Sigmatropic rearrangements orbital symmetry

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

The following rearrangement was devised and carried out to prove the stereochemistry of [1,5] sigmatropic hydrogen shifts. Explain how the observed result confirms the predictions of orbital symmetry. 

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). 

In any given sigmatropic rearrangement, only one of the two pathways is allowed by the orbital-symmetry rules the other is forbidden. To analyze this situation we first use a modified frontier-orbital approach. We will imagine that in the transition state the migrating H atom breaks away from the rest of the system, which we may treat as if it were a free radical. 

The orbital symmetry rules also help us to explain, as on pages 1083 and 1433, the unexpected stability of certain compounds. Thus, 102 could, by a thermal [1,3] sigmatropic rearrangement, easily convert to toluene, which of course is far more stable because it has an aromatic sextet. Yet 102 has been prepared and is stable at dry ice temperature and in dilute solutions. ... 

Besides the selection rules, which are based on the conservation of orbital symmetry, for sigmatropic rearrangements of order [ij] it is possible to demonstrate that the following correlations will always apply ... 

The mechanistic basis of sigmatropic rearrangements was introduced in Chapter 11 of Part A. The sigmatropic process that is most widely applied in synthesis is the [3,3] sigmatropic rearrangement. The principles of orbital symmetry establish that concerted... 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

Sandmeyer reaction, 306 Sandwich compoimds, 275 Sawhorse projections, 7 Saytzev elimination, 249, 256 Schiff bases, 221 Schmidt rearrangement, 122 Selectivity, 156, 169, 326 a, 362 a, 370 372 aj,385 a bonds, 6 o complexes, 41,131 Sigmatropic rearrangements, 352-357 antarafacial, 353 carbon shifts, 354 hydrogen shifts, 352 orbital symmetry in, 352 photochemical, 354 suprafadal, 353 thermal, 353... 

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. 

Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

The approach of the two orbitals on the same side of the surface is called supra , that on opposite sides is called antara . The above rules, 1 and 2, also apply in the case of sigmatropic rearrangements, since the changes of orbital symmetries are the same on going from a HOMO to a LUMO or from a chain of 4n to a chain of 4n + 2 tt electrons. 

Woodward and Hoffmann give an enlightening discussion of sequential sigmatropic rearrangements in The Conservation of Orbital Symmetry, p. 132. 

Orbital symmetry tells us that [3,3]-sigmatropic rearrangements are allowed but says nothing about which way they will go. They are allowed in either direction. So why does the Claisen-Cope rearrangement always go in this direction ... 

Berson, J. A. Nelson, G. L. Steric prohibition of the inversion pathway. Test of the orbital symmetry prediction of the sense of rotation in thermal suprafacial 1,3-sigmatropic rearrangements, 7. Am. Chem. Soc. 1970, 92,1096-1097. 

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