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Cycloaddition reactions concerted process

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... [Pg.415]

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... [Pg.636]

Classify the following reactions as electrocyclizations, sigmatropic rearrangements, cycloadditions, etc., and give the correct symbolism for the electrons involved in each concerted process. Some of the reactions proceed by two sequential processes. [Pg.656]

Mechanistically the 1,3-dipolar cycloaddition reaction very likely is a concerted one-step process via a cyclic transition state. The transition state is less symmetric and more polar as for a Diels-Alder reaction however the symmetry of the frontier orbitals is similar. In order to describe the bonding of the 1,3-dipolar compound, e.g. diazomethane 4, several Lewis structures can be drawn that are resonance structures ... [Pg.74]

Concerted (Section 30.1) A reaction that Lakes place in a single step without intermediates. For example, the Diels-Alder cycloaddition reaction is a concerted process. [Pg.1238]

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. [Pg.544]

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. [Pg.591]

Many such reactions may indeed be carried out preparatively under photochemical conditions, though, for reasons that cannot be gone into here (the detailed mechanism of photochemical changes), they are often not concerted but proceed via biradical intermediates. One photochemical (2n + 2n) cycloaddition that does, however, proceed via a concerted process is the example we have already referred to ... [Pg.349]

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. [Pg.205]

Other examples are given in Table 12.3. The cycloaddition reactions take place even at 0 °C. A regioisomeric mixture was obtained from 1,1-dimethylallene [50]. As for the mechanism of the allene-ketene [2 + 2]-cycloaddition, it is not clear whether the reaction proceeds via a concerted process (ketene antrafacial) or a two-step process. [Pg.744]

The [3 + 2]-cycloaddition reactions of allenes with 1,3-dipoles are useful for the construction of a variety of five-membered heterocycles with a high degree of regio- and stereochemical control [67]. Generally, the dipolar cycloaddition reactions are concerted and synchronous processes with a relatively early transition state. The stereoselectivities and regiochemistries are accounted for by the FMO theory The reaction pathway is favored when maximal HOMO-LUMO overlap is achieved. [Pg.750]

Abstract 1,3-Dipolar cycloaddition reactions (DCR) are atom-economic processes that permit the construction of heterocycles. Their enantioselective versions allow for the creation of up to four adjacent chiral centers in a concerted fashion. In particular, well-defined half-sandwich iridium (111) catalysts have been applied to the DCR between enals or methacrylonitrile with nitrones. Excellent yield and stereoselectivities have been achieved. Support for mechanistic proposals stems from the isolation and characterization of the tme catalysts. [Pg.209]

The Diels-Alder reaction, inverse electronic demand Diels-Alder reaction, as well as the hetero-Diels-Alder reaction, belong to the category of [4+2]-cycloaddition reactions, which are concerted processes. The arrow pushing here is merely illustrative. [Pg.199]

A third mechanistically distinct [3 -1- 2] cycloaddition between vinyl ethers and vinyl-carbenoids was discovered and reported in 2001 [26]. This reaction is remarkable because when Rh2(S-DOSP)4 is used as the catalyst, the cis-cyclopentenes 142 are formed in up to 99% enantiomeric excess. The reaction occurs between vinylcarbenoids unsubstituted or alkyl-substituted at the vinyl terminus and vinyl ethers substituted with an aryl or vinyl group. Some illustrative examples are shown in Tab. 14.12. The reaction is considered to be a concerted process, which would be consistent with the highly stereoselective nature of the reaction [26]. Contrary to the [3-1-2] cycloaddition derived by means of vinylogous carbenoid reactivity, this latest [3 -1- 2] cycloaddition is not influenced by solvent effects. Due to steric demands on the carbenoid, the [3-1-2] cycloaddi-tion only occurs with cis-vinyl ethers. [Pg.323]

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry restrictions in the same way that cycloadditions and sigmatropic processes are. [Pg.403]

The reason for this difference is that if 16 were to undergo a concerted elimination, it would have to follow the forbidden (high-energy) [2ns + 2ns] pathway. For 17, the elimination can take place by the allowed [2ns + 4rcv] pathway. Thus, these reactions are the reverse of, respectively, the [2 + 2] and [4 + 2] cycloadditions, and only the latter is an allowed concerted process. The temperature at which 16 decomposes is fairly typical for strained azo compounds, and the decomposition presumably proceeds by a noncon-certed diradical mechanism. Because a C—N bond must be broken without concomitant compensation by carbon-carbon bond formation, the activation energy is much higher than for a concerted process. [Pg.406]

One of the most widely applied cycloaddition techniques for the preparation of thietanes is the reaction of sulfenes with enamines. The stereochemistry of these reactions has been extensively investigated by Truce and Rach. Whether the mechanism is a two-step or a concerted process, both in accordance with the stereoselective formation of the cis form in Scheme 1, is still unresolved. The special orientation of the 1,4-dipolar intermediate 64, in which the charged phenyl and dimethylamino moieties are in proximity, enforces the cis geometry of the resulting thietane dioxide. In the concerted mode of reaction, formation of the orthogonal oriented unsaturated system, 65 should also yield the cis cycloadduct. [Pg.214]

Several computational studies have addressed whether the dipolar cycloaddition of nitronates is a concerted or stepwise process (93,100). Natural population analysis reveals that their is very little zwitterionic character in the transition state. The formation of the C C bond marginally precedes the C—O bond on the basis of calculated bond lengths and orders in the transition structure. These calculations also show that the reaction is a concerted process that is shghtly asynchronous. In addition, the cycloaddition likely proceeds through an early transition state and is overall an exothermic process. [Pg.114]

The complete stereoselectivity of the reaction, however, is difficult to reconcile with a two-step process. This earlier controversy, however, has long since been resolved. For example, when considering results of the cycloaddition of p-nitrobenzonitrile oxide with cis- and trani-l,2-dideuterioethylene (111), the experiments clearly established that, within experimental limits of detection, the reaction is > 98% stereoselective. If diradical intermediates were operative, significant scrambling of configuration should be observed in the products. These and other results confirm a concerted mechanism for the 1,3-dipolar cycloaddition reaction (15). [Pg.375]

Intramolecular ionic Diels-Alder reactions were carried out in highly polar media to afford carbocyclic ring systems. The strategy, which obviates the need for high temperatures and pressures, features in situ generation of heteroatom-stabUized allyl cations that undergo subsequent (4 + 2) cycloaddition at ambient temperature. Typically, reactions were complete within 1 hour after addition of substrate. Some cycloadducts were the result of a concerted process, whereas others were formed via a stepwise reaction mechanism (Grieco, 1996). [Pg.162]


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See also in sourсe #XX -- [ Pg.443 ]




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