Analysis Allowed or Forbidden

Examine the reactants and products identifying the active components of each. The active components are those a and/or n bonds which are broken in the reactants and formed in the products. 

Draw the constituent orbitals for each component. These are p orbitals at each end of the n system and (usually) sp3 hybrid orbitals at each end of components (if the bond terminates at H, the orbital there is an s orbital). Disregard orbital 

Identify which orbitals of the reactants must overlap to form the newly formed bonds of the products and connect these with a curved line. When orbitals overlap in a n fashion, one may arbitrarily choose the pair of lobes (top or bottom) when making the connection. 

4n + 2 with n — 0. A component may have no electrons, in which case it is a 4w component = 

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Use frontier orbital analysis to decide whether the dimeriza-tion of 1,3-butadiene shown here is symmetry allowed or forbidden. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

As such, we will not consider photochemical processes in this chapter, deferring such topics to Chapter 16, which is devoted entirely to photochemistry. When we make tables to present rules for various types of reactions, describing them as allowed or forbidden, we will only be addressing thermal conversions. The photochemical part of such tables has always been redundant you just reverse the thermal predictions. However, on a more basic level we feel that predictions about photochemical reactions based on the level of analysis presented in this chapter are risky and fail to take into account the many subtleties of photochemistry. If you want to consider a photochemical pericyclic reaction, it is best to consider it in the context of the entire field of photochemistry, rather than as the opposite of a thermal process. 

In each case the ring closure involves the rotation of the terminal carbons so that the p orbitals of the tt system, which are necessarily parallel to each other in the polyene, point toward each other and make a new bond. It is the direction of rotation (clockwise or counterclockwise) that changes depending upon electron count and orbital interactions. This direction of rotation influences the stereochemical outcome of the reactions, and therefore our analysis really focuses on rationalizing and predicting stereochemistry. Before examining which rotations are allowed or forbidden, we must introduce some new terminology. 

PROBLEM 20.8 What would happen if the light were used to promote an electron from the HOMO of the alkene (jt) to the LUMO of the aUcene (tt ) Would the Diels-Alder reaction be allowed or forbidden Show your analysis. 

A firm understanding of concerted cycloaddition reactions developed as a result of the formulation of the mechanism within the framework of molecular orbital theory. Consideration of the molecular orbitals of reactants and products revealed that in some cases a smooth transformation of the orbitals of the reactants to those of products is possible. In other cases, reactions that appear feasible if no consideration is given to the symmetry and spatial orientation of the orbitals are found to require high-energy transition states when the orbitals are considered in detail. (Review Section 11.3 of Part A for a discussion of the orbital symmetry analysis of cycloaddition reactions.) These considerations permit description of various types of cycloaddition reactions as allowed or forbidden and permit conclusions as to whether specific reactions are likely to be energetically feasible. In this chapter, the synthetic applications of cycloaddition reactions will be emphasized. The same orbital symmetry relationships that are informative as to the feasibility of a reaction are often predictive of the regiochemistry and stereochemistry of the process. This predictability is an important feature for synthetic purposes. Another attractive feature of cycloaddition reactions is the fact that two new bonds are formed in a single reaction. This can enhance the efficiency of a synthetic process. 

Analysis of the fragmentation reaction of PR5 from the point of view of the conservation of orbital symmetry led Hoffmann et al. to suggest a simple answer. The least-motion mode of concerted departure from the stable Dsh structure, coupling between an apical group and an equatorial group, is symmetry-forbidden, or WH (Woodward-Hoffmann) forbidden. On the other hand, both equatorial-equatorial and apical-apical fragmentations are symmetry-allowed, or WH (Woodward-Hoffmann) allowed (Scheme 2.3). 

As a prelude to further analysis, it is useful to review one important property of molecular orbitals. As noted in Chapter 1, symmetry-correct molecular orbitals must be either symmetric or antisymmetric with respect to the full symmetry of the basis set of atomic orbitals that are used to construct the molecular orbitals. In the analysis of orbital symmetries, we will need to consider only the number of molecular S5nnmetry elements that are sufficient to distinguish between allowed and forbidden pathways. Also, it is not necessary to consider here the minor perturbation of molecular orbital symmetry that results from isotopic or alkyl substitution. In other words, to a first approximation the basis set orbitals of any conjugated diene are considered to be the same as those for 1,3-butadiene. Figtue 11.13 shows the... 

Still another useful viewpoint of concerted reactions is based on the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground-state molecules. A stabilized or aromatic transition state will result in a low activation energy, i.e., an allowed reaction. An antiaromatic transition state will mean that a high energy barrier exists, and the reaction will be unfavorable or forbidden. With this idea as a basis, it is possible to analyze potential transition states for concerted reactions and draw conclusions about their stability. This analysis directly parallels that used in deciding on the aromaticity or antiaromaticity of ground-state molecules. 

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

We have seen from Coulson s equations (p. 18) that for conjugated polyenes, the odd-numbered MOs are symmetrical and the even-numbered MOs antisymmetrical. Hence the symmetry properties of the frontier orbitals of the k fragment alternate as the polyene gains or loses a double bond. This allows us to generalize the analysis above into the selection rule thermal disrotatory ring openings are forbidden for 4n electron and allowed for 4n + 2 electron systems. The opposite is true for conrotatory opening. 

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