Molecular reactions Electrocyclic

Concerted electrocyclic reactions are faster than molecular reactions which involve separate bond-breaking or -making steps. 

Solution (a) This is a photochemical electrocyclic reaction. Electrocyclic reactions are concerted, therefore they are completely stereospecific. The exact stereochemistry of the product depends upon the number of double bonds in the polyene molecular orbital theory allows us to predict this stereochemistry. Let us look at the electron configuration of butadiene, a four tt electron system, in the ground state and in the first excited state (achieved by the absorption of radiation) ... 

Conjugated polyene, electrocyclic reactions of, 1181-1186 molecular orbitals of, 1179-1180 Conjugated tricne, electrocyclic reactions of, 1182 Conjugation, 482... 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

The basis of electrocyclic reactions can be considered in terms of the molecular orbitals involved. By considering the phasing of the molecular orbitals it is possible to say whether a reaction can proceed (only orbitals of the same phase can overlap and bond) and to predict the stereochemistry of the reaction. This approach is called the frontier orbital model. 

Thus, the apparent paradox lies in the fact that radical and radical-ion electrocyclic reactions are all forbidden in the Woodward-Hoffinann sense because the symmetry of the singly occupied molecular orbital (SOMO) changes... 

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

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... 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

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. 

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