Reaction Woodward-Hoffmann forbidden

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... 

Foldy-Wouthuysen transformation, 215 Forbidden reaction, Woodward-Hoffmann... 

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. 

In reality, the reaction could not be persuaded to go exactly as shown in Scheme 21.1, because the Cl—C3 bond would certainly break at very nearly the same rate as Cl—C2. In the experiments actually conducted by Baldwin et al., this problem was resolved by deuterium labeling both C2 and C3—creahng diastereomericaUy pure, but achiral molecules. Even then, there remained a large number of technical difficulties, which in the end the researchers were able to overcome. Their results indicated that the four stereochemical courses for the reaction run at 300 °C were sr 23%, si 40%, ar 13%, and ai 24%. These numbers do not ht the expectations from either mechanism. Clearly, the Woodward-Hoffmann forbidden and allowed products are formed in nearly equal amounts ([sr] + [ai] =47% [si] + [ar] = 53%)— hardly what one would expect for a pericyclic reaction. On the other hand, the stereochemical paths do not show the pairwise equalities expected from the stepwise mechanism. 

In previous sections we have seen how the CM model may be utilized to generate reaction profiles for ionic reactions, and it is now of interest to observe whether the same general principles may be applied to the class of pericyclic reactions, the group of reactions that is governed by the Woodward-Hoffmann (1970) rules. In other words, the question we ask is whether the concept of allowed and forbidden reactions may be understood within the CM framework. 

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 Woodward-Hoffmann pericyclic reaction theory has generated substantial interest in the pathways of forbidden reactions and of excited state processes, beginning with a paper by Longuet-Higgins and Abrahamson,54 which appeared simultaneously with Woodward and Hoffmann s first use of orbital correlation diagrams.55 We have noted in Section 11.3, p. 586, that the orbital correlation diagram predicts that if a forbidden process does take place by a concerted pericyclic mechanism,56 and if electrons were to remain in their original orbitals, an... 

Give examples of possible reactions of each of the following types, and determine whether each is forbidden or allowed in the ground state, using both the Woodward-Hoffmann and the Dewar-Zimmerman rules. 

All recent ab initio studies on the head-to-tail dimerization reaction for the parent silene H2Si=CH2 predict a very exothermic reaction with a low barrier, despite the fact that the reaction is formally forbidden12. It is believed that the strong polarization of the Si=C double bond leads to a relaxation of the Woodward-Hoffmann rales. A detailed analysis of the symmetry of the head-to-tail silene dimerization reveals, however, that a concerted [2jrs + 2jrs] reaction in the appropriate point group is not forbidden by symmetry (see Figure l)186. 

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. 

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