Allowed reaction, Woodward-Hoffmann rules

Allowed reaction, Woodward-Hoffmann rules. 188 Claisen reaction, 304 CVFF force eld, 40 ... 

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... 

What is Symmetry Allowed and Symmetry Forbidden Reactions Woodward Hoffmann Rule o Bonds involved in Cycloaddition Reactions Some more Examples of 2 + 2 Cycloadditions Photochemical Cycloadditions 2 + 3 Cycloadditions 2, 1 Cycloaddition... 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

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. 

The concerted mechanism shown above is allowed by the Woodward-Hoffmann rules. The TS involves the tt electrons of the alkene and enophile and the cr electrons of the allylic C-H bond. The reaction is classified as a [tt2 + tt2 + cr2] and either an FMO or basis set orbital array indicates an allowed concerted process. 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

The spontaneous oxepin-benzene oxide isomerization proceeds in accordance with the Woodward-Hoffmann rules of orbital symmetry control and may thus be classified as an allowed thermal disrotatory electrocyclic reaction. A considerable amount of structural information about both oxepin and benzene oxide has been obtained from theoretical calculations using ab initio SCF and semiempirical (MINDO/3) MO calculations (80JA1255). Thus the oxepin ring was predicted to be either a flattened boat structure (MINDO/3) or a planar ring (SCF), indicative of a very low barrier to interconversion between boat conformations. Both methods of calculation indicated that the benzene oxide tautomer... 

Are intermediates permissible in reactions allowed according to the Woodward-Hoffmann rules ... 

A second point that needs to be clarified is why allowed reactions actually have a barrier at all. In fact, some so-called allowed reactions possess particularly high barriers so that they hardly proceed. One such case, discussed by Houk et al. (1979), concerns the allowed trimerization of acetylene to yield benzene. Despite being extremely exothermic (AH° = — 143 kcal mol ) this reaction appears to have an activation barrier in excess of 36 kcal mol"1. Since the orbitals of the reactants correlate smoothly with those of the products, in accord with the Woodward-Hoffmann rules, the origin of barrier formation in allowed reactions generally, needs to be clarified. 

To summarize, we note two key points. Firstly, that the favourable interaction between DA and D + A in allowed reactions does not preclude barrier formation. The CM model, by its very nature, emphasizes barrier formation through the avoided crossing of reactant and product configurations. Second, the CM model gives rise to the Woodward-Hoffmann rules through consideration of the symmetry properties of the DA and D+A-configurations. This is as it should be. As we have noted previously, a key test of the CM model is whether it blends in naturally with existing theories that focus on specific areas of reactivity. 4... 

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... 

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. 

The Woodward-Hoffmann rule states that the reaction is allowed in the ground state if m + u is odd. The Dewar-Zimmerman rule states that the reaction is forbidden in the ground state if... 

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

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. 

The parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

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. 

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