Cycloaddition reaction antarafacial geometry

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. 

Figure 30.10 (a) Interaction of a ground-state HOMO and a ground-state LUMO in a potential [2 - 2] cycloaddition does not occur thermally because the antarafacial geometry is too strained, (b) Interaction of an excited-state HOMO and a ground-state LUMO in a photochemical [2 r 2] cycloaddition reaction is less strained, however, and occurs with suprafacial geometry. 

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. 

Interaction of HOMO and LUMO in a potential thermal [2 + 2] cycloaddition. The reaction does not occur because antarafacial geometry is too strained. 

Let s examine an alternative collision geometry for the 4 + 2 reaction. In an anta-rafacial interaction, one reactant collides in such a manner that the opposite faces of the p orbitals in the tt bonds interact. Show that the 4 + 2 cycloaddition reaction is forbidden when butadiene interacts in an antarafacial manner. 

Any of the combinations s + s, s + a, a + s, a + a is conceivable for a cycloaddition of two components. Comparison of cis-trans stereochemistry of substituents in product to that in reactants establishes which occurred. In additions of relatively short chains, the antarafacial interaction is difficult for the molecule to attain, but when systems with appropriate geometry are contrived, it is found that reactions in which one component acts in the antarafacial manner exhibit an inverted preference with respect to ring size The 2 + 2 additions are now favorable and the 4 + 2 not. 

It is not clear how many photochemical reactions are in fact pericyclic, and it is not easy to use the usual methods of physical organic chemistry to prove it. What is clear is that a great many photochemical reactions do obey the photochemical rule, and it therefore seems likely that some of them have pericyclic character at a critical stage. Let us look at two possible geometries for [2+2] cycloadditions. Both components can act suprafacially 3.28, or one can be suprafacial and one antarafacial 3.29. The first 3.28 is a [ g+ J cycloaddition with an even number of (4q+2)s components and no (4r)a... 

In all of the above discussion we have assumed that a given molecule forms both the new ct bonds from the same face of the n system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [ 2s + 4J-cycloaddition (the subscript 71 indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2 + 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 photoche-mically allowed. Thus in order for a [fZs + -reaction to proceed, overlap between the highest occupied n orbital of the alkene and the lowest unoccupied 71 orbital of the diene would have to occur as shown in Fig. 15.10, with a + lobe... 

We have already mentioned the binary nature of the pericyclic reaction rules. If we change the mode of interaction of one of the reactants, we will reverse the allowed / forbidden nature of the reaction. For example, if we change the interaction mode of one of the reaction partners in the cycloaddition from suprafacial to antarafacial, now the [4+2) cycloaddition is forbidden, and the [2 + 2] cycloaddition is actually allowed. The [2+2] cycloaddition is now designated as [ 2 + T 2a]. In Figure 15.10 A we define the [ 2 + 2j] reaction, and in Figure 15.10 B we show a realistic geometry that could achieve the necessary orbital interactions. The two TT systems approach in a perpendicular orientation, and the lines define the suprafacial and antarafacial interactions. 

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