Antarafacial addition mode

Woodward and Hoffman described addition modes based on the facial approach of the reactive termini as suprafacial and antarafacial. "A suprafacial process is one in which bonds made or broken lie on the same face of the system undergoing reaction", as in 11. 2 antarafacial process has the "newly formed or broken bonds on opposite sides of the reaction system," as in 12. 2 Woodward and Hoffman then described several... 

Additions are somewhat less common than [4+2]-cycloadditions because they are allowed only when one component reacts in the antarafacial mode. Most examples of [2+2]-additions that are antarafacial in one component have been observed with ketenes. Ketenes are ideal antarafacial components in reactions of this type, since they offer a minimum of steric hindrance to antarafacial addition because one of the carbon atoms involved is s p-hybridized. This results in a substantial decrease in the degree of crowding in the transition state. 

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. 

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... 

There are two modes of orbital overlap for the simultaneous formation of two cr bonds—suprafacial and antarafacial. Bond formation is suprafacial if both cr bonds form on the same side of the tt system. Bond formation is antarafacial if the two cr bonds form on opposite sides of the tt system. Suprafacial bond formation is similar to syn addition, whereas antarafacial bond formation resembles anti addition (Section 5.19). 

As cycloadditions occur between two Ti-systems, each can undergo either suprafacial (s) or antarafacial (a) addition to give four different possibilities, s+s,a+a,s+a and a+s. The Woodward-Hoffmann rules for cycloadditions distinguish between these four modes according to the numbers of electrons involved. 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

Since in a typical cycloaddition reaction there is addition of two unsaturated systems, it is logical to expect the addition to occur on the same or the opposite faces of the system involved. Furthermore, as both the Tu-systems are undergoing addition, it is necessary to specify these modes of addition on each of them especially to discuss and understand the stereochemistry of the product. These different modes have been termed as suprafacial (on the same side) and antarafacial (on the opposite side) and are shown in Figure 4.1. 

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). 

Additions are less common than [4 + 2] cycloadditions because they are allowed only when one component reacts in the antarafacial mode. Most examples of [2 -f 2] additions that are antarafacial in one component have been observed with... 

Suprafacial and Antarafacial Processes These are two stereochemical modes of cycloaddition. Here two systems containing double bonds add up, therefore it is logical to expect that addition occurs either at the same side of opposite side of the system. As both the Jt-systems are undergoing addition, it is... 

We have seen that the stereochemically distinguisliable modes of addition on a single olefin component are but two - suprafacial and antarafacial. For a two component +, rp] cyclo-addition the maximum number of distinguishable modes is therefore 2, for a three component addition it is 2, ... 

An antarafacial interaction on one, or both, of the molecules requires that the approach be orthogonal. Because of the high symmetry of the system the supra-antara and antara-supra modes are identical. Correlation diagrams for the [ 2j + 2, ] addition are to be found in the literature (Woodward and Hoffmann, 1965, 1969). We will concentrate here on the other two modes of addition. 

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