Mixed pericyclic reactions

A simple example is the chlorination of methane (CH4), in which CH4 and elemental chlorine are mixed and irradiated to yield a mixture of chlorohydrocarbons, such as CH3C1 and CCI4. The energy for reaction comes from the UV photons. Diels-Alder and other pericyclic reactions also require photons of light. 

This lack of mixing of DA and D+A for two ethylenes, in contrast to the situation for ethylene and butadiene, where mixing can take place, will be utilized (Section 3) to analyse the relationship of allowed and forbidden pericyclic reactions in configuration terms. 

Please note—these orbitals are just p Ofbitals, and do not make up HOMOs or LUMOs or any particular molecular orbital. Do not attempt to mix frontier orbital and Wo dward--Hbffmann descriptions of pericyclic. reactions. 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericyclic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericyclic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

This chapter includes additional problems related to the material in Chapters 3-6. Some of the mechanisms are mixed for example, there might be a pericyclic reaction followed by a hydrolysis in either acid or base. If a reaction appears to be pericyclic, be sure to determine whether the reaction is symmetry-allowed or symmetry-forbidden under the reaction conditions. If it is symmetry-forbidden, a nonconcerted reaction pathway through radical or charged intermediates will be the most likely mechanism. 

Throughout this book we have often referred to the importance of the HOMO and LUMO in understanding a molecule s properties. Fukui reasoned that these frontier molecular orbitals might play an especially important role in concerted, pericyclic reactions. We have also noted before that it is always favorable to mix filled molecular orbitals wi th empty molecular orbitals. Combining these ideas led to frontier molecular orbital (FMO) theory, an elegant analysis of the types of reactions of importance here. Quite simply, frontier molecular orbital theory states that if we can achieve a favorable mixing between the HOMO of one reactant and the LUMO of another reactant, a reaction is allowed. If we cannot, the reaction is forbidden. 

In summary, all pericyclic reactions can be examined simply by writing the HOMO of one component, the LUMO of the other component (where "component" is defined as "separate interacting orbitals" see below), and determining whether, at the geometry being assumed, the orbitals can produce a mixing that is in-pha.se. Often, as we ll explore below, the HOMO and LUMO to be analyzed are within the same molecule, and maybe even in conjugation. 

As always, stereochemistry has proven to be a crucial indicator of mechanism. Many examples of highly stereospecific SET reactions have been found. An example is the Diels-Alder reaction of the l,2-di(aryloxy)-ethylenes shown below. Mixing the dienophile with cyclopentadiene and the very convenient SET reagent tris(p-bromophenyl)aminiiim (1 ) gives the cycloaddition adducts with high stereospecificity (first two examples below). The observation of several cases like this led many to conclude that the SET reactions really were concerted, pericyclic processes. However, more recent work has found clear exceptions. The deuterated 4-methoxy-styrene shown adds to cyclopentadiene under the same conditions with extensive loss of stereochemistry (third example). These systems are more complicated than conventional pericyclic reactions. 

Since the number of domino processes which start with a Diels-Alder reaction is rather large, we have subdivided this section of the chapter according to the second step, which might be a second Diels-Alder reaction, a 1,3-dipolar cycloaddition, or a sigmatropic rearrangement. However, there are also several examples where the following reaction is not a pericyclic but rather is an aldol reaction these examples will be discussed under the term Mixed Transformations . 

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