Woodward-Hoffmann rules examples

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Fukui at Kyoto Imperial University in Japan introduced a simplified version. According to Fukui, we need to consider only two molecular orbitals, called the frontier orbitals. These frontier orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In ground-state 1,3,5-hexa-triene, for example, 1//3 is the HOMO and excited-stale 1,3,5-hexatriene, however, 5 is the LUMO. 

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

These reactions are characterized by the phenomenon that the frontier orbitals of the reactants maintain a defined stereochemical orientation throughout the w hole reaction. Most noteworthy in this respect, is the principle of orbital symmetry conservation ( Woodward-Hoffmann rules la), but the phenomenon is much more general, as shown by the following examples of Self-Immolative Stereoconversion or Chirality Transfer . This term describes processes by which a stereocenter in the starting material is sacrificed to generate a stereocenter in the product in an unambiguous fashion. This is, of course, the case in classical SN2-displacements. 

Similarly, electrocyclic reactions7 follow the Woodward-Hoffmann rules and proceed in either a con- or disrotatory manner. For example, the diene 5 cyclizes to the cyclobutene 6 or 7, which are enantiomers. No diastereomers with R1/Ri trams geometry are formed. 

The Diels-Alder reaction is a concerted reaction in which four re-electrons from the diene and two re-electrons from the dienophile participate in the transition state. The Woodward-Hoffmann Rules provide a theoretical framework for these reactions.24 They suggest that those reactions are thermally allowed which have 4n + 2 pericyclic electrons, i.e. 6, 10, 14, etc. The Diels-Alder reaction is an example where n = 1, i.e. (4 + 2) re-electrons. 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

The Alder-ene reaction is an atom-economic reaction which forms a new carbon carbon-bond from two double bond systems (alkenes, carbonyl groups, etc.) with double bond migration [5]. This reaction follows the Woodward-Hoffmann rules if the reaction is performed under thermal conditions. However, when transition metal catalysts are involved, thermally forbidden Alder-ene reactions can also be realized (Scheme 9.1). Examples of such processes are the formal [4 + 4]-Alder-ene reaction catalyzed by low-valent iron catalysts. 

No scientific discovery seems to be totally new, as has been discussed superbly well by Berson (1992) with reference to the discoveries of the Diels-Alder reaction and the Woodward-Hoffmann rule. In the case of C60, the near misses by Iijima (1987) and the unpublished work by Chapman (Diederich 1992), are more pre-eminent examples of the precedence than those described above. 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

Extraction of information from p may not be as elegant as from P. For example, the Woodward-Hoffmann rules follow fairly transparently from the symmetries of molecular orbitals (wavefunctions), but deriving them from p requires using a dual descriptor function [1]. 

The Woodward-Hoffmann rules cannot always be applied directly. For example, the intracyclic double bond complicates the analysis of Reaction (4.1), which could be an eight-electron (forbidden) or a six-electron (allowed) process. 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

We will not develop all of the Woodward-Hoffmann rules, but we will show how the molecular orbitals can indicate whether a cycloaddition will take place. The simple Diels-Alder reaction of butadiene with ethylene serves as our first example. The molecular orbitals of butadiene and ethylene are represented in Figure 15-18. Butadiene, with four atomic p orbitals, has four molecular orbitals two bonding MOs (filled) and two antibonding MOs (vacant). Ethylene, with two atomic p orbitals, has two MOs a bonding MO (filled) and an antibonding MO (vacant). 

This is a key difference. The Woodward-Hoffmann rules (Chapters 35 and 36) were deduced from theory, and examples were gredually discovered that fitted them. They cannot be violated a reaction that disobeys the Woodward-Hoffmann rules is getting around them by following a different mechanism. Baldwin s rules were formulated by making observations of reactions that do, or do not, work. 

Note that the stereospecificity of the reaction in Scheme 2 follows the orbital symmetry predicted by the Woodward-Hoffmann rules [2] the oxide and ylide interconvert via a disrotatory mode. As is the case with all three-membered heterocycles mentioned here, UV irradiation of the heterocycle generates a highly colored ylide intermediate. Unfortunately, the stability of the ylides derived from monocyclic oxiranes is poor and photochromic behavior is evident only upon irradiation at low temperatures (77 K) [3]. This drawback has been somewhat circumvented in a few cases by annulation of the ylide functionality, which increases its stability. For example, 5-oxabicyclo[2.1.0] pentane develops a purple color when irradiated at 253.7 nm at room temperature [4], This reaction is shown in Scheme 3. 

The effect described here seems fully capable of rationalizing the phenomena discussed. However, at least one other effect parallels the changes in polarization described in the last section. Like any qualitative quantum mechanical effect, generalizations and predictions are easily made, but experiments must be devised, or quantitative computations attempted, in order to determine whether the effect is of chemical significance. For example, the basis of the Woodward-Hoffmann rules is certainly correct, and allowed reactions appear to be 10—15 kcal/mol more favored than similar forbidden reactions in many cases. It would have been possible, in principle, that allowed reactions would have been 0.1 kcal/mol more favorable than forbidden. If such had been the case, then the Woodward-Hoffmann Rules would have been of no chemical significance The magnitude of the effects, not the correctness of the arguments, is what is in question for the phenomena discussed here. 

What is Symmetry Allowed and Symmetry Forbidden Reactions Woodward Hoffmann Rule o Bonds involved in Cycloaddition Reactions Some more Examples of 2 + 2 Cycloadditions Photochemical Cycloadditions 2 + 3 Cycloadditions 2, 1 Cycloaddition... 

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