Cycloaddition reaction suprafacial 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. 

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

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. 

The symmetries of the terminal lobes of the HOMO and LUMO of the reactants in a [4 + 2] thermal cycloaddition allow the reaction to proceed with suprafacial geometry. 

The Diels-Alder reaction is a thermal [4 + 2] cycloaddition, which occurs with suprafacial geometry. The stereochemistry of the diene is maintained in the product. 

The reaction of cyclopentadiene and cycloheptatrienone is a [6 + 4] cycloaddition. This thermal cycloaddition proceeds with suprafacial geometry since five electron pairs are involved in the concerted process. The n electrons of the carbonyl group do not take part in the reaction. 

Interaction of excited-state HOMO and LUMO in photochemical [2 -L 2] cycloaddition reactions. The reaction occurs with suprafacial geometry. 

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. 

In all of the above discussion, we have assumed that a given molecule forms both the new s bonds from the same face of the 7t 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 [7t2s+,i4s] cycloaddition (the subscript n 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. 

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... 

The preservation ofalkene geometry in these concerted cycloadditions implies a mode of reaction in which, for each alkene unit, the two new bonds are formed to the same face of the Jt-bond. This mode is called suprafacial-suprafacial cycloaddition, and is one of... 

Reactions of Ketenes, AUenes and Carbenes which Appear to be Forbidden. Some [2 + 2] cycloadditions only appear to be forbidden. One of these is the cycloaddition of ketenes to alkenes. These reactions have some of the characteristics of pericyclic cycloadditions, such as being stereospecifi-cally syn with respect to the double bond geometry, and hence suprafacial at least on the one component, as in the reactions of the stereoisomeric cyclo-octenes 6.110 and 6.112 giving the diastereoisomeric cyclobutanones 6.111 and 6.113. However, stereospecificity is not always complete, and many ketene cycloadditions take place only when there is a strong donor substituent on the alkene. An ionic stepwise pathway by way of an intermediate zwitterion is therefore entirely reasonable in accounting for many ketene cycloadditions. 

Under thermal conditions, the [2+2]-cycloaddition of olefins is symmetrically forbidden, according to the Woodward-Hoffmann rules. However, under photochemical conditions, [2+2]-cycloadditions become a suprafacial process for both components The orbital geometry of the interacting orbitals is equal and therefore the entire reaction is symmetrically allowed. 

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