Cycloaddition reactions classification

The different possibilities for the creation of the pyrazole ring according to the bonds formed are shown in Scheme 46. It should be noted that this customary classification lacks mechanistic significance actually, only two procedures have mechanistic implications the formation of one bond, and the simultaneous formation of two bonds in cycloaddition reactions (disregarding the problem of the synchronous vs. non-synchronous mechanism). 

The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

Fig. 11.11. Classification of cycloaddition reactions with respect to basis set orbitals.

For a system of classification of cycloaddition reactions, see Huisgen, R. Angew. Chem. Int. Ed. Engl., 1968, 7, 321. For a review of certain types of cycloadditions leading to three- to six-membered rings involving 2, 3, or 4 components, see Posner, G.H. Chem. Rev, 1986, 86, 831. See also the series Advances in Cycloaddition. 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

A modern valence-bond description of the Diels-Alder reaction has been presented. The method of reaction classification by similarity has been expanded to include the effect of steric congestion in the classification of cycloaddition reactions. ... 

In a cycloaddition reaction, two sigma bonds are formed to give a new ring of atoms, and one classification of these reactions is based on the number of ring atoms thal derive from each of the reacting units. One of the simplest classes are (2 + 2 cycloadditions, such as... 

Figure 1 Frontier molecular orbital classification of cycloaddition reactions...

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

From another viewpoint, the value of the arene-alkene meta cycloaddition arises from its capacity to produce a cycloadduct (66 equation 14) with three new rings and up to six new stereocenters, an impressive feat even when compared with the highly regarded Diels-Alder cycloaddition. Moreover, the cycloadduct can be used in the synthesis of a variety of commonly encountei structural types including cyclopentanes, cycloheptanes, bicyclo[3.2.1]octanes and bicyclo[3.3.0]octanes. While fr uently overlooked in some discussions of reaction classification, the overall processes leading to cycloheptanes, bi-cyclo[3.2.1]octanes and bicyclo[3.3.0]octanes are clearly classifiable as [5C + 2C], [3C + 2C] and [3C + 2C] cycloadditions, respectively. Examples of these types will be given in the following section. 

As was mentioned in the earlier review, considerable difficulties arise in the classification of imidazole syntheses. There is a great variety of methods, few of which have wide general application, and in many instances the mechanism is poorly understood. For example, some syntheses almost certainly involve concerted cycloaddition reactions, but sound evidence for this process is lacking. In addition, numerous examples have appeared of conversions of other heterocyclic compounds into imidazoles. While the... 

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 is another aspect of cycloaddition TS structure that must be considered. It is conceivable that some systems might react through an arrangement with Mobius rather than Hiickel topology (see p. 716). Mobius systems can also be achieved by addition to opposite faces of the tt system. This mode of addition is called antarafacial and the face-to-face addition is called suprafacial. In order to specify the topology of cycloaddition reactions, subscripts s and aare added to the numerical classification. For systems of Mobius topology, as for aromaticity, 4n combinations are favored and 4n- -2 combinations are unfavorable... 

In CHEC-I, cycloaddition reactions involving thiophene were further classified according to the number of n electrons of the thiophene ring taking part in the process. Since there are few reactions in which different modes of cycloaddition have been observed with the same reactants, such a classification has been dispensed with in the present review. Instead, an extra section has now been included, which deals with the cycloaddition reactions of 3,4-dimethylenethiophene. An exhaustive review of the cycloaddition reactions of thiophenes appeared in 1985 <85HC(44/l)67l>. 

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