Electrocydic reactions examples

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. 

To date, only a few examples are known where a domino reaction starts with an electrocydic reaction, although the value of this approach is clearly demonstrated by the beautiful synthesis of estradiol methyl ether 4-319 through a domino elec-trocyclic/cycloaddition process. There is also an impressive example of a double thermal electrocyclization being used however, the starting material for this domino reaction was prepared in situ by a transition metal-catalyzed transformation, and is therefore discussed in Chapter 6. 

Electrocydic reactions, then, are completely stereospecific. The exact stereochemistry depends upon two things (a) the number of double bonds in the polyene, and (b) whether reaction is thermal or photochemical. It is one of the triumphs of the orbital symmetry approach that it can account for all these facts indeed, most of the examples known today were predicted by Woodward and Hoffmann before the facts were known. 

A cycloaddition reaction is one in which two unsaturated molecules add to one another, ieJding a C3 clic product. As with electrocydic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed processes often take place readily, but symmetry-disallowed processes take place with great difficulty, if at all, and then only by non-concerted pathways. Let s look at two examples to see how they differ. 

Another reaction that can be described in terms of orbital symmetry is the ring closing reaction of cis—1,3—butadiene to produce cyclobutene. The reaction is an example of a class of reactions known as electrocydic reactions. There are two different pathways that can be imagined for this reaction, which can be illustrated as shown in Figure 9.5. Both mechanisms involve rotation of the terminal CH2 groups but in different ways. 

Electrocydization is another major mode of reaction utilizing the vinylidene complexes of Group 6 metals. It should be noted that the first example of this type of reaction was reported by Merlic et al. using a ruthenium complex employing nonaromatic diene-yne substrates [26]. 

These reactions, called electrocydic rearrangements,365 take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereo-specificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cw-3,4-dimethylcyclobutene gave only cis,trans-2,4-hexadiene, while the trans isomer gave only the trans-trans diene 366... 

Ketone semicarbazones undergo acid catalyzed reversible cyclization to 3,3-disubstituted 1,2,4-triazolidinones (76JHC1257). The only example available in the literature where a fused triazoline was obtained involves the nitrosation of ethyl 2-isopropylidenehydrazonothiazole-4-acetate, resulting in the formation of a 3,3-dimethyl-2H-s-triazolo[3,4-f ]thiazole (Scheme 14) (78JHC40l).The reaction may be classified as a 1,5-electrocydization. 

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