Other Contributors 1] Exocylcic Double Bonds Exocyclic doube bond by definition is a double bond where one of the participating carbon atoms is a part of a ring, while the other carbon atom is not part of the same ring. From the name we can understand that exo-cyclic would stand for a double bond outside the ring and endo-cyclic would stand for a double bond within the ring. Below are a few examples as to what are exocyclic and what are endocyclic double bonds. Examples of Exocyclic and Endocyclic Double Bonds The above figure differentiates between exocyclic shown in red and endocyclic shown in green double bonds. In example 1, the double bond present within ring A is exocyclic to ring B as it is attached to an atom which is shared between ring A and ring B, while the double bond present in ring B is not connected to any ring A atoms and is within just one ring, hence making it endocyclic.
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In the language of orbital symmetry, a pericyclic reaction is termed symmetry-forbidden if there is an additional symmetry-imposed energetic barrier arising from the intended correlation of the ground state electron configuration of the starting material with an excited state electron configuration of the product and vice versa.
Although the non-crossing rule forbids such a correlation, the rise in energy as the intended crossing is approached results in an additional energy barrier nonetheless. A pericyclic reaction is classified as symmetry-allowed if no such symmetry-imposed barrier exists. Thus, these terms do not imply whether a reaction in question will actually take place.
Rather, with all other energetic factors being equal, a symmetry-forbidden process will be impeded by an additional energetic barrier. Although the symmetry-imposed barrier is often formidable up to ca. Likewise, a symmetry-allowed reaction may be preempted by an insurmountable energetic barrier resulting from factors unrelated to orbital symmetry.
The Woodward—Hoffmann rules were first formulated in to explain the striking stereospecificity of electrocyclic reactions under thermal and photochemical control. The interconversion of model cyclobutene and butadiene derivatives under thermal heating and photochemical Ultraviolet irradiation conditions is illustrative.
Similarly, thermolysis of cis-1,2,3,4-tetramethylcyclobutene 3 afforded only E,Z isomer 4. Thermolysis of 6 follows the same stereochemical course as 3: electrocyclic ring opening leads to the formation of E,Z -2,4-hexadiene 7 and not 5. The terms conrotatory and disrotatory were coined to describe the relative sense of bond rotation involved in electrocyclic ring opening and closing reactions. When the two ends of the breaking or forming bond rotate in the same direction both clockwise or both counterclockwise — as in the case of the ring opening of 1, 3 or 6 under thermal conditions , the process is termed conrotatory.
When the two ends rotate in opposing directions one clockwise, one counterclockwise — as in the photochemical ring closing of 5 , the process is termed disrotatory. This pattern was first explained in , when Woodward and Hoffmann proposed the conservation of orbital symmetry see below as a key principle that governs the stereochemical course of electrocyclic reactions.
Eventually, it was recognized that thermally-promoted pericyclic reactions in general obey a single set of generalized selection rules, depending on the electron count and topology of the orbital interactions. The key concept of orbital topology or faciality was introduced to unify several classes of pericyclic reactions under a single conceptual framework. In short, a set of contiguous atoms and their associated orbitals that react as one unit in a pericyclic reaction is known as a component, and each component is said to be antarafacial or suprafacial depending on whether the orbital lobes that interact during the reaction are on the opposite or same side of the nodal plane, respectively.
The older terms conrotatory and disrotatory, which are applicable to electrocyclic ring opening and closing only, are subsumed by the terms antarafacial and suprafacial, respectively, under this more general classification system. Given these general definitions, the Woodward—Hoffmann rules can be stated succinctly as a single sentence:  Generalized pericyclic selection rule.
A ground-state pericyclic process is brought about by addition of thermal energy i. In contrast, an excited-state pericyclic process takes place if a reactant is promoted to an electronically excited state by activation with ultraviolet light i. It is important to recognize, however, that the operative mechanism of a formally pericyclic reaction taking place under photochemical irradiation is generally not as simple or clearcut as this dichotomy suggests.
