Woodward-Hoffmann rules sigmatropic rearrangements

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). 

An intramolecular rearrangement of the conjugate acid of the triazene compound to form the oc-complex without an additional molecule of amine would correspond to a thermal [l,3]-sigmatropic rearrangement. However, such a mechanism can be ruled out on the grounds of the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

The following reactions take place with one or more sigmatropic rearrangements. Identify the reactions, and show that they obey the Woodward-Hoffmann rule. 

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 possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

When we come to use the Woodward-Hoffmann rules on these [2,3]-sigmatropic rearrangements, we find something new. We have a K bond and a o bond and a carbanion. How are we to represent a carbanion (or a carbocation) that is just a p orbital on an atom The new symbol we use for a simple p orbital is to. A carbanion is an component and a carbocation is an m0 component as it has zero electrons. If the two new bonds are formed to the same lobe of the p orbital of the carbanion, we have an m2s component but, if they are formed to different lobes, we have an m2a component. 

This rotation is the reason why you must carefully distinguish electrocyclic reactions from all other pericyclic reactions. In cycloadditions and sigmatropic rearrangements there are small rotations as bond angles adjust from 109° to 120° and vice versa, but in electrocyclic reactions, rotations of nearly 90° are required as a planar polyene becomes a ring, or vice versa. These rules follow directly from application of the Woodward-Hoffmann rules—you can check this for yourself. 

Carbamimidoyl isothiocyanates on heating easily undergo irreversible isomerization to afford quinazoline-4(3//)-thiones. The reaction comprises two sigmatropic rearrangements (a Claisen rearrangement and a Cope rearrangement) both are allowed according to the Woodward-Hoffmann rules. 

In a sigmatropic rearrangement, bonds are made and broken at the ends of two conjugated systems. The Woodward-Hoffmann rules for sigmatropic rearrangements must take both components into account. 

In summary, the Woodward-Hoffmann rules for sigmatropic rearrangements (Table 4.5) are as follows. Both components of a sigmatropic rearrangement involving an odd number of electron pairs are suprafacial under thermal condi-... 

Table 4.5. Woodward-Hoffmann Rules for Sigmatropic Rearrangements...

The Stevens rearrangement and the Wittig rearrangement (nonallylic version) (Chapter 4) can be classified as four-electron [1,2] sigmatropic rearrangements. The Woodward-Hoffmann rules state that for a four-electron sigmatropic rearrangement to be allowed, one of the components must be antarafacial, yet it is... 

The rearrangement step is a ground state thermal process and may be classified as a [l,4]-sigmatropic shift of carbon across the face of a 2-oxybutenyl cation. The Woodward-Hoffmann rules require a sigmatropic shift of this type to proceed with inversion of configuration. The orbitals involved in a [1,4]-sigmatropic shift are shown below. 

Table 4.3 The Woodward-Hoffmann rules for sigmatropic rearrangements...

This reaction is an example of a 1,3-shift that is suprafacial for both components and involves two 7i-systems, each with 3 electrons. The MOs of 1,5-hexadiene and the Cope-rearrangement transition state show the reacting orbitals. Table 4.3 gives the Woodward-Hoffmann rules for sigmatropic rearrangements between 7C-systems with I and / electrons. 

Let s start with the hexatriene ring closure from the beginning of this section, first looking at the orbitals and then following the same procedure that we taught you for cycloadditions and sigmatropic rearrangements to see what the Woodward-Hoffmann rules have to say about the reaction. 

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