Forbidden reaction, Woodward-Hoffmann rules

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... 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

Refer to the molecular orbital diagrams of allyl cation (Figure 10.13) and those presented earlier in this chapter for ethylene and 1,3-butadiene (Figures 10.9 and 10.10) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Hoffmann rules. 

What makes photoexcited lepidopterene and its derivatives undergo adiabatic cycloreversion with so high quantum efficiency The answer to this question must be linked with fact that the formation of lepidopterene from its cycloreversion product A is a highly efficient ground state process, viz. an intramolecular Diels-Alder reaction, which is symmetry-allowed by Woodward-Hoffmann rules. By the same token, the excited state 4jm-2ji cycloreversion of lepidopterene L is a symmetry-forbidden process. Thus, it is... 

The Woodward-Hoffmann rule states that the reaction is allowed in the ground state if m + u is odd. The Dewar-Zimmerman rule states that the reaction is forbidden in the ground state if... 

The Woodward-Hoffmann rule states that the reaction is forbidden in the ground state if m + u is even. 

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. 

There are many conceivable analogues of this reaction. We mention only the famous H2 + I2 reaction, long supposed to proceed via a rectangular transition state, but shown by Sullivan7 to involve iodine atoms. The rectangular transition state is, in the light of the Woodward-Hoffmann rules, obviously forbidden. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

This reaction is forbidden by the Woodward-Hoffmann rules. Both interactions involving the ends of the dienes need to be bonding for concerted cycloaddition to take place. Here, one is bonding and the other is antibonding. 

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... 

Since the detailed calculation of these matrices is sufficiently described in the original literature [33, 58], it is possible to present directly the final results first for the case of concerted reactions for which there are two alternative reaction mechanisms, conrotatory and disrotatory. The first of these mechanisms is allowed by the Woodward-Hoffmann rules while the second one is forbidden. 

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 predictions one can make about electrocyclic processes are given in Table 1. Although this is a Table of both allowed and forbidden one-step processes, this does not rule out other reaction paths, e.g. via several steps by free radicals. Furthermore, forcing conditions may provide sufficient energy so that a forbidden path may become allowed. Considering the type of system, there are perhaps more predictions in the Table than experimental facts. Nevertheless, the success of the Woodward-Hoffmann rule has been remarkable. 

According to the Woodward-Hoffmann rules, the concerted [2S + 2J cycloaddition with two alkenes is photochemically symmetry-allowed, but is symmetry-forbidden at the ground state [9]. Photochemical [2 + 2] cycloaddition, in which one of two alkene partners is electronically excited, has been applied to the synthesis of cage hydrocarbons [10]. In such transformations, the intramolecular version of the reaction is particular efficient. The transformation of compound 1, in which two... 

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. 

The 1,5 cyclooctadiene complex [Cp Ru(jj rf--C Yiu) (CO)]OTf was isolated upon treatment of Cp Ru(j)" -butadiene)X (X = Cl, Br) with butadiene, AgOTf, and CO. A similar [4-1-4] cycloaddition (a thermally forbidden reaction see Woodward-Hoffmann Rules)) is observed when Cp Ru(isoprene)Cl is treated with iso-prene, AgOTf, and CO. Likewise, the reaction of 1,3-pentadiene with Cp Ru( ) -l,3-pentadiene)Cl results in linear dimerization to form [Cp Ru(4-methyl-(l,3-jj 6-8-j) )-nonadienediyl)]OTf. These types of dimerization occur with both stoichiometric and catalytic amounts of the ruthenium complex. ... 

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