Molecular rearrangement electrocyclic

An electrocyclic reaction is a molecular rearrangement that involves the formation of a cT-bond between the termini of a fully conjugated linear rr-electron system and a decrease by one in the number of 7U-bonds, or the reverse of that process. Thus if the open chain partner contains n 7u-electrons, the cyclic partner has (n — 2) 7c-electrons and two electrons in a new a-bond. For example, let us consider electrocyclization of butadiene and hexatriene systems as shown in Scheme 2.1. 

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. 

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. 

Enantiopure epoxides (3/ ,4Y)-dibenz[ 7, ]anthracene 3,4-oxide and (3iJ,4Y)-phenanthrene 3,4-oxide were synthesized via involved routes and were observed to spontaneously racemize. This racemization of arene oxides is in accordance with perturbation molecular orbital predictions based on resonance energy considerations, and presumably occurs via an electrocyclic rearrangement to the corresponding (undetected) oxepine tautomer (Scheme 17) <2001J(P1)1091>. 

The stereospecificity of the cyclobutene isomerization was rationalized in the first of the Woodward-Hoffmann papers in 1965 which outlined the principle of Conservation of Orbital Symmetry in Concerted Reactions. This isomerization was defined as a conrotatory electrocyclic process. Note should be made of the possibility that the conrotatory pathway could also result in a cw,cw-2,4-hexadiene from a ran -3,4-disubstitued cyclobutene, but unfavorable steric interactions apparently intervene when a sterically large group rotates inward on the molecular system. Thus ran.y-l,2,3,4-tetramethylcyclobutene undergoes rearrangement with log k = 13.85 — 33 600/2.3R7 while the cis isomer reacts with log A = 14.1 — 31000/2.3RT so at 177°C the trans material reacts 49 times faster than the cis isomer. 

Mass Spectrometry. Green has given an exhaustive review of the consequences of molecular stereochemistry on organic mass spectrometry. In this, the stereoisomeric dependence of the electron-impact-induced rearrangements of alcohols and carbonyl compounds, the apparent electrocyclic fragmentations, and bond cleavage reactions are all covered in detail. 

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