Pericyclic reactions concerted nature

With allylic alcohols, there is the possibility of a [2,3] variant of the Wittig rearrangement that can compete with the [1,2] rearrangement described above (Eq. 22, the indicated electron flow is for the [2,3] rearrangement). The reaction is expected to be a one-step, pericyclic process without a distinct carbanion intermediate. This rearrangement has proven to be useful synthetically because its concerted nature can lead to high stereoselectivity. ... 

Pericyclic reactions are concerted reactions that take place in a single step without any intermediates, and involve a cyclic redistribution of bonding electrons. The concerted nature of these reactions gives fine stereochemical control over the generation of the product. The best-known examples of this reaction are the Diels-Alder reaction (cyclo-addition) and sigmatropic rearrangement. 

Although such an understanding of the reaction mechanism is in principle applied in the theory of pericyclic reactions, the above general picture is in this case slightly complicated by the specific (introduced in the course of historical development) classification of reaction mechanisms in terms of concertedness and/or nonconcertedness. Concerted reactions are intuitively understood as those reactions for which the scission of old bonds and the formation of the new ones is synchronised, whereas for nonconcerted reactions the above bond exchange processes are completely asynchronised. Moreover, since the above asynchronicity is also intuitively expected to induce the stepwise nature of the process, the nonconcertedness is frequently believed to require the presence of intermediates, whereas the concerted reactions are believed to proceed in one elementary step. 

Pericyclic reactions in which p-orbitals at the ends of the rt-component of each system overlap and form the new a-bonds on the same surface are called suprafacial cycloaddition. Almost aU pericyclic cycloaddition reactions are suprafacial on both systems and thus the stereochemistry is maintained due to their concerted nature. This specification is usually made by placing a suitable subscript (s or a) after the number referring to the Tr-component. For example,... 

It appears that nature rarely uses concerted, pericyclic reactions in biosynthesis or metabolism. However, in at least one instance a Claisen rearrangement is key to a biosynthetic pathway. It i.s, in fact, a crucial pathway, the one that biosynthesizes the amino acids phenylalanine (Phe) and tyrosine (Tyr). The pathway only exists in plants— our bodies cannot synthesize Phe and Tyr, making Phe and Tyr so-called essential amino acids. 

A further criticism of this approach is its purely qualitative nature. There is no way of telling how large or how small will be the factors preventing violations of the rules derived from it. This has led to an exaggerated estimate of the potency of the factors distinguishing allowed pericyclic reactions from forbidden ones and the consequent belief that forbidden reactions cannot occur as concerted pericyclic processes. 

Cycloaddition is a member of pericyclic reactions in which reactive components possessing conjugated jr-electrons (e.g., diene/dienophile, 1,3-dipole/dipolarophile, and ketene/imine) transform into cyclic molecules. These reactions proceed in a concerted mechanism with a high degree of regio- and stereoselectivity. Thus, they have been widely used for the construction of cyclic skeletons of numerous natural products and pharmacologically active molecules. Based on the jr-electron systems of reactants, they can be further classihed into [4 + 2], [3 + 2], and [2 + 2] cycloadditions that produce six-, five-, and four-membered rings, respectively. ... 

In a more polar solvent, Favorskii reactions cease to be stereospecific, and presumably take place by ionisation of the chloride to give the same cation from each diastereoisomer. Whether the reaction takes place by way of the cation or with concerted loss of the chloride ion, this reaction presented a serious puzzle before its pericyclic nature was recognised. The a overlap of the p orbital on C-2 of the enolate with the p orbital at the other end of the allyl cation 6.340 or with the orbital of the C—Cl bond 6.341 looked forbiddingly unlikely—it is 3-endo-trig at C-2. It is made possible by its pericyclic nature, where the tilt of the orbitals can begin to sense the development of overlap. The torquoselectivity in the development of overlap 6.341, however improbable it looks, corresponds to inversion of configuration at the carbon atom from which the chloride departs. 

A unified picture of the mechanism of this cycloaddition for various structural types of nitroso dienophiles is not available. Depending upon the electronic and steric nature of the particular nitroso compound, the reaction apparently may vary from a concerted pericyclic process to one involving discrete dipolar intermediates. The net result of this mechanistic diversity is a few anomalous regiochemi-cal results in the cycloaddition. 

Controversy continues over the precise nature of the transition state in the cis thermolytic ester elimination reaction. Whereas one group interpret their data as indicative of a symmetrical, non-heterolytic, non-planar transition state (12) analogous to other pericyclic fragmentations, another author proposes charge development (13), defining the order of extent of movement of electrons as 1 > 2 > 3. An unsymmetrical concerted transition state (14) has been assigned to the thermal / -elimination reaction of alkyl... 

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