Woodward-Hoffmann rules sigmatropic reaction

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). 

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. 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

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. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

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. 

The Woodward—Hoffmann rules predict that [5,5] sigmatropic shifts would proceed suprafacially through a ten-membered transition state. A common example of [5,5] sigmatropic shift is benzidine rearrangement. This reaction is an acid-catalyzed rearrangement of hydrazobenzenes (in general, the NJ f-diarylhydrazines) to 4,4-diaminobiphenyls (benzidines) (Scheme 3.27). 

Computational and experimental studies of pericyclic and pseudopericyclic reactions that show sequential transition structures on the potential energy surfaces have been reviewed. The reformulation of the Woodward-Hoffmann rules for sigmatropic reactions in a conceptual density functional theory (DFT) context has been reported. Considering reaction coordinates and intrinsic reaction coordinates, the allowed mode of the sigmatropic rearrangement corresponds to the largest value of the initial hardness response. ... 

Based on the orbital relations discussed in the text and extensions of these relations to other cases, formulate generalized verbal rules (Woodward-Hoffmann rules) for cycloadditions and sigmatropic shift reactions. 

Treatment for the Rate of Bimolecular, Gas Phase Reactions , if The symmetry rules allowing some reactions and forbidding others were first proposed by Robert B. Woodward and Roald Hoffmann in two letters to the editor Stereochemistry of Electrocyclic Reactions and Selection Rules for Sigmatropic Reactions , Journal of American Chemical Society, 87 (1965) 395, 2511 as well as by Kenichi Fukui and Hiroshi Fujimoto in an article published... 

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