Components Woodward-Hoffmann rule

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. 

Cycloadditions of ketenes and alkenes have synthetic utility for the preparation of cyclobutanones.163 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.164 To be an allowed process, the [2ir + 2-tt] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the TS is a 2irs + (2tts + 2tts) addition.165 Figure 6.13 illustrates these combinations. Note that both representations predict formation of the d.v-substituted cyclobutanone. 

Thermal concerted 2 + 2 reactions are predicted to occur between an alkene and a ketene. According to Woodward-Hoffmann rules, addition must be suprafacial to one component and antarafacial to the other if the process is to be concerted. 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

When we come to use the Woodward-Hoffmann rules on these [2,3]-sigmatropic rearrangements, we find something new. We have a K bond and a o bond and a carbanion. How are we to represent a carbanion (or a carbocation) that is just a p orbital on an atom The new symbol we use for a simple p orbital is to. A carbanion is an component and a carbocation is an m0 component as it has zero electrons. If the two new bonds are formed to the same lobe of the p orbital of the carbanion, we have an m2s component but, if they are formed to different lobes, we have an m2a component. 

If you use the Woodward—Hoffmann rules, you need to note that the hydrogen atom must react with retention. The Is orbital is spherically symmetrical and has no node, so wherever you draw the dotted line from that orbital it always means retention. Choosing the components is easy—the diene is a n4 and the C-H bond a c2 component. 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

Then there is the problem of assessing whether the reaction is symmetry-allowed or not using the Woodward-Hoffmann rule. All reactions using (An + 2) electrons (an odd number of curly arrows) are allowed in the all-suprafacial mode, and so it is helpful to draw the dashed or solid lines (or better still use a line with a distinctive colour) to show the developing overlap with only suprafacial components. The (4q + 2)s components will then add up to an odd number, and the task is done. 

Under thermal conditions, the [2+2]-cycloaddition of olefins is symmetrically forbidden, according to the Woodward-Hoffmann rules. However, under photochemical conditions, [2+2]-cycloadditions become a suprafacial process for both components The orbital geometry of the interacting orbitals is equal and therefore the entire reaction is symmetrically allowed. 

The Woodward-Hoffmann rules for cycloadditions (Table 4.4) are as follows. Both components of a cycloaddition involving an odd number of electron pairs are suprafacial under thermal conditions under photochemical conditions, one component must be antarafacial. Both components of a cycloaddition involving an even number of electron pairs are suprafacial under photochemical conditions under thermal conditions, one component must be antarafacial. 

Problem 4.13. What do the Woodward-Hoffmann rules suggest about the facial reactivity of the components of the following thermal [6 + 4] cycloaddition and thermal [4 + 3] cationic cycloaddition ... 

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

In a sigmatropic rearrangement, bonds are made and broken at the ends of two conjugated systems. The Woodward-Hoffmann rules for sigmatropic rearrangements must take both components into account. 

Thermal [1,3] H shifts such as the conceited rearrangement of enols to carbonyl compounds are disallowed. The allylic C-C-O unit itself can only react suprafa-cially, as it is geometrically impossible for the H(ls) orbital to bond simultaneously to a top lobe on one terminus and a bottom lobe at the other terminus, and the H atom itself must also react suprafacially, as the H(ls) orbital has only one lobe. The Woodward-Hoffmann rules, though, say that one of the two components of this four-electron rearrangement must react antarafacially for it to be allowed. Therefore this rearrangement reaction always proceeds through a nonconcerted mechanism and requires acidic or basic catalysis. 

The Stevens rearrangement and the Wittig rearrangement (nonallylic version) (Chapter 4) can be classified as four-electron [1,2] sigmatropic rearrangements. The Woodward-Hoffmann rules state that for a four-electron sigmatropic rearrangement to be allowed, one of the components must be antarafacial, yet it is... 

In discussing the Woodward-Hoffmann rules, we were indifferent as to which of the two components of a cycloaddition would provide the HOMO and which the LUMO. The two pairs of frontier orbitals bear a complementary relationship to each other they both invariably give the same answer, and we could safely make an arbitrary choice. To explain the effects of substituents on the rates of Diels-Alder reactions, however, we need to know which is the more... 

It is more difficult to explain the effect of substituents on the rates, and on the regio- and stereoselectivities of the unimolecular pericyclic reactions. We cannot strictly look at the HOMO and LUMO of each component, as we could with bimolecular reactions, and therefore cannot properly use frontier orbitals to explain the effects of electron-donating and electron-withdrawing substituents on the rates.905 The effects are profound, sometimes even strong enough to override the Woodward-Hoffmann rules.906... 

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