Photochemical cycloadditions electrocyclic reactions

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

Photochemical activation can be used to achieve forward or reverse cycloadditions and electrocyclic reactions that are thermodynamically unfavorable or have unfavorable concerted thermal mechanisms. Thus the... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

This chapter follows on from chapter 12 where we introduced some basic ideas on stereocontrol. Since then we have met many stereospecific reactions such as pericyclic reactions including Diels-Alder (chapter 17), 2 + 2 photochemical cycloadditions (chapter 32), thermal (chapter 33) cycloadditions, and electrocyclic reactions (chapter 35). Then we have seen rearrangements where migration occurs with retention at the migrating group such as the Baeyer-Villiger (chapters 27 and 33), the Amdt-Eistert (chapter 31) and the pinacol (chapter 31). 

Classify these reactions as electrocyclic reactions, [x + y] cycloadditions, or [/,/] sigmatropic rearrangements and explain whether each is allowed thermally or photochemically. 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

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. 

Generally, the electrocyclic cycloaddition reactions are performed thermally, without catalysis. Diels-Alder reactions are further facilitated by high pressure in hydrophobic structures, suffering hydrophobic collapse within an aqueous environment. Photochemical cycloadditions are often limited by the penetration of the polymeric support by the light of the desired wavelength. However, especially in case of hetero Diels-Alder reactions, Lewis acid catalysis has been applied. On a solid support the use of acid catalysis is often limited by the stability of the support linker, since most common linkers have been designed to be cleavable under acidic conditions. Table 4 shows some of the Lewis acids and the support link used. 

The conclusion is general reactions which are forbidden thermally are allowed photochemically and vice versa. For a particular number of electrons, cycloadditions that go entirely suprafacially under thermal conditions will require an antarafacial component photochemically and vice versa. For electrocyclic reactions that are conrotatory under thermal conditions, the corresponding photochemical reactions will be disrota-tory and vice versa. 

Heteroaromatic N-imines and IV-aminoazonium salts show a variety of reactivities, depending on the nature of the heteroaromatic ring and the substituents on the imino or amino nitrogen. The most important types of the reactions are (i) reactions with electrophiles at the imino or amino nitrogen, (ii) reactions with nucleophiles on the heteroaromatic ring, (iii) 1,3-dipolar cycloaddition, (in) 671-electrocyclic reaction of 1,5-dipoles (mainly thermal reaction), (n) 47t-electrocyclic reaction (mainly photochemical reaction), and (vi) N—N bond cleavage (by thermolysis, photolysis, oxidation, and reduction). 

The only pericyclic reactions we have used so far have been cycloadditions the Diels-AIder reaction (Chapter 17) and photochemical (Chapter 32) or thermal t Chapter 33) 2 + 2 cycloadditions. Electrocyclic and sigmatropic reaaions are also useful in synthesis and as each is the basis of a method of five-membered ring synthesis, they are conveniently grouped into one chapter here. 

Optical purity, by NMR, 13, 14 Orbital correlation diagrams, 196-203 cycloaddition reactions, 197-196 Diels-Alder, 198 ethylene -E ethylene, 198 electrocyclic reactions, 198-200 butadienes, 199 hexatrienes, 199 limitations, 203 photochemical, 201 Woodward-Hoffinann, 197 Orbital energies, see also Energies, orbital degeneracy, 27, 90 Orbital interaction theory, 34-71 diagram, 40, 42, 47 limitations, 69-71 sigma bonds, 72-86 Orbitals... 

The thiophene ring is less aromatic than benzene this allows for a rich potential of chemistry in electrocyclic reactions. Thiophene derivatives may be induced to react as monoene (o) or diene (6) components in these reactions by proper control of functionalization and activation (Scheme 7). Photochemical [2 + 2] cycloadditions are also possible if ethylene is sufficiently activated by... 

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