Antarafacial bonds

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). 

A cycloaddition reaction that forms a four-, five-, or six-membered ring must involve suprafacial bond formation. The geometric constraints of these small rings make the antarafacial approach highly unlikely even if it is symmetry-allowed. (Remember that symmetry-allowed means the overlapping orbitals are in-phase.) Antarafacial bond formation is more likely in cycloaddition reactions that form larger rings. 

A bonding interaction can be maintained only in the antarafacial mode. The 1,3-suprafacial shift of hydrogen is therefore forbidden by orbital symmetry considerations. The allowed... 

For a successful cycloaddition to take place, the terminal tt lobes of the two reactants must have the correct symmetry for bonding to occur. This can happen in either of two ways, called supra facial and antara facial. Suprafacial cycloaddjtions 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. Antarafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (Figure 30.8). 

How can we predict whether a given cycloaddition reaction will occur with suprafacial or with antarafacial geometry According to frontier orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of the other. An intuitive explanation of this rule is to imagine that one reactant donates electrons to the other. As with elec-trocyclic reactions, it s the electrons in the HOMO of the first reactant that are least tightly held and most likely to be donated. But when the second reactant accepts those electrons, they must go into a vacant, unoccupied orbital—the LUMO. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

The stereochemistry of any pericyclic reaction can be predicted by counting the total number of electron pairs (bonds) involved in bond reorganization and then applying the mnemonic "The Electrons Circle Around. " That is, thermal (ground-slate) reactions involving an even number of electron pairs occur with either conrotatory or antarafacial stereochemistry. Exactly the opposite rules apply to photochemical (excited-state) reactions. 

This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2s + 4a] cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photochemically allowed. Thus in order for a reaction to proceed,... 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

We have been discussing a cycloaddition where bonds are made or broken on the same face (suprafacial process). The alternative process is one where the bonds are made or broken on opposite faces of the reacting system (antarafacial) ... 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

Antarafacial use of a o bond involves retention at one end and inversion at the other. 

The reaction via a planar transition state is n2s + n2s. Here only one of the two new C—C bonds can be formed. This will raise its activation energy impossible to be reached. So there are two (4q + 2) electron suprafacial components and no antarafacial component. Since the total number of counting components is two, an even number, the reaction is thermally disallowed. 

The above interaction is suprafacial with respect to one component and antarafacial with respect to the other and is therefore a k2s + n2a] process. A bonding interaction could occur between two pairs of lobes of the same sign, but the nearer alkene molecule would need to twist about its original n bond, and so it will make it geometrically inaccessible. 

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. 

In antarafacial cycloaddition new bonds develop on opposite sides of the n system. If the number of components in a cycloaddition is considered, the possible pathways become still more numerous. For m+n cycloadditions, where m and n are different, four different pathways are conceivable, distinguished by the stereochemistry of the resulted adducts ms + ns, ma + na and... 

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