Pericyclic reactions features

Some special features arise from pericyclic reactions. In the reaction of an a-methyl-branched aldehyde with ( )-crotylboronates, Cram selectivity is enhanced, whereas the Z-isomers show moderate anti-Cram selectivity23 - 25 (see Section D.1.3.3.3.3.1.3.). These findings can most likely be applied generally. 

The orbital phase theory includes the importance of orbital symmetry in chanical reactions pointed out by Fukui [11] in 1964 and estabhshed by Woodward and Holiimann [12,13] in 1965 as the stereoselection rule of the pericyclic reactions via cyclic transition states, and the 4n + 2n electron rule for the aromaticity by Hueckel. The pericyclic reactions and the cyclic conjugated molecules have a conunon feature or cychc geometries at the transition states and at the equihbrium structures, respectively. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

A sequential use of two pericyclic reactions involving sulfur compounds is also implied in a general allylic amination of alkenes with Kresze s reagent, 7V,A -bis(methoxycarbonyl)suIfurdiimide (8). A [2,3] sigmatropy, following an ene reaction of the S-allyl sulfinamidine (9) intermediary formed, affords the diamino sulfane (10), easily converted to the carbamate (11) and then to the amine (12) or (13) [511]. The reactions are featured with 2-methyl-2-butene as the alkene to be functionalized. They are described in Organic Syntheses [512]. 

Pericyclic reactions represented for many years a difficult mechanistic problem because the apparent absence of intermediates left few concrete features that could be subjected to experimental study. Application of some fundamental principles of orbital theory, initiated in 1965 by Woodward and Hoffmann1 and since developed extensively by them2 and by others,3 have provided new... 

All pericyclic reactions share the feature of having a cyclic transition... 

Cycloadditions are characterized by two components coming together to form two new o-bonds, at the ends of both components, joining them together to form a ring, with a reduction in the length of the conjugated system of orbitals in each component. Cycloadditions are by far the most abundant, featureful, and useful of all pericyclic reactions. 

Cycloadditions are the most useful of all pericyclic reactions in organic synthesis. This chapter describes the wide range of known cycloadditions, identifies the conditions under which they take place, draws attention to their regio- and stereochemistry, and gives the simple rules for which of them take place and which do not. The explanations for most of these features, based on the molecular orbitals involved, will then be covered in the following chapter. 

The characteristic feature of all pericyclic reactions is the concertedness of all the bond making and bond breaking, and hence the absence of any intermediates. Naturally, organic chemists have worked hard, and devised many ingenious experiments, to prove that this is true, concentrating especially on the Diels-Alder reaction. The following is an oversimplified description of some of the most telling experiments. 

A problem with this explanation is that it is a bit more difficult to explain those pericyclic reactions that we shall come to in Chapter 4, which smoothly take place in spite of their having a total of 4n electrons. We shall find that these all show stereochemistry involving an antarafacial component. It is possible to include this feature in the aromatic transition state model—if the... 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

Perhaps the most remarkable feature of this reaction is that a bond has formed between C-l and C-5, both of which are positively charged. Any attempt to think of this reaction as the combination of a nucleophilic and an electrophilic carbon would not make proper sense, yet the reaction occurs easily. Pericyclic reactions really are a distinctly different class of reactions from ionic and radical reactions. Since this reaction is also 5-endo-trig at both ends, it would appear to be also deeply forbidden by Baldwin s rules— which evidently do not apply with any great force to electrocyclic reactions. 

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. 

By the end of the 1950s the main features of ionic and radical reactions were reasonably well understood, but pericyclic reactions were not even recognized as a separate class. Diels-Alder reactions, and a good many others, were known individually. Curly arrows were used to show where the bonds went to in these reactions, but the absence of a sense of direction to the arrows was unsettling. Doering provocatively called them no-mechanism reactions in the early 1960s. 

The X-ray crystal structure for AZ-28 has a variety of structural features that are consistent with the proposed mechanism operative for the oxy-Cope rearrangement. The antibody binds the transition stage analog in a chair-like conformation, consistent with the preferred chair transition state for this pericyclic reaction (Doering and Roth, 1962). The positions of the C-2 and C-5 atoms are fixed in the antibody-bound hapten molecule in a similar fashion, the C-2 and C-5 positions in the hexadiene substrate should be held in a fixed position by conserved van der Waals interactions locking in the two phenyl substituents in the antibody combining site. This bound conformation of the acyclic (47T + 2er) system of the hexadiene substrate should enforce a molecular conformation close to the transition state for the rearrangement reaction, consistent with the catalysis observed for AZ-28. 

This chapter is divided into two sections, largely separating stereospecific reactions from the merely stereoselective. The first deals with the ionic stereospecific reactions, and the explanations based on molecular orbital theory for the sense of the stereospecificity. The second deals with stereoselective reactions, in which a new stereocentre is created selectively under the influence of one or more existing stereochemical features, which is also sometimes a question of how the orbitals interact. The stereospecificity that is such a striking feature of pericyclic reactions is covered in the next chapter. 

The characteristic feature of all pericyclic reactions is the concertedness of all the bond making and bond breaking, and hence the absence of any intermediates. 

The Woodward-Hoffmann rules arise fundamentally from the conservation of orbital symmetry seen in the correlation diagrams. These powerful constraints govern which pericyclic reactions can take place and with what stereochemistry. As we have seen, frontier orbital interactions are consistent with these features,... 

Instead of proceeding with a historical presentation, we discuss the computational results by methodology. Using HF/6-31G, the only feature located on the 2h sUce through the PES is a transition state with /ijg = 2.046 This geometry looks quite reasonable however, as is found with many other pericyclic reactions, the activation barrier is dramatically overestimated AH, = 55.0 kcal moE versus... 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

It is now well established that the Nazarov cyclization is a pericyclic reaction belonging to the class of electrocyclizations. As with all pericyclic reactions, mectuuiism and stereochemistry are inexorably coupled and any discussion of one feature must invoke the other. In this section the stereospecific aspects of the Nazarov cyclization are discussed, the stereoselective aspects of the reaction are dealt with individually in each of the following sections. 

Each of these theoretical approaches leads to the same predictions regarding reaction conditions and stereochemistry. For a wide range of reactions, the selection rules can be used empirically, based on a simple method of electron counting, without regard to their theoretical basis. The selection rules for pericyclic reactions relate three features ... 

Pericyclic mechanisms are undoubtedly the hardest for students to draw. The superficial similarity of the mechanistic types, the way in which seemingly reasonable steps are disallowed by theoretical considerations, the simultaneous formation of many bonds, the lack of a clearly reactive center— all these features of pericyclic reactions combine to make them anathema to many students. You can learn some useful techniques to help you work through... 

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