Pericyclic transformations

The connection of radical and pericyclic transformations in one and the same reaction sequence seems to be on the fringe within the field of domino processes. Here, we describe two examples, both of which are highly interesting from a mechanistic viewpoint. The first example addresses the synthesis of dihydroindene 3-326 by Parsons and coworkers, starting from the furan 3-321 (Scheme 3.79) [128]. Reaction of 3-321 with tributyltin hydride and AIBN in refluxing toluene led to the 1,3,5-hexatriene 3-324 via the radicals 3-322 and 3-323. 3-324 then underwent an elec-trocyclization to yield the hexadiene 3-325 which, under the reaction conditions, aromatized to afford 3-326 in 51 % yield. 

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. 

In a series of publications over the past few years, the group of Barriault has reported on microwave-assisted tandem oxy-Cope/Claisen/ene and closely related reactions [175-178], These pericyclic transformations typically proceed in a highly stereoselective fashion and can be exploited for the synthesis of complex natural products possessing decalin skeletons, such as the abietane diterpene wiedamannic... 

Performiate free radical, 742 Performic acid, determination, 698, 699 Pericyclic transformation, olefins, 94 Peroxalate, hydrogen peroxide determination, 643, 648, 649 Peroxidase... 

The process is induced photochemically and involves the single-electron transfer oxidation of cubane then completed with a backward electron transfer to the transient radical cations. A Li+ salt with a weakly coordinating anion is able to induce pericyclic transformations, including the rearrangement of cubane to cuneane, quadricyclane to norbomadiene, and basketene to Nenitzescu s hydrocarbon 392... 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

This argument demonstrates in an entirely general way that the number of phase inversions in the interaction diagram for a simply connected pericyclic transformation is zero if the number of antarafacial components is even and one if the number of antarafacial components is odd. In order to complete the link between the Woodward-Hoffmann and Dewar-Zimmerman points of view, it remains only to find the total number of electrons in the completed ring. A 4q system contains an even number of pairs and a 4q + 2 system an odd number of pairs therefore any system obtained by joining components will have a total number of electrons satisfying the formula 4q (even number of pairs total) if there are an even number of 4q + 2 components and a total number satisfying 4 + 2 (odd number of pairs total) if there are an odd number of 4q + 2 components. 

From a thermodynamic standpmnt, one can expect that for the thermal conversion of (1) to (2) and related derivatives, unless the temperature is sufficiently high or strain factors become predominant, cycloreversion will not be observed. A more complete discussion of the factors affecting the thermal equilibration of strained hexatrienes and cyclohexadienes will be discussed in Section 6.2.2.2.I. A kinetic study of the pericyclic transformation of the parent triene (1) to 1,3-cyclohexadiene (2) provided an activation energy of 29.9 kcal mol. Taking into account the heats of formation of reactant and product (40.6 kcal mol for (Z)-l,3,5-hexatriene and 25.4 kcal mol" for 1,3-cyclohexadiene), the... 

In their reactions, 2//-pyran-2-ones behave like 1,3-dienes and also like lactones. This is manifested above all in a series of intra- and intermolecular pericyclic transformations. 

Among the various preluding and guiding reaction steps, an ene reaction can also adjust a substrate for fiudher pericyclic transformations such as a cycloadditon. 

So far, we have shown in this subchapter ene reactions that adjust the starting material for a subsequent pericyclic step. Of course, also numerous other reaction types are able to pave the way for a pericyclic transformation. In Scheme 6.34 we present a domino three-component synthesis of a-spiro-S-lactams. This transformation is a prime example to demonstrate how difficult it might be to really distinguish what kind of pericyclic reaction is involved or - furthermore - to distinguish whether any pericyclic reaction is involved or whether the course of... 

In the ground state, if the symmetry of MOs of the reactant matches that of the products that are nearest in energies, then reaction is thermally allowed. However, if the symmetry of MOs of the reactant matches that of the product in the first excited state but not in the ground state, then the reaction is photochemically allowed (Figure 1.12). When symmetries of the reactant and product molecular orbitals differ, the reaction does not occur in a concerted manner. It must be noted that a symmetry element becomes irrelevant when orbitals involved in the reaction are aU symmetric or antisymmetric. In conclusion, we can say that in pericyclic transformations, symmetry properties of the reactants and products remain conserved. 

It has been proposed that these complex structures arise from various electro-cyclisation and cycloaddition reactions of linear polyene side-chains, undergone post-pyrone formation. For example, Trauner pointed out the wide variety of skeletal types that can be theoretically obtained from pericyclic transformations of a relatively simple E,Z,Z,E)-tQttait Q 21, further observing that isomerisations of the alkene geometries lead to an even greater number of potential cyclisation products (Scheme 1.4) [14],... 

In the following, we are going to consider calculation data on the PES and MERP of some representative pericyclic reactions. The emphasis will be placed on the specific features of the intrinsic mechanism which do not follow from the above-examined qualitative notions on the steric course and the one-step character of pericyclic reactions. As a matter of fact, the data from rigorous quantum chemical calculations have assisted in forming novel ideas concerning the detailed mechanism of these reactions thus modifying quite substantially the assertion that the pericyclic transformations represented the reactions without a mechanism . 

These results led Dewar to formulate a new rule the synchronous multibond reactions are usually forbidden [30]. Even though this rule cannot be deduced from strict theoretical premises as, for example, the rule governing the selection of transition state structures by their symmetry properties (see Sect. 1.3.3.2), it has been verified, according to an analysis in Ref. [30], for all the multi-bond reactions for which a reliable examination of the PES s has been carried out. This fact requires a certain reassessment of the customary view in regard to the pericyclic transformations as necessarily synchronous reactions in those cases when they are allowed by the rules of orbital symmetry conservation. 

In practical terms, particularly important are the cycloaddition reactions catalyzed through the formation of cation-radicals. A theoretical analysis of the PES of these reactions [94] has helped to evolve some general notions concerning the mechanism of ionradical pericyclic transformations. 

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