Several modes of electronic excitation are usually possible, and electronically excited molecules may undergo intersystem crossing , radiationless decay, or relax to an unfavorable equilibrium geometry before the excited-state pericyclic process can take place. Thus, many apparent pericyclic reactions that take place under irradiation are actually thought to be stepwise processes involving diradical intermediates.
Nevertheless, it is frequently observed that the pericyclic selection rules become reversed when switching from thermal to photochemical activation. This can be rationalized by considering the correlation of the first electronic excited states of the reactants and products.
Pericyclic reactions involving an odd number of electrons are also known. With respect to application of the generalized pericyclic selection rule, these systems can generally be treated as though one more electron were involved. A process in which the HOMO-LUMO interaction is constructive results in a net bonding interaction is favorable and considered symmetry-allowed, while a process in which the HOMO-LUMO interaction is non-constructive results in bonding and antibonding interactions that cancel is disfavorable and considered symmetry-forbidden.
Importantly, though conceptually distinct, aromatic transition state theory Zimmerman and Dewar , frontier molecular orbital theory Fukui , and the principle of orbital symmetry conservation Woodward and Hoffmann make identical predictions. Although orbital "symmetry" is used as a tool for sketching orbital and state correlation diagrams, the absolute presence or absence of a symmetry element is not critical for the determination of whether a reaction is allowed or forbidden.
That is, the introduction of a simple substituent that formally disrupts a symmetry plane or axis e. Instead, the symmetry present in an unsubstituted analog is used to simplify the construction of orbital correlation diagrams and avoid the need to perform calculations.
Moreover, orbital correlations can still be made, even if there are no conserved symmetry elements e. For this reason, the Woodward—Hoffmann, Fukui, and Dewar—Zimmerman analyses are equally broad in their applicability, though a certain approach may be easier or more intuitive to apply than another, depending on the reaction one wishes to analyze.
Original formulation[ edit ] The Woodward—Hoffmann rules were first invoked to explain the observed stereospecificity of electrocyclic ring-opening and ring-closing reactions at the ends of open chain conjugated polyenes either by application of heat thermal reactions or application of light photochemical reactions.
In a photochemical reaction an electron in the HOMO of the reactant is promoted to an excited state leading to a reversal of terminal symmetry relationships and stereospecificity. Using this formulation it is possible to understand the stereospecifity of the electrocyclic ring-closure of the substituted buta-1,3-diene pictured below.
The Woodward—Hoffmann rules say nothing about the position of equilibrium for pericyclic processes. Without loss of generality, all analyses here are performed in the ring closing direction. Conversely in the electrocyclic ring-closure of the substituted hexa-1,3,5-triene pictured below, the reaction proceeds through a disrotatory mechanism. Organic reactions that obey these rules are said to be symmetry allowed. Reactions that take the opposite course are symmetry forbidden and require substantially more energy to take place if they take place at all.
Correlation diagrams[ edit ] As shown by Longuet-Higgins and E. Abrahamson, the Woodward—Hoffmann rules can also be derived by examining the correlation diagram of a given reaction. If a symmetry element is present throughout the reaction mechanism reactant, transition state, and product , it is called a conserved symmetry element.
Then, throughout the reaction, the symmetry of molecular orbitals with respect to this element must be conserved. That is, molecular orbitals that are symmetric with respect to the symmetry element in the starting material must be correlated to transform into orbitals symmetric with respect to that element in the product. Conversely, the same statement holds for antisymmetry with respect to a conserved symmetry element.
A molecular orbital correlation diagram correlates molecular orbitals of the starting materials and the product based upon conservation of symmetry. From a molecular orbital correlation diagram one can construct an electronic state correlation diagram that correlates electronic states i. Correlation diagrams can then be used to predict the height of transition state barriers. MOs of butadiene are shown with the element with which they are symmetric. They are antisymmetric with respect to the other.
Considering the electrocyclic ring closure of the substituted 1,3-butadiene, the reaction can proceed through either a conrotatory or a disrotatory reaction mechanism. In order to correlate orbitals of the starting material and product, one must determine whether the molecular orbitals are symmetric or antisymmetric with respect to these symmetry elements.
