Cycloaddition terminology

The main purpose of this chapter is to introduce the various aspects of the Diels-Alder cycloaddition and the terminology employed. 

We note that in a more generalized sense the electrocyclic reactions can be regarded also as cycloadditions if one agrees to apply the suprafacial-antarafacial terminology to a bonds as well as to tt bonds in the manner illustrated by 5 and 6. 

Some confusion has been occasioned by the introduction of another kind of terminology based on counting the number of atoms of the component systems taking part in the cycloaddition. In this system, the total number of atoms in each component is counted. This number includes both termini and all of the atoms in between. The description of a cycloaddition based on the number of atoms does not give the same designation as that based on the number of electrons. 

The past classification of the [4 + 2] cycloaddition reactions involving die 4it participation of a cationic heterodiene among the polar cycloadditions is derived not from an implied stepwise addition-cy-clization reaction mechanism but was terminology introduced to distinguish cycloadditions employing cationic or anionic components from those employing dipolar or uncharged components. Herein the reactions are simply referred to as [4 + 2] cycloaddition reactions. 

There are some anomalies in Table 10.12 for example, the nitro group is listed as strongly electrophilic, but in fact is not reactive at all in normal 1,3-dipolar cycloadditions. The 0) scale is also applicable only to the reactions in which the dipolarophile is acting as the electrophilic component that is, in FMO terminology, the LUMO pp, -HOMOjipoij ophiie interaction is dominant. 

The addition of alkenes to dienes is a very useful method for the formation of six-membered carbocyclic rings. The reaction is known as the Diels-Alder reaction. The concerted nature of the mechanism was generally agreed on and the stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In the terminology of orbital-symmetry classification, the Diels-Alder reaction is a [ 4,+ 2 ] cycloaddition, an allowed process. The stereochemistry of both the diene and the alkene (the alkene is often called the dienophile) is retained in the cyclization process. The transition state for addition requires the diene to adopt the s-cis conformation. The diene and alkene approach... 

The schematic reaction in equation 11.29 is often termed a (3 + 2) cycloaddition. It is important to note that this terminology designates the number of atoms in each of the reactants that undergo concerted bonding changes. All of these reactions are categorized as [4 + 2] cycloadditions in the Woodward-Hoffmann formalism because the 1,3-dipole provides four n electrons and the olefin provides two k electrons. Thus, terms in brackets typically indicate the number of electrons in the cyclization, while terms in parentheses indicate the number of atoms This convention is not always... 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

When Woodward and Hoffmann developed the conservation of orbital symmetry, they introduced the terms "allowed" and "forbidden" to describe reactions such as the [4+2] and [2+2] cycloadditions, respectively. This terminology caught on, and has become fairly standard in the field. With the benefit of a historical perspective, though, we can now see that these terms are too definitive. 

Electrocyclic reactions can be labeled with the suprafacial and antarafa-cial terminology used for cycloadditions. The disrotatory opening of a cyclohexadiene has the front lobes on both ends (or back lobes on both ends) of the o bond attaching to the same face of the ji system, and this is a 2 process (Eq. 5.16). 

The world of phenomena does not provide us with definition and classification principles. These are created by the scientist and are not free from arbitrariness. A terminology is suggested for cycloadditions. These reactions do not all proceed by the same mechanism. Since ideas about reaction mechanism can change with the introduction of new criteria, it is shortsighted to base definitions and classifications on mechanism alone." (Huisgen, 1968) [15]... 

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