Suprafacial and antarafacial

A suprafacial process is one in which the bonds made or broken lie on the same face of the system undergoing reaction. When the newly formed or broken bonds lie on the opposite faces of the reacting systems, it is known as an antarafacial process. 

In the course of a pericyclic cycloaddition, the interacting terminal lobes of each component may overlap either in a suprafacial mode or in an antarafacial mode. If both the new bonds form from the same face of the molecule it is known as a suprafacial mode (also known as supra-supra). It is antarafacial if one bond forms from one surface and the other bond forms from the other surface (also known as supra-antara) (Fig. 8.13). 

The bracketed numbers that designate reactions of this kind sometimes carry subscripts (s or a) that specify their configuration. Thus, the Diels-Alder reaction (section 8.3.1) may be termed as [4s+2s] process under thermal conditions. 

In electrocyclic reactions, the suprafacial mode involves each component of the new CT-bond being formed from the same face of the reactant Tr-system (this is equivalent to disrotation). The antarafacial mode involves twisting of the orbitals so that the two components of the new a-bond form from the opposite face of the reactant TT-system (this is equivalent to conrotation) (Fig. 8.14). 

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Note that both suprafacial and antarafacial cycloadditions are symmetry-allowed. Geometric constraints often make antarafacial reactions difficult, however, because there must be a twisting of the it orbital system in one of the reactants. Thus, suprafacial cycloadditions are the most common for small tt systems. 

Both suprafacial and antarafacial sigmatropic rearrangements are symmetry-allowed, but suprafacial rearrangements are often easier for geometric reasons. The rules for sigmatropic rearrangements are identical to those for cycJoaddition reactions (Table 30.3). 

The symbols ji, o and (0 are given respectively to the n systems, o bonds and lone p orbitals which participate in the transition state and the symbols (s) and (a) are indicated for their suprafacial and antarafacial use. The notation is completed by the number of electrons supplied by each component. Thus n2s denotes a two electron n system used in a suprafacial way. woa indicates a vacant p orbital used in an antarafacial way and so on. 

Figure 12.3. Examples of suprafacial and antarafacial reactions of n systems. Notice that the examples serve to describe the stereochemical course of the reaction only. No mechanism is implied by these examples.

So far we have only defined what suprafacial and antarafacial mean on rc-systems (Fig. 2.7), but we need to see how o-bonds are treated by the Woodward-Hoffmann rules. Just as a suprafacial event on a rc-bond has overlap developing to the two upper lobes that contribute to the bond, so with o-bonds (Fig. 3.5a), overlap that develops to the two large lobes of the sp3... 

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. 

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... 

In order to describe the ring opening of the aziridine 6.55, we need to define what suprafacial and antarafacial mean when applied to a p orbital. This is shown in Fig. 6.9, and applied there to the conrotatory aziridine opening. When both lines are drawn into the same lobe it is suprafacial, and when there is one line drawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a n nor a a orbital, it is given the Greek letter uj. The same designations apply whether the orbital is filled (on the left) or unfilled (on the right), and whether it is a p orbital or any of the sp" hybrids. 

Fig. 6.9 Suprafacial and antarafacial defined for a p orbital, and the allowed conrotatory interconversion of an aziridine with an azomethine ylid...

This type of condensation is of great interest in connection with the Woodward-Hoffmann selection rules for symmetry-allowed concerted suprafacial and antarafacial cycloaddition reactions.284 The generalized rules for cycloaddition of an m- to an n-electron system predict that the concerted supra-supra or antara-antara dimerization is allowed in the excited state (i.e., photochemically) when m + n = 4q, and in the ground state (i.e., thermally) when to + n = 4q + 2, where to and n are the numbers... 

Figure 8.13 Suprafacial and antarafacial modes in a cycloaddition reaction.

When classifying cyclizations, the subscripts s and a are used to designate suprafacial and antarafacial, respectively. Thus, a more complete... 

The selection rules predict an antarafacial process. In this case, suprafacial and antarafacial processes would lead to the same product. 

These terms resemble the familiar ones syn and anti, but with this difference. Syn and anti describe the net stereochemistry of a reaction. We have seen anti addition, for example, as the overall result of a two>step mechanism. Suprafacial and antarafacial, in contrast, refer to actual processes the simultaneous making (or breaking) of two bonds on the same face or opposite faces of a component. 

For a successful cycloaddition to take place, the terminal lobes of the two reactants must have the correct symmetry for bonding to occur. This can happen in either of two ways, denoted suprafacial and antarafacial. Suprafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant (Figure 30.8). 

The Woodward-Hoffmann rules for electrocyclic reactions can also be formulated using the terms suprafacial and antarafacial (Table 4.3). A it system is said to react suprafacially in a pericyclic reaction when the bonds being made to the two termini of the it system are made to the same face of the 77 system. It reacts an-tarafacially when the bonds are made to opposite faces of the 7r system. In electrocyclic reactions, disrotatory reactions are suprafacial, and conrotatory reactions are antarafacial. 

The value of the terms suprafacial and antarafacial is that, unlike disrotatory and con-rotatory, they can also be used to describe the way that 7r systems react in cycloadditions and sigmatropic rearrangements. Most importantly, when a 77 system reacts suprafacially, its out groups become cis in the product when it reacts antarafacially, they become trans. Note that in disrotatory electrocyclic reactions, the out groups become cis, and in conrotatory electrocyclic reactions, the out groups become trans. 

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