Pericyclic reactions electrocyclic transformations

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocyclic 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. 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Altenbach used the Michael addition of sodium methyl malonate to allene (206) for a dia-stereoselective spiroannulation to a steroid (equation 72). Or, in imaginative work by Okamura, allenyl sulfoxides were transformed into enantiomerically pure hydrocarbons by pericyclic reactions like electrocyclic ring closure (equation 73) or intramolecular cycloaddition (equation 74). Note that the starting materials (propargylic alcohols) are readily accessible as single enantiomers. 

Among the numerous reactions that have been developed to date, a certain class, namely the pericyclic reactions, especially stands out for several reasons. Originally termed no-mechanism reactions, these transformations have occupied one of the most prominent positions in organic synthesis. Beside ionic and radical reactions, they constitute the third distinct class of reaction mechanisms. In all kinds of pericyclic reactions - cycloadditions, sigma tropic rearrangements, electrocyclizations, and ene reactions - one can observe cyclic transition states. No intermediates are formed, and all bond-forming and bond-breaking processes take place in concert [1]. 

The inconsistency in the details of photochemical transformations are not peculiar to electrocyclic reactions alone, but are relevant to other photo-pericyclic reactions as well. For example, calculation shows that reaction (4.16) is also highly endothermic, and hence the product cannot be formed in the... 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

The following thermal transformation involves three pericyclic changes. PROBLEM 6.19 The first two are electrocyclic and the third is a sigmatropic rearrangement. Give structures for the two intermediates in the reaction. 

The Diels-Alder reaction couples the ends of two separate tt systems. Can rings be formed by the linkage of the termini of a single conjugated di-, tri-, or polyene Yes, and this section will describe the conditions under which such ring closures (and their reverse), called electrocyclic reactions, take place. Cycloadditions and electrocyclic reactions belong to a class of transformations called pericyclic (peri, Greek, around), because they exhibit transition states with a cyclic array of nuclei and electrons. 

In an electrocyclic reaction (Woodward and Hoffmann, 1970) a molecule with a conjugated system of mw electrons cyclizes to form a ring ofm—2w electrons and one a bond. Forthis transformation to happen, the two ends of the relevant w system must approach each other in such a way as to enable the end p orbitals to overlap constmctively. To form a a bond the two terminal p orbitals need to rotate. When the molecule is substituted the rotations can be in the same or in opposite directions. This leads to different stereoisomers and is of much interest to physical organic chemists. Therefore, such reactions are extensively studied. They are a special case of a pericyclic isomerization where the transition state is cyclic. 

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