Diels-Alder reactions Woodward-Hoffmann rules

Frontier orbital analysis is a powerful theory that aids our understanding of a great number of organic reactions Its early development is attributed to Professor Kenichi Fukui of Kyoto University Japan The application of frontier orbital methods to Diels-Alder reactions represents one part of what organic chemists refer to as the Woodward-Hoffmann rules a beautifully simple analysis of organic reactions by Professor R B Woodward of Harvard University and Professor Roald Hoffmann of Cornell University Professors Fukui and Hoffmann were corecipients of the 1981 Nobel Prize m chemistry for their work... 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

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. 

Theoretical considerations in the same fashion enable predication of the possible configuration of the transition state. Eq. (3.25 b) for the multicentre interaction is utilized. Hoffmann and Woodward 136> used such methods to explain the endo-exo selectivity of the Diels-Alder reaction (Fig. 7.28). The maximum overlapping criteria of the Alder rule is in this case valid. The prevalence of the endo-addition is experimentally known 137>. 

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. 

Problem 9.33 When applying the Woodward-Hoffmann rules to the Diels-Alder reaction, (a) would the same conclusion be drawn if the LUMO of the dienophile interacts with the HOMO of the diene (b) Would the reaction be light-catalyzed ... 

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. 

The suprafacial thermal addition of an allylic cation to a diene (a 3 + 4 cycloaddition) is allowed by the Woodward-Hoffmann rules (this reaction would be expected to follow the same rules as the Diels-Alder reaction1095). Such cycloadditions can be carried out1096 by treatment of a diene with an allylic halide in the presence of a suitable silver salt, e.g,1097... 

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 Diels-Alder reaction is a concerted reaction in which four re-electrons from the diene and two re-electrons from the dienophile participate in the transition state. The Woodward-Hoffmann Rules provide a theoretical framework for these reactions.24 They suggest that those reactions are thermally allowed which have 4n + 2 pericyclic electrons, i.e. 6, 10, 14, etc. The Diels-Alder reaction is an example where n = 1, i.e. (4 + 2) re-electrons. 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

No scientific discovery seems to be totally new, as has been discussed superbly well by Berson (1992) with reference to the discoveries of the Diels-Alder reaction and the Woodward-Hoffmann rule. In the case of C60, the near misses by Iijima (1987) and the unpublished work by Chapman (Diederich 1992), are more pre-eminent examples of the precedence than those described above. 

The Diels-Alder reaction and related pericyclic reactions, which can be treated qualitatively by the Woodward-Hoffmann rules (Section 4.3.5), have been reviewed in the context of computational chemistry [39]. The reaction is clearly nonionic, and the main controversy was whether it proceeds in a concerted fashion as indicated in Fig. 9.5 or through a diradical, in which one bond has formed and two unpaired electrons have yet to form the other bond. A subtler question was whether the reaction, if concerted, was synchronous or asynchronous whether both new bonds were formed to the same extent as reaction proceeded, or whether the formation of one ran ahead of the formation of the other. Using the CASSCF method (Section 5.4.3), Li and Houk [40] concluded that the butadiene-ethene reaction is concerted and synchronous, and chided Dewar and Jie [41] for stubbornly adhering to the diradical (biradical) mechanism. 

Cycloadditions are easier to treat than unimolecular reactions they only require an evaluation of the best FO overlap (rule 4). Let us look at the cyclodimerization of butadiene. Woodward and Hoffmann suggested that the experimentally observed endo compound is due to secondary interactions (shown by the double arrows above), which increase the stabilizing the FO s interaction.23 Cisoid configurations are often adopted by the dienophile in Diels-Alder reactions,24 as first suggested by Dewar.20 For... 

Scheme 6.3 applies these rules by showing the HL structures for two cycloaddition reactions 11R and 11P are the structures for the reactants and products of the Woodward-Hoffmann forbidden 2 + 2 reaction, while 12R and 12P are the structures for the Woodward-Hoffmann allowed Diels—Alder reaction. In both cases, the difference in the HL structure is only the mode of spin coupling, and therefore the promotion energy G will involve only singlet-triplet excitations. In accord, we drew in Fig. 6.5 the corresponding... 

We will not develop all of the Woodward-Hoffmann rules, but we will show how the molecular orbitals can indicate whether a cycloaddition will take place. The simple Diels-Alder reaction of butadiene with ethylene serves as our first example. The molecular orbitals of butadiene and ethylene are represented in Figure 15-18. Butadiene, with four atomic p orbitals, has four molecular orbitals two bonding MOs (filled) and two antibonding MOs (vacant). Ethylene, with two atomic p orbitals, has two MOs a bonding MO (filled) and an antibonding MO (vacant). 

There are a number of concerted reactions involving cyclic transition states which are characterized by the maintenance throughout of an overlap between orbitals of the correct symmetry. These reactions are known as pericyclic reactions and the rules that govern them are known as the Woodward-Hoffmann rules. A typical example of a reaction of this type is the Diels-Alder reaction of a diene and a dienophile. 

Another question posed in Chapter 1 was Why does the Diels-Alder reaction give endo adducts Whereas the Woodward-Hoffmann rules have been explained in several (related154) ways, the frontier orbital method is virtually the only one to have been used to account for secondary effects like this.155... 

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... 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

Later, the structural chemists determined the structure of 1,3-butadiene (the bond lengths Cl—C2 and C2—C3 are 1.467 and 1.349 A, respectively, which can be compared with the corresponding values of fraw -2-butene (1.508 and 1.347 A), respectively). Another significant aspect of dienes is the Diels-Alder reaction, the reaction between a diene and an olefin with electron-withdrawing substituents to give a six-membered ring. The reaction is designated as 4 - - 2 cycloaddition since the diene has fonr carbon atoms while the olefin, a dienophile, may represent a two-carbon nnit. The mechanism of this useful reaction was not clear until 1964, when Woodward and Hoffmann proposed the so-called Woodward-Hoffmann rule. This proposal has opened a wide world of electrocyclic reactions in which the symmetry of orbitals plays an important role. 

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