Pericyclic reactions sigmatropic, examples

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

Since the number of domino processes which start with a Diels-Alder reaction is rather large, we have subdivided this section of the chapter according to the second step, which might be a second Diels-Alder reaction, a 1,3-dipolar cycloaddition, or a sigmatropic rearrangement. However, there are also several examples where the following reaction is not a pericyclic but rather is an aldol reaction these examples will be discussed under the term Mixed Transformations . 

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. 

Sigmatropic, electrocyclic, cycloaddition and cycloelimination, and cheletropic reactions have all been carried out with organosulfur compounds and often used for synthetic puiposes. A chapter of Block s monograph (203] is devoted to this topic, and most of the pericyclic processes include examples with sulfur compounds. The treatises by Barton and Ollis [482], Trost and Fleming [483] and Klamann [484] are guides to the more specialized literature. Some reviews deal with specific cases thiocarbonyl compounds [120] or cycloaddition reactions [485],... 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

So far, cycloadditions have been our only examples of pericyclic reactions. There are several other classes of pericyclic reactions, of which the most notable are cheletropic reactions, sigmatropic rearrangements and electrocyclic reactions. In essence, frontier orbital theory treats each of them as a cycloaddition reaction. 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

The radical cations of conjugated systems can also take part in pericyclic reactions. Examples are known of cycloadditions, electrocyclic reactions and sigmatropic rearrangements. One noticeable feature of some of... 

Abstract The integration of conservation of orbital symmetry and the orbital overlap effect serves as a powerful tool to reliably predict the stereochemical course of pericyclic reactions as exemplified in this chapter. The orbital overlap factor has been discussed with a variety of examples such as the thermal fragmentations of cyclopropanated and cyclobutanated r .v-3,6-dimethyl-3,6-dihydropyridazine, and [1,5] sigmatropic shifts in c/.v-2-alkenyl-1 -alkylcyclopropanes and civ-2-alkenyl-1 -alkylcyclobutanes. 

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. 

Pericyclic reactions, most notably the Diels-Alder reaction, other cycloadditions, and certain sigmatropic rearrangements in which two or more electron pairs move in a more or less concerted manner along a cyclic pathway are a cornerstone of organic synthesis. Much of their importance derives from the efficiency with which they create two or more bonds in one step and also in a stereospecific manner. Some examples are as follows ... 

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