Pericyclic patterns

Conceptually, these studies demonstrated that effective asymmetric MCRs could be designed using aminocatalytic intermediates (dienamine 21) and reactivities (pericyclic patterns) other than an enamine and the nucleophiUc addition manifold. A similar domino sequence served for the development of a formal direct... 

A set of electrocyclic ring closures is the subject of recent controversy because their mechanism lies in the borderline between pericyclic and pseudopericyclic reactions [123-127], The mechanisms were clarified by means of ELF analyses [121,122]. As shown in Figure 28.4, connected patterns (C) are... 

The appearance in the previous section of the 4 + 2 and 4r formulas brings to mind the criteria for aromatic and antiaromatic systems discussed in Chapter 1. Furthermore, the HOMO-LUMO interaction patterns discussed in Section 11.2 are reminiscent of those used in Section 10.4 to analyze aromatic stabilization. In this section, we trace the connection between aromaticity and pericyclic reactions, and show how it leads to a third approach to the pericyclic theory. 

We now turn to the ideas, based on MO theory, which have been advanced to explain the patterns of reactivity that all pericyclic reactions show. 

The symmetry properties of orbitals, and the pattern of electron occupation of the orbitals, is important in concerted reactions, that is, those that transform substrates directly into products without going through intermediates. A large group of concerted reactions called pericyclic reactions have been widely studied for alkenes and... 

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

The reactions of alkenes are determined by the electron-rich Ji-system of the alkene. Mechanistically, these reactions may be divided into a number of groups. The ji-system may react with electrophiles, with radicals and as a component in pericyclic reactions, including acting as a dienophile in Diels-Alder reactions. The alkene may also stabilize an adjacent electron-deficient carbon either as a radical or as a carbocation. This pattern of reactivity is summarized in Box 3.1. 

Identifying pericyclic reactions takes care. They may be executed under acidic, basic, or neutral conditions, just like polar reactions. Many reactions involve several polar steps as well as one pericyclic step. Also, sometimes it s quite difficult to figure out the relationship between the starting materials and the products because of the extensive changes in bonding patterns that often occur with pericyclic reactions. 

It has also been previously shown73 that the fluctuation pattern of electron density in the ELF basins provides a consistent description of pseudopericyclic and pericyclic bonding in concerted processes such as thermal chelotropic decarbonilation reactions.74 Experimental support for planar pseudopericyclic transition states in chelotropic decar-bonilations has been recently reported.75 ELF picture of bonding reveals that for the eight transition states analysed (see Scheme 3), the departing CO can be visualized in terms of a carbon monoxide structure with a Tone pair region on the carbon atom. 

Regioselectivity has been previously described in terms of a local hard and soft acid and base (HSAB) principle, and some empirical rules have been proposed to rationalize the experimental regioselectivity pattern observed in some DA reactions.71,72 There is not a single criterion, however, to explain most of the experimental evidence accumulated in cycloaddition processes involving four centre interactions. An excellent source for the discussion of regioselectivity in concerted pericyclic reactions is given in reference.73... 

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