The same analysis can be carried out for the molecular orbitals of cyclobutene. The result of both symmetry operations on each of the MOs is shown to the left. Correlation lines are drawn to connect molecular orbitals in the starting material and the product that have the same symmetry with respect to the conserved symmetry element.
This would lead to a significantly higher transition state barrier to reaction. However, as reactions do not take place between disjointed molecular orbitals, but electronic states, the final analysis involves state correlation diagrams.
A state correlation diagram correlates the overall symmetry of electronic states in the starting material and product. The overall symmetry of the state is the product of the symmetries of each filled orbital with multiplicity for doubly populated orbitals.
A second excited state ES-2 of butadiene. These correlations can not actually take place due to the quantum-mechanical rule known as the avoided crossing rule. This says that energetic configurations of the same symmetry can not cross on an energy level correlation diagram. In short, this is caused by mixing of states of the same symmetry when brought close enough in energy. In the diagram below the symmetry-preferred correlations are shown in dashed lines and the bold curved lines indicate the actual correlation with the high energetic barrier.
However, if the molecule is in the first excited state i. These are not completely distinct as both the conrotatory and disrotatory mechanisms lie on the same potential surface.
Thus a more correct statement is that as a ground state molecule explores the potential energy surface, it is more likely to achieve the activation barrier to undergo a conrotatory mechanism.
See below "General formulation" for a detailed description of the generalization of WH notation to all pericyclic processes. This mechanism leads to a retention of stereochemistry in the product, as illustrated to the right.
In the correlation diagram, molecular orbitals transformations over the course of the reaction must conserve the symmetry of the molecular orbitals. Thus a high barrier is predicted. Thus these two excited states correlate.
The ground state of the starting materials only attempts to correlate with the second excited state as there is an avoided crossing in the middle due to the states possessing the overall same symmetry. Thus in actuality, the ground state of the reactants is transformed into the ground state of the products only after achieving a high energetic barrier. However, there is no large activation barrier if the reactants are in the first excited state. Thus this reaction proceeds easily under photochemical control, but has a very high barrier to reaction under thermal control.
The simplest case is the reaction of 1,3-butadiene with ethylene to form cyclohexene shown to the left. There is only one conserved symmetry element in this transformation — the mirror plane through the center of the reactants as shown to the left.
From this we can assign the symmetry of the molecular orbitals of the reactants very simply. Correlating the pairs of orbitals in the starting materials and product of the same symmetry and increasing energy gives the correlation diagram to the right. As such this ground state reaction is not predicted to have a high symmetry-imposed barrier. One can also construct the excited-state correlations as is done above. Here, there is a high energetic barrier to a photo-induced Diels-Alder reaction under a suprafacial-suprafacial bond topology due to the avoided crossing shown below.
Group transfer reactions[ edit ] Transfer of a pair of hydrogen atoms from ethane to perdeuterioethane. The symmetry-imposed barrier heights of group transfer reactions can also be analyzed using correlation diagrams. A model reaction is the transfer of a pair of hydrogen atoms from ethane to perdeuterioethane shown to the right. The only conserved symmetry element in this reaction is the mirror plane through the center of the molecules as shown to the left. Conserved mirror plane in transfer reaction.
The full molecular orbital correlation diagram is constructed in by matching pairs of symmetric and asymmetric MOs of increasing total energy, as explained above. As can be seen in the adjacent diagram, as the bonding orbitals of the reactants exactly correlate with the bonding orbitals of the products, this reaction is not predicted to have a high electronic symmetry-imposed barrier. Each of these particular classes is further generalized in the generalized Woodward—Hoffmann rules.
The more inclusive bond topology descriptors antarafacial and suprafacial subsume the terms conrotatory and disrotatory, respectively. A suprafacial transformation at a chiral center preserves stereochemistry, whereas an antarafacial transformation reverses stereochemistry. Electrocyclic reactions[ edit ] The selection rule of electrocyclization reactions is given in the original statement of the Woodward—Hoffmann rules.