What are electrocyclic reactions?
Electrocyclic reactions are characterized by the opening or closing of a ring within a single molecule by interconversion of σ bonds into π or vice versa.
Typical examples of electrocyclic reactions are:
This would be an allyl cation delocalized between the three centers.
Where we can also make a simile of arrows and logically that are balances so that the reactions are given in both directions.
In all cases, what we have in the transition state is a cyclic loop of interacting electrons where π and σ electrons are participating, but in the transition state the σ and π bonds are losing their sense because what we are left with is an intermediate between the two:
If we look here we can also have 2 substituents below the plane of the ring and 2 above, and in the process of opening or closing, the bond between the carbons forming the σ carbon is broken and they become sp2 carbons.
This implies that they will be planar, and therefore the substituents that were originally attached to those carbons in the opening or closing process have to rotate along those bonds so that those substituents that were below or above the plane become in-plane.
That is to say, in the process of opening or closing, a process of rotation along the links that become π-links must take place simultaneously.
Therefore, the stereochemistry of electrocyclic reactions is characterized by the direction of rotation of these bonds.
Let's see it with an example. Suppose the following molecule:
The breakage of that bond can occur in such a way that the rotation on those bonds is towards the same side, then we speak of a conrotatory rotation, where both bonds rotate towards the same side. Therefore, we will have a representation of the transition state where we see that the two are going towards the same side and then one of the a is inward and the other outward and likewise for the b.
There can also be another conrotatory rotation in the opposite direction, which will be distinguished from the previous one if the substituents are different. Or, a disrotatory rotation can occur, in which both bonds rotate in the opposite direction.
And this is going to give rise to a representation of the TS that subsequently evolves into the product of the reaction with the two a's to one side and the b's to the other.
In this case, the products can be different when another disrotatory movement occurs when the two rotate in opposite directions.
Thus a conrotatory or disrotatory movement implies a particular stereochemistry depending on the substituents which may have a particular configuration on the carbons of the reaction products since if a is more or less priority than b it will give rise to the isomer E or Z. As we have already said it is useful to try to see that all pericyclic reactions are particular cases of cycloadditions, for which we must try to associate the stereochemistry antarctically or suprafacially for the different types of bonds or electronic systems that may interact.
Definición de supra antara en sistemas σ
We have seen sterochemistry in π systems, when it was supra or antarafacial, but in electrocyclic reactions not only π systems are involved but also σ systems are involved and then it may be worthwhile for us to be able to define a supra or antara stereochemistry for the different types of systems.
Thus, any system π of any length is supra when intervening above or below:
And antara when they intervene simultaneously from above and below:
A σ bond can be represented as the interaction of two sp3 or sp2 hybrid orbitals through such an interaction:
Then a σ bond can also intervene supra or antarafacially, it must be taken into account that the supra or antara interaction implies much on the symmetry of the system and of the signs of the lobes involved, but on a σ system a supra interaction is one that occurs on lobes of the same sign, while an interaction on one of the large lobes and one of the small lobes is a case of antara interaction. One can also consider a single p orbital that can also interact supra o antara, so that for a supra interaction to occur there must be two interactions over one of the lobes, while in an antara it occurs simultaneously over one lobe and the other.
Definition of electrocyclic reaction as a cycloaddition
We are trying to see that electrocyclic reactions can be considered as a particular case of cycloadditions. Let's assume an electrocyclic reaction in a 6-membered system in which we are going to have on the one hand the σ orbital and we have the dienic system with its corresponding π orbitals.
We have already seen that there are 2 possibilities of stereochemistry, since when this bond is broken, a twist has to occur along the others so that the triene can be formed and the σ bond with the π system overlaps the lobes.
One possibility is a disrotatory motion such that you see the type of interaction that will occur.
Likewise, the small lobes will interact on the underside. Therefore, a disrotatory motion forces us to say that in the σ system both lobes of the same sign interact and the π system interacts with the 2 lobes on the same side.
Therefore, this electrocyclic reaction can be considered to be a cycloaddition of a σ system with two electrons interacting suprafacially with a π system of 4 electrons also suprafacially.
If we have substituents, they will have the proper stereochemistry according to the disrotatory procedure. If what is produced is a conrotatory motion:
When rotating in the direction of the figure, the lobes interact in that way, so this reaction is also considered as σ2s since two lobes of the same sign interact plus π4a since the system π on the one hand interacts with the upside and on the other hand with the downside. Also, the same conrotatory motion can be considered if we notice that when moving upward the small lobe on the left can interact with the top and on the right the big one also interacts with the top. In this case we would have an interaction σ2a+π4s.
