Organic chemistry cycloaddition reaction

Although not as common as in organic chemistry, cycloaddition reactions provide a useful approach to the synthesis of inorganic heterocycles when the appropriate building blocks may be obtained readily (see Section 2.5). For... 

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle. The classic example of such a process is the Diels-Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry. Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes, representative examples of which are given in Schemes 3.1-3.3. This chapter will discuss these reactions and their use in synthesis. 

If the analogy that is drawn between the Si=Si dimer on the Si(100)-2 x 1 surface and an alkene group is reasonable, then certain parallels might be expected to exist between cycloaddition reactions in organic chemistry and reactions that occur between alkenes or dienes and the silicon surface. In other words, cycloaddition products should be observed on the Si(100)-2 x 1 surface. Indeed, this prediction has been borne out in a number of studies of cycloaddition reactions on Si(100)-2x1 [14], as well as on the related surfaces of Ge(100)-2 x 1 (see Section 6.2.1) and C(100)-2 x 1 [192-195]. On the other hand, because the double-bonded description is only an approximation, deviations from the simple picture are expected. A number of studies have shown that the behavior differs from that of a double bond, and the asymmetric character of the dimer will be seen to play an important role. For example, departures from the symmetry selection rules developed for organic reactions are observed at the surface. Several review articles address cycloaddition and related chemistry at the Si(100)-2 x 1 surface the reader is referred to Refs. [10-18] for additional detail. 

As emphasized by Fukui, the mechanism of chemical reactions can often be understood in terms of frontier orbitals—the highest occupied molecular orbitals (HOMO S) and lowest unoccupied molecular orbitals (LUMO s) of reacting molecules. Ideally, the frontier orbitals of the reactants interact to form the MO s of the products. And it is in such transformations that orbital symmetry is conserved. We will consider two relevant examples from organic chemistry electrocyclic reactions and cycloadditions. 

Cycloaddition reactions are one of the most important organic chemical reactions. Diel s-Alder reaction, is the most popular example of cycloaddition reaction and known to every serious student of organic chemistry. Cycloaddition provides route to synthesise cyclic compounds from acyclic reactants. 

In the [2+4] pencyclic 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 exeellent summary of this work [9] Additional sources for early work in this area are reviews in Organic Reactions [61] and Fluorine Chemistry Reviews [62]... 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

See e.g. (a) W. Cahhuthehs, Cycloaddition Reactions in Organic Synthesis, Tetrahedron Organic Chemistry Series Vol. 8 Pergamon Press Elmsford, NY 1990 (b) I. OjiMA, Catalytic Asymmetric Synthesis, VCH Publishers. Inc. New York. 1993 ... 

The 1,3-dipolar cycloaddition reaction of nitrones with alkenes gives isoxazolidines is a fundamental reaction in organic chemistry and the available literature on this topic of organic chemistry is vast. In this reaction until three contiguous asymmetric centers can be formed in the isoxazolidine 17 as outlined for the reaction between a nitrone and an 1,2-disubstituted alkene. The relative stereochemistry at C-4 and C-5 is always controlled by the geometric relationship of the substituents on the alkene (Scheme 8.6). 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

The 1,3-dipolar eyeloaddition, also known as the Huisgen cycloaddition, is a elassie reaetion in organic chemistry consisting in the reaetion of a dipolar-ophile with a 1,3-dipolar compound that allows the produetion of various five-membered heteroeyeles. This reaction represents one of the most productive fields of modern synthetic organic chemistry. Most dipolarophiles are alkenes, alkynes, and molecules possessing related heteroatom functional... 

Carruthers, W., Cycloaddition Reactions in Organic Synthesis, Tetrahedron Organic Chemistry Series, Vol. 8, Baldwin, J. E., and Magnus, P. D., eds., Pergamon Press, 1990. 

Diels-Alder reactions are one of the most fundamental and useful reactions in synthetic organic chemistry. Various dienes and dienophiles have been employed for this useful reaction.1 Nitroalkenes take part in a host of Diels-Alder reactions in various ways, as outlined in Scheme 8.1. Various substituted nitroalkenes and dienes have been employed for this reaction without any substantial improvement in the original discovery of Alder and coworkers.2 Nitrodienes can also serve as 4ti-components for reverse electron demand in Diels-Alder reactions. Because the nitro group is converted into various functional groups, as discussed in Chapters 6 and 7, the Diels-Alder reaction of nitroalkenes has been frequently used in synthesis of complex natural products. Recently, Denmark and coworkers have developed [4+2] cycloaddition using nitroalkenes as heterodienes it provides an excellent method for the preparation of heterocyclic compounds, including pyrrolizidine alkaloids. This is discussed in Section 8.3. 

The [S2N]+ cation is an important reagent in S-N chemistry,63 especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes, which give quantitative yields of heterocyclic cations (Scheme 3). It is conveniently prepared by reaction of S3N2C12 with AsF5 and S8 in liquid S02.63b The [SNS]+ cation is linear with S-N bond distances in the range indicating a bond order of two, i.e., S=N+=S. 

CARRUTHERS Cycloaddition Reactions in Organic Synthesis CLARIDGE High-Resolution NMR Techniques in Organic Chemistry FINET Ligand Coupling Reactions with Heteroatomic Compounds GAWLEY AUBE Principles of Asymmetric Synthesis... 

Therefore, the ability of (530a) to be involved in [4+ 2]-cycloaddition reactions with some olefins and dienes is very important in dihydrooxazines chemistry, although the ene reactions limit the use of this nitrosoalkene in organic synthesis. 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

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