Pericyclic reactions addition

Dinitrobenzofuroxan (DNBF) is known as a superelectrophile due to its high reactivity both as an electrophile and in its pericyclic addition reactions. NMR studies show that reaction with 2-aminothiazole and its 4-methyl derivative yield anionic carbon-bonded adducts such as (11) by reaction at the 5-position, whereas the 4,5-dimethyl derivative reacts via the exocyclic amino group. Kinetic studies of the first two compounds, both in acetonitrile and in 70 30 (v/v) water-DMSO, have been used to assess their carbon nucleophilicities and place them on the Mayr nucleophilicity scale.55 In a related study, the nucleophilic reactivity, in acetonitrile, of a series of indoles with both DNBF and with benzhydryl cations have been compared and used to determine nucleophilicity parameters for the indoles.56... 

Pericyclic addition reactions are attractive C-C, C-N, and C-O bond-formation reactions for many reasons. They involve ... 

Example Anoth er example of Iron tier orbital theory uses the reaction ol phenyl-butadiene with ph en ylethylene. This reaction is a [4-1-21 pericyclic addition to form a six-membered ring. It could proceed with the two phenyl rings close to each other (head to head) or further away front each other (head to tail). 

Since the double-bond configuration is established in the final elimination step from a /t-silicon-(or tin-) substituted carbenium ion in a conformation of lowest energy, often high E selectivity is observed. In reactions of allylstannanes, catalyzed by tin(TV) chloride or titanium(IV) chloride, occasionally a metal exchange occurs, followed by the pericyclic addition pathway leading to the iwti-diastereomers17 19. A more detailed discussion is given in Section D.1.3.3.3.5. 

An elegant approach to polyquinanes has recently been described by Paquette et aL1281 By means of an anionic-pericyclic domino reaction four new C-C bonds are formed in one process. First, a two-fold addition of a vinyl lithium reagent, e.g. cyclopentyl lithium to diisopropyl squarate takes... 

The [3+2] cycloaddition strategy provides an effective method to access valuable intermediates for the construction of biologically important alkaloids, amino acids, amino carbohydrates and P-lactams [58-62]. The reaction involves the concerted pericyclic addition of a dipole and a dipolarophile and considerable efforts have been made to render these reactions asymmetric nsing Lewis acid catalysis and chiral anxiliaries [63]. 

In that report, electron-rich olefins were found to react with Rh(I) vinylidenes differently from the manner in which electron-neutral olefins do (Scheme 9.5). A nucleophilic addition reaction, rather than a pericyclic ring closure, was evident. Because the reaction is strongly promoted by organic base, proton-shuttling is believed to account for the conversion of zvfitterionic intermediate 26 to the observed... 

We already have discussed a few addition reactions that appear to occur in a concerted manner. These include the addition of diimide, ozone, and boron hydrides to alkenes (Sections 11-5, 11-7A, and 11-6B). Concerted reactions that have cyclic transition states often are called pericyclic reactions. Other examples will be considered in later chapters. 

The first chapter, High Pressure Synthesis of Heterocycles Related to Bioactive Molecules by Kiyoshi Matsumoto, presents a unique high-pressure synthetic methodology in heterocyclic chemistry. Basic principles and fruitful examples for pericyclic reactions, such as Diels-Alder reactions, 1,3-dipolar reactions, and also for ionic reactions, such as Sn and addition reactions, are discussed. The review will be of considerable interest to heterocyclic chemists and synthetic chemists. 

Many types of pericyclic and cycloaddition reactions have been documented. Although there are no general guidelines for the asymmetric preparation, reagents such as chiral catalysts are providing more general routes. Many of the reactions discussed rely on the use of low temperature. Although it is expensive to conduct low temperature reactions on an industrial scale, reactions that need temperatures of down to -105°C can be conducted. It should be noted that at such temperatures only stainless steel vessels, which require neutral or basic conditions, can be used at these extreme temperatures. In addition, reactions that involve the use of a metal can cause contamination problems in wastewater or the product. 

In the [2-i4] pericyclic cycloaddition reaction known as the Diels-Alder reaction, fluonne-containing compounds have been widely used as dienes, dieno-philes, or both Much of the fundamental work, including many comprehensive and systematic studies, was done before 1972, and Hudlicky provides an excellent summary of this work [9] Additional sources for early work in this area are reviews in Organic Reactiom [61] and Fluorine Chemistry Reviews [62]... 

Pericyclic Reactions of Anion Radicals General aspects of anion radical pericyclic additions... 

A 1,3-dipolar cycloaddition is a pericyclic addition in which a 1,3-dipole reacts with a dipolarophile to form a five-membered ring. Common dipolarophiles are alkenes and alkynes. Since 1,3-dipoles typically contain heteroatoms, these reactions are often used to produce five-membered heterocycles. 

Finally, in a remarkable series of recent papers, Bernard , Olivucci, Robb and their collaborators (1990-1994) demonstrated that the S -S touching actually is not avoided even in the low-symmetry case of real organic molecules, and they confirmed the earlier conjectures by computing the actual geometries of the funnels (conical intersections) in the S, surface at a reasonable level of ab initio theory. They also pointed out that still additional reactions can proceed through the same pericyclic funnel, such as the cis-trans isomerization of butadiene. 

The C,C-bond-forming reactions can be classified as polar reactions, in which an activated donor reacts selectively with an acceptor or vice versa radical reactions, which are increasingly applied in coupling or addition reactions because they do not interfere with polar groups in the molecule and thus save steps as no protections are needed [43] pericyclic reactions, especially cycloadditions, for the construction of ring compounds with high stereoselectivity [45] and transition metal-catalyzed reactions [46]. 

Some other methods, although pertinent to the subject of this section, are also covered as partial aspects of other sections in this volume, e.g., asymmetric cyclopropanations (Section D.l.6.1.5.), [3 + 2] cycloadditions (Section 1.6.1.2.) and other metal-induced cycloaddi-tion-lype reactions (Sections D.l.6,1.3., D.l.6.1.4, D.l.6.1.6.). Enantioselective versions of metal-induced formal pericyclic C —C bond forming addition reactions, such as ene reactions (Section D.l.6.2.), metallo-ene reactions (SectionD.l.6.2.), Cope and related reactions (Sections D.l.6.3., D.l.6.4.) are also covered elsewhere in this volume. 

Cycloaddition reactions involve the combination of two molecules to form a new ring. Concerted pericyclic cycloadditions involve reorganization of the Tr-electron systems of the reactants to form two new a bonds. Examples might include cyclodimerization of alkenes, cycloaddition of allyl cation to an alkene, and the addition reaction between alkenes and dienes (Diels-Alder reaction). 

As with the first edition, the first five chapters of this book consider structure and bonding of stable molecules and reactive intermediates. There is a chapter on methods organic chemists use to study reaction mechanisms, and then acid-base reactions, substitution reactions, addition reactions, elimination reactions, pericyclic reactions, and photochemical reactions are considered in subsequent chapters. In each case I have updated the content to reflect developments since publication of the first edition. 

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