Cycloaddition reactions antarafacial process

A cycloaddition reaction can be classified not only by the number of electrons in the individual components, but also by the stereochemistry of the reaction with regard to the plane of the tt system of each reactant. For each component of the reaction, there are two possibilities the reaction can take place on only one side of the plane or across opposite faces of the plane. If the reaction takes place across only one face, the process is called suprafacial if across both faces, antarafacial. The four possibilities are shown in the following diagram ... 

Moving on to reactions in which antarafacial processes occur, we find that if we incorporate one antarafacial process, and keep the other suprafacial, then those reactions in which a total of 4n electrons are involved become allowed, and those in which a total of (4n + 2) electrons are involved become forbidden. To take just one of the relatively small number of reactions in this class, we can consider the remarkable reaction of heptafulvalene (129) with tetracyano-ethylene (130).128 This reaction is a [14 + 2] cycloaddition in other words,... 

Similarly, in cycloreversion or retro cycloaddition reactions, c-bonds take part in bond reorganization process. The cycloreversion of Diels—Alder reaction [7r" s + TT s] and of [tt s + tt s] cycloaddition may be designated as [cr s + CT s + TT s] and [ct s + c s] cycloaddition, respectively. In suprafacial cycloreversion, either retention or inversion at both the ends of other hand, antarafacial process provides retention at one end and inversion at the other end of the a-bond (Figure 4.3). 

Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-cycloaddition reactions are very common. This fact can be explained by analyzing these cycloaddition reactions using Woodward-Hofifmann selection rules. In frontier orbital approach, the thermal reaction of two ethene molecules (one is HOMO and other is LUMO) is orbital symmetry forbidden process for its suprafacial-suprafacial [7t s+7t s]-cycloaddition, but a suprafacial-antarafacial [jt s+jt a]-cycloaddilion reaction is symmetry allowed process (Fig. 3.1). It signifies that the cycloaddilion of one two-7t electron system with another two-ji electron system will be a thermally allowed process when one set of orbitals is reacting in a suprafacial mode and other set in an antarafacial mode ( s means suprafacial and a means antarafacial). Thermal [7t s+Ji a]-reactions usually occur in the additions of alkenes to ketenes, when alkene is in the ground state and ketene in the excited state [1] (Fig. 3.2). 

Some cycloaddition reactions of more than six ji electron systems have been reported. Thermal suprafacial [its+ s] [ s + s] [its + 7Cs]" y lo ditions are forbidden according to Woodward-Hofifmann rules. These cycloadditions are photochemically allowed processes. Thermal antarafacial [itj + 7c" a] addition is possible, but is rare. The following examples are illustrative for [4+4]- and [6+61-cycloadditions ... 

The selection rules for cycloaddition reactions can also be derived from consideration of the basis set orbitals from which the transition state for the cycloadditions v/ould arise (Fig. 10.10). For [4 + 2]-suprafacial addition, the transition state is aromatic for [2 H- 2]-suprafacial addition, it is antiaromatic. On the other hand, a [2 + 2]-addition that is antarafacial in one component is an allowed process. 

Among the cycloaddition reactions that have been shown to have general synthetic utility are the [2 + 2] cycloadditions of ketenes and alkenes. The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules. To be an allowed process, the [ 2 + 2] cycloaddition must be suprafacial in one component and antarafacial in the other. Figure 6.5 illustrates the transition state. The ketene, utilizing the low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the stereochemistry observed in these reactions. 

If a cycloaddition reactions results in bridging between opposite faces of the two 7t systems, the process is antarafacial. Antarafacial addition is symmetry allowed if one molecular orbital is symmetric and the other is antisymmetric. However, this type of cycloaddition has geometric constraints. A bridge that contains many atoms is required to permit simultaneous bonding to the opposite sides of a ti system. So, antarafacial additions are rare. 

The two modes of addition and the associated stereochemistry resemble other addition reactions we studied earlier. The suprafacial addition is a concerted syn addition to one of the ti systems. The antarafacial addition corresponds to a concerted anti addition. Although anti addition reactions are common in the chemistry of alkenes, the two groups that add to the alkene are not bonded to each other in the transition state. In cycloaddition reactions, both atoms of the molecule that bond to the terminal atoms of the second molecule are also connected to each other. Thus, only if the number of atoms in each of the two molecules is quite large can one molecule add to the other in an antarafacial process. 

Cycloadditions give rise to four-membered rings. Thermal concerted [2+2] cycloadditions have to be antarafacial on one component and the geometrical and orbital constraints thus imposed ensure that this process is encountered only in special circumstances. Most thermal [2+2] cycloadditions of alkenes take place by a stepwise pathway involving diradical or zwitterionic intermediates [la]. Considerably fewer studies have been performed regarding the application of microwave irradiation in [2+2] cydoadditions than for other kinds of cydoaddition (vide supra). Such reactions have been commonly used to obtain /1-lactam derivatives by cycloaddition of ketenes with imines [18-20,117,118],... 

The rules based on the Hiickel-Mobius concept have their counterpart among the Woodward-Hoffmann selection rules. There was a marked difference between the suprafacial and antarafacial arrangements in the application of the Woodward-Hoffmann treatment of cycloadditions. The disrotatory and conrotatory processes in elec-trocyclic reactions presented similar differences. The suprafacial arrangement in both of the reacting molecules in the cycloaddition as well as the disrotatory ring closure in Figure 7-25 correspond to... 

This [2 + 2] reverse cycloaddition is not likely to occur as a concerted process because the antarafacial geometry required for the thermal reaction is not possible for a four n-electron system. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

Essentially, a suprafacial-suprafacial or an antarafacial-antarafacial cycloaddition is equivalent to a concerted syn addition. A suprafacial-antarafacial or an antarafacial-suprafacial process is equivalent to a concerted anti addition. The Diels-Alder reaction is suprafacial for both components, so that the stereochemical relationships among the substituents are maintained in the product. In Example 6.6, suprafacial addition to the dienophile component means that the two carbomethoxy groups that are cis in the starting material also are cis in the product. Suprafacial reaction at the diene component leads to a cis orientation of the two methyl groups in the product. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

Occasionally, though, you will run across a more exotic pericyclic process, and will want to decide if it is allowed. In a complex case, a reaction that is not a simple electrocyclic ringopening or cycloaddition, often the basic orbital symmetry rules or FMO analyses are not easily applied. In contrast, aromatic transition state theory and the generalized orbital symmetry rule are easy to apply to any reaction. With aromatic transition state theory, we simply draw the cyclic array of orbitals, establish whether we have a Mobius or Hiickel topology, and then count electrons. Also, the generalized orbital symmetry rule is easy to apply. We simply break the reaction into two or more components and analyze the number of electrons and the ability of the components to react in a suprafacial or antarafacial manner. 

The reason for this anomalous behavior of halogenated olefins lies not in antarafacial cycloaddition, but rather in the fact that these reactions are not concerted. They are stepwise processes that proceed by way of a diradical intermediate stabilized by the halogen substituents ... 

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