The reaction is the same since the motion is conrotatory on the same system and gives the same products, however, it can be considered in two different ways.
These two reactions are fully equivalent. If we look at the electrocyclic reactions they can be considered as an example of cycloaddition reactions in which the interacting orbitals do not necessarily belong to different molecules, but can belong to the same molecule and the interaction is not limited to π-type orbitals but σ-type orbitals can also interact. The interesting thing is that in all these reactions an TS is produced with a closed loop of interacting electrons.
Particularities of electrocyclic reactions
In electrocyclic reactions, as in cycloadditions, certain preferences of a stereochemistry are observed according to the number of electrons involved in the reaction. Thus, when the number of electrons is 2, disrotatory stereochemistry is always obtained, whereas if the number of electrons is 4, the stereochemistry is conrotatory.
When the number of electrons is 6, disrotatory stereochemistry returns and so on.
That is, as in the case of cycloaddition reactions, when we change the number of electrons (increasing or decreasing by two) the stereochemistry changes, and this is reversed when the reactions are performed photochemically, i.e., in the excited state the preferred stereochemistry with two electrons is the conrotatory one and the opposite thermally. Let us now see that sigmatropic transposition reactions are also a particular example of cycloadditions.
Suppose a transpose [1,5] in which we have a 4-membered π-system and we have a σ-bond in which it is going to transpose along a π-system.
Therefore, we can have two types of transposition according to the sterochemistry in which the migrating group intervenes, if for example we have a transposition in which that group migrates to the other position, it does so while maintaining the stereochemistry of that center.
So that reaction is actually a cycloaddition involving a σ orbital with 2 electrons and a π orbital with 4 electrons.
In such a way that in the σ orbital the 2 lobes of the same sign intervene and therefore it will be an interaction σ2s+π4s (2 lobes of the π system below). But it could also be produced an interaction in which instead of producing the interaction of that same lobe to break the bond could be producing the interaction with the lobe below so that we could have an intermediate situation.
As this group migrates, it would be transformed from an sp3 orbital to a p orbital, so that in this intermediate situation we would have something in between the two types of orbitals that would be interacting with one part of the π system and the other.
If we look at that center, configuration inversion occurs, it is very similar to an SN2 type transition state in which configuration inversion must occur in the carbon that undergoes the substitution.
That will be a logically σ2a+π4s interaction. In reactions in the ground state, it is observed that there are sigmatropic reactions that occur and others that do not. For example, these reactions [1,5] occur with retention of the configuration on the migrating carbon, which indicates that it must be a σ2a+π4s reaction.
However, in the excited state, configuration inversion is observed in the migrating group.
This indicates that the migrating group does so with inversion of the configuration, so that this synteracting orbital interacts in an antarafacial manner.
Whenever an antara interaction occurs, it must be associated with an inversion of the configuration at one of the chiral centers formed or involved in the configuration. Therefore, we see that these are still particular examples of cycloaddition reactions and also what we said for cycloaddition reactions and electrocyclic reactions is still true.
In general, pericyclic reactions meet the conditions we have mentioned. They proceed or not according to the number of electrons, and when they occur the stereochemistry is also defined by this. Moreover, all these rules change when we pass from the ground state to the excited state.
They are characterized by the opening or closing of a ring within a single molecule. The stereochemistry of these reactions was developed by Woodward and Hoffmann, constituting the first achievement of orbital symmetry. Two types of ring closure can be identified.
- Conrotatory: the p orbitals of the π electron system rotate in the same direction to form the σ bond.
- Disrotatory: the p orbitals of the π electron system rotate in the opposite direction.
The Woodward and Hoffmann rules establish the preferred closure type and predict the stereochemistry of the product. This stereochemistry is determined by the number of electrons π of the open system, and whether the reaction is given in the ground state (thermal), or in the excited state of the polyene (photochemical). If we consider the 2,4-hexadiene.
In other words, the stereochemistry will depend on the type of closure, and this will depend on whether the reaction is carried out by heat or light. If we consider the fundamental state of a conjugated diene, such as, for example, butadiene, of the two thermal closure processes (butadiene in the fundamental state) only the conrotatory process of the HOMO orbital would occur because it ends in the bonding interaction of the orbitals at the ends.
If the reaction takes place photochemically, upon absorption of light the butadiene passes to its excited state and the reacting orbital is the LUMO. In this case the closure allowed by orbital symmetry is the disrotatory process where the bonding interaction of the end orbitals occurs.
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For example, hexatriene has 6 π electrons (4n+2=6, for n=1) and the thermally allowed mechanism would be the disrotatory one, while photochemically the conrotatory one occurs.