Woodward-Hoffman

For Woodward-Hoffman allowed thermal reactions (such as the conrotatory ring opening of cyclobutane), orbital symmetry is conserved and there is no change in orbital occupancy. Even though bonds are made and broken, you can use the RHF wave function. 

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 photocycloaddition of two identical olefins is allowed to be a concerted [27TS+2 s] reaction according to the Woodward-Hoffman 57> rules if the factor of excited state spin multiplicity is ignored. The construction of a simple interaction diagram, Fig. 7, with arbitrary... 

There is an important class of rearrangements of strained cyclic a-bonded systems to give less strained ir-bonded qrstems which occur under the influence of transition metal catalysts although the uncatalysed proce is Woodward-Hoffman forbidden and slow. Examples are the conversion of cubanes XXII and bis-homocubanes XXIll to syn-tricyclooctadienes XXIV and related species XXV and of quadricyclene (XXVI) to norbomadiene (XXVII) [Ag, however, converted cubane and related species to the previously unrecognised species cuneane (XXVIII) and its relatives as do some electrophiles with incompletely filled d-subshells ... 

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. 

A further possibility to interpret cis addition is the so-called cA-concerted mechanism74,145. It assumes that the addition of the two hydrogen atoms takes place in a single step in a concerted fashion on a single 3M site possessing three coordinative unsaturations. The transfer of the two hydrogens to the double bond through a concerted process, where the interaction with the catalyst removes the symmetry restrictions imposed by the Woodward-Hoffman rales, leads directly to alkane formation. 

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