction. This new possibility, however attractive for synthetic purposes, is successful, with a few exceptions, only with 7r-excessive five-membered heterocyclic derivatives. As is usual in this kind of reaction, Michael additions, ene reactions, [2 + 2]-cycloadditions, and polymerization compete with the Diels-Alder cycloaddition. 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Among these reactions, the photochemical cycloadditions of C=C bom which can create up to four asymmetric carbons during the photochemical sti are particularly interesting, and numerous synthetic applications of this react have been reported. Advances in the understanding of the origin of asymmefa induction, during addition of alkenes with carbonyl derivatives, cyclic enom and aromatic compounds, will be discussed in detail. 

The accumulation of the cycloaddition product is related to its thermal stability in regard to nitrogen elimination. Here, elimination of nitrogen is even more pronounced because of two reasons the presence of the double C-C bond instead of a cyclopropane moiety (Scheme 11) and because it can produce corresponding furan derivatives. Furan is actually one of the rare aromatic heterocyclic compounds that easily participates in Diels-Alder reactions as a moderately active diene. Therefore, it is also reasonable to postulate that the furan derivative obtained after elimination of nitrogen is more reactive than 2,5-bis(trifluoromethyl)-l,3,4-oxadiazole. Additionally, the cycloadduct with a second molecule of cyclooctyne would be a final product of the cycloaddition reaction. To explore this possibility further, a semiempirical study of cycloadduct stability and activation barrier needed for cyclooctyne to react with furan was performed. 

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