Woodward-Hoffmann rules, for cycloaddition

Table 7.1 Woodward-Hoffmann rules for cycloaddition reactions...

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. 

Table 29.3 Woodward- -Hoffmann Rules for Cycloaddition Reactions ...

As cycloadditions occur between two Ti-systems, each can undergo either suprafacial (s) or antarafacial (a) addition to give four different possibilities, s+s,a+a,s+a and a+s. The Woodward-Hoffmann rules for cycloadditions distinguish between these four modes according to the numbers of electrons involved. 

Fig. 4.24b The interaction between the butadiene LUMO and the ethylene HOMO Table 4.2 The Woodward-Hoffmann rules for cycloaddition reactions...

Based on the orbital relations discussed in the text and extensions of these relations to other cases, formulate generalized verbal rules (Woodward-Hoffmann rules) for cycloadditions and sigmatropic shift reactions. 

The unsubstituted benzo-TA 263 that was generated in situ from 2-ethoxy-2,3-dihydrobenzo-TA 262 reacts with 1,3-dienes to give the Diels-Alder adducts 264 and 265 (73JHC149) possessing a cis-configuration according to the Woodward-Hoffmann rules for [2 + 4]-cycloaddition (Scheme 103). 

Fig. 12.4 shows two possible ways for this to happen the HOMO of the diene can combine with the LUMO of the dienophile or the LUMO of the diene can combine with the HOMO of the dienophile. The thermal reaction with this six-electron transition state is allowed, but the corresponding photochemical mechanism is forbidden. More generally, the Woodward-Hoffmann rule for concerted cycloaddition reactions can be stated If the number of electrons in the transition state equals An [An + 2], then thephotochemical [thermal] reaction will be allowed, but the thermal [photochemical] reaction will be forbidden. 

The generalized Woodward-Hoffmann rules for concerted cycloaddition can be summarized as follows ... 

The idea that transition states can be of Mobius type, in which the relative stability of AN- and 4A- -2-electron systems is reversed, was developed and systematized by Zimmerman [33], who derived the Woodward-Hoffmann Rules for the various thermo-rearrangements in terms of the Hiickel or Mobius nature of their transition states. As shown in (a) and (b) of Fig. 1.4, a cycloaddition in which one of the reaction partners reacts suprafacially and the other antarafacially mimics a Mobius surface, so [t 2s +7r2a]-cycloaddition is allowed whereas reaction along the [ Aa pathway is forbidden, as is the... 

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. 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offiilminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4-6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

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. 

Such cycloadditions involve the addition of a 2tt- electron system (alkene) to a 4ir- electron system (ylide) and have been predicted to occur in a concerted manner according to the Woodward-Hoffmann rules. The two most important factors involved in the cycloaddition reactions are (i) the energy and symmetry of the reacting orbitals and (ii) the thermodynamic stability of the cycloadduct. The reactivity of 1,3-dipolar systems has been successfully accounted for in terms of HOMO-LUMO interactions using frontier MO theory (71TL2717). This approach has been extended to explain the 1,3 reactivities of the nonclassical A,B-diheteropentalenes <77HC(30)317). 

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. 

Much of what we have said about the electronic factors controlling whether a cycloaddition reaction can be concerted or not originally was formulated by the American chemists R. B. Woodward and R. Hoffmann several years ago, in terms of what came to be called the orbital symmetry principles, or the Woodward-Hoffmann rules. Orbital symmetry arguments are too complicated for this book, and we shall, instead, use the 4n + 2 electron rule for-normal Hiickel arrangements of tt systems and the An electron rule for Mobius arrangements. This is a particularly simple approach among several available to account for the phenomena to which Woodward and Hoffmann drew special attention and explained by what they call conservation of orbital symmetry.- ... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

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