Woodward-Hoffman rules

For many reasons, including the Woodward-Hoffman rules that describe the likelihood of reaction based on arguments about the shapes of orbitals, it is desirable to be able to visualize molecular orbitals. 

The Woodward-Hoffman rules (1) state that "A ground state pericyclic change is symmetry allowed when the total number of (4q + 2) suprafacial and (4r) antarafacial components is odd". 

In summary, the A1- and A2-dialin isomers have been shown to be appreciably more active than etralin (and decalin) in transferring hydrogen to anthracene and phenanthrene. The observed selectivity of this hydrogen transfer is in accord with the Woodward-Hoffman rules for group transfer reactions, anthracene conversions being in the ratio ( 3 / 0 ) = 12/1 >> 1 while phenanthrene conversions are in the ratio ( 0/(33 ) = 0.6/1 < 1. The quantitative differences in the selectivities observed with anthracene and phenanthrene are being further explored. 

At typical coal liquefaction conditions, namely temperatures from 300 to 400 C and reaction times on the order of 1 hr, hydrogen transfer from model CIO donors, the A1- and A2-dialins, to model C14 acceptors, anthracene and phenanthrene, occurs in the sense allowed by the Woodward-Hoffman rules for supra-supra group transfer reactions. Thus, in the conversion of the C14 substrates to their 9, 10 dihydro derivatives the dialins exhibited a striking reversal of donor activity, the A dialin causing about twice as much conversion of phenanthrene but only one-tenth as much conversion of anthracene as did A2-dialin. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Decomposition of oxetanes is still another chemiluminescent reaction. On the basis of Woodward-Hoffman rule of conservation of orbital symmetry, the concerted bond cleavage of dioxetane, a 4-membered ring peroxide, should yield one carboxyl moiety in the excited state... 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbomadiene to quadricyclene conversion.

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

In essence, jSel is the interaction between the electronic wave functions of A and B. Obviously there must be some spatial overlap between the two in order to give rise to a finite value. Furthermore, if 0A. and />B fall in the same symmetry class or, in complex molecules, have similar local symmetry, the value of j8 may be relatively large. This will depend upon whether or not the perturbation operators, H, have preferred symmetry properties. The Woodward-Hoffman rules suggest that these operators can... 

Vibronic mixing of levels, leading to an overall finite value of a transition probability, even when /9el is zero because of symmetry, may occur if nonsymmetric vibrational levels of the product are used in the internal conversion process. The effect is well known in the case of optical transitions and may be the best interpretation of the Woodward-Hoffman rules. 

C. L. Perrin, "The Woodward-Hoffman Rules—An Elementary Approach," Chemistry in Britain 8, 163 (1972). 

A total synthesis of vitamin B12 was announced in 1972, as the result of a collaborative effort between R. B. Woodward (Harvard) and A. Eschenmoser (Zurich). The synthesis was completed after 11 years of effort involving 100 co-workers from 19 countries. A number of important techniques and reactions of synthetic value were developed during the course of this work, including the principle of conservation of orbital symmetry (the Woodward-Hoffman rules, Section 21-10). The biochemical action of vitamin B12 is considered in Chapter 31. 

Woodward-Hoffman rules define photochemical [4+2] cycloadditions as disallowed based on orbital symmetry considerations, whereas [4+4] cycloadditions are allowed [10]. This is only true, however, when the... 

Perhaps the most important consequence of the Woodward-Hoffman rules is its predictions of allowed and forbidden reactions. In particular, reactions involving An (An + 2) electrons are allowed if there are an odd (even) number of antarafacial two-electron components. A conseqnence of this symmetry property is that changing the number of electrons will alter whether the reaction is allowed or forbidden. 

The concept of torquoselectivity is now accepted as an extension of the Woodward-Hoffman rules. It has been used as a guide for synthetic chemists to prepare the appropriate stereoisomer. Examples include Danishefsky s exploitation of the stereoselective ring opening of fran5 -l,2-disiloxybenzocyclobutenes 133 to prepare idarubicin, Paquette s use of the electrocyclization of 134 that ultimately leads to a very efficient synthesis of pentalene, and Murakami s ... 

In considering the mechanism of these reactions it is important to stress that they are [2tt - - 2tt] additions, which are formally forbidden as thermally induced [2tts + 2tts] processes according to the well-established Woodward-Hoffman rules for pericyclic reactions. Consequently, it is much more likely that these reactions proceed via a pathway that involves radical intermediates, although concerted processes have been claimed [238]. 

The Woodward-Hoffman rules also predict that in a given cyclization mode a permutation of alkene geometry must be reflected in the configuration of the products. This test is precluded under the normal reaction conditions (acid, light) which would isomerize the dienone double bonds. However, Corey recently reported the formation of a c -disubstituted cyclopentenone from a (Z, )-precursor, derived fiom an allene oxide, which cyclizes via the 2-oxido pentadienylic cation (Section 6.3.8). ... 

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