Pseudopericyclic reactions

Perhaps the most important consequence of the Woodward-Hoffman rules is its predictions of allowed and forbidden reactions. In particular, reactions involving An (An + 2) electrons are allowed if there are an odd (even) number of antarafacial two-electron components. A conseqnence of this symmetry property is that changing the number of electrons will alter whether the reaction is allowed or forbidden. 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

The electrocyclization of 5-oxo-2,4-pentadienal (89) to pyran-2-one (90) is an example of a psendopericyclic reaction. As shown in Fignre 4.21, the disrotatory electrocyclization of 88 occurs with a closed loop of the p-orbitals in the transition state. On the other hand, the electrocyclization of 89 has no such closed loop. Rather, two orbital disconnections interchange the role of bonding and nonbonding orbitals. 

making use of experiments and ab initio and DFT computations, has been the principal champion of pseudopericyclic reactions. He examined the electrocyclization of 89 and could find a transition state only at the HF level at MP2, all attempts to locate a stable structure of 89 failed, collapsing directly to 90. This was later confirmed with a B3LYP/6-31G study that also failed to locate a barrier for this reaction. This lack of a barrier (or more generally, a small barrier) for a pseudopericyclic reaction is not unique. 

Cycloaddition reactions can also be pseudopericyclic. Bimey examined a number of these and a few examples involving the reactions of formylketene (91) are covered here. Formylketene reacts with alcohols to produce p-ketoesters from the enols 92. Bimey examined the model reaction of formylketene with water (Reaction 4.6). The reactants first come together to form a hydrogen-bonded complex (93) before passing though the transition state 94 to give the enol product 95. The activation barrier, defined as the energy for the reaction 93 94, is 6.4 kcal 

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The participation of the lone-pair orbital in the cyclization process allows its classification as a so-called pseudopericyclic reaction (76JA4325 97JA4509), which is a subset of a general type of pericyclic reactions... 

The present section is organized as follows. Firstly, the reactivity and aromaticity of the different rings that compose an acene system as a reactant is analyzed, and secondly, the aromaticity of the TS structures of pericyclic and pseudopericyclic reactions is discussed. 

In a pericyclic reaction, the electron density is spread among the bonds involved in the rearrangement (the reason for aromatic TSs). On the other hand, pseudopericyclic reactions are characterized by electron accumulations and depletions on different atoms. Hence, the electron distributions in the TSs are not uniform for the bonds involved in the rearrangement. Recently some of us [121,122] showed that since the electron localization function (ELF), which measures the excess of kinetic energy density due to the Pauli repulsion, accounts for the electron distribution, we could expect connected (delocalized) pictures of bonds in pericyclic reactions, while pseudopericyclic reactions would give rise to disconnected (localized) pictures. Thus, ELF proves to be a valuable tool to differentiate between both reaction mechanisms. 

A set of electrocyclic ring closures is the subject of recent controversy because their mechanism lies in the borderline between pericyclic and pseudopericyclic reactions [123-127], The mechanisms were clarified by means of ELF analyses [121,122]. As shown in Figure 28.4, connected patterns (C) are... 

An important consequence of the pseudopericyclic mechanism is that the planar (or nearly planar) transition states preclude orbital overlap between the a- and Jt-orbitals. This implies that all pseudopericyclic reactions are allowed. Therefore, Jt-electron count, which dictates whether a pericyclic reaction will be allowed or forbidden, disrotatory, or conrotatory, is inconsequential when it comes to pseudopericyclic reactions. Bimey demonstrated this allowedness for all pseudopericyclic reactions in the study of the electrocyclic reactions 4.11-4.14. 

The transition state of Reaction 4.15 is much more planar than is typical for a [3,3]-rearrangement. Dihedral angles are about 20° in the TS for Reaction 4.15, while the dihedral angles in the TSs of the other two reactions are about 50°. This is consistent with Bimey s contention that pseudopericyclic reactions have nearly planar TSs. Furthermore, the activation barrier for Reaction 4.15 is also quite small,... 

A referee of this communication, later identified as Dan Singleton, proffered an alternative mechanism (Scheme 8.14) for the creation of 102. This is apseudoper-icyclic reaction (see Section 4.5 for a detailed discussion of this type of reaction) that leads from 98 directly to 102. In fact, the TS for this pseudopericyclic reaction is 19.0 kcal mol (MPWBIK/ 6-31-l-G(d,p)) lower in energy than the TS for the retro-Diels-Alder reaction of Scheme 8.11. 

In a pseudopericyclic reaction there is no continuous orbital overlap around the ring of breaking and forming bonds. All pseudopericyclic reactions are allowed there are no anti-aromatic transition states. 

The Dewar thiophene 1 was oxidized to its S-Oxide. 19F NMR of the latter compound showed an automerization through the 1,3-shift of the S-oxide group.21 Later, the Dewar thiophene itself was found to automerize at much higher temperature.22 The lone pair on the sulfur atom was proposed to participate in the 1,3-shift. Lemal named it a pseudopericyclic reaction (Scheme 5). The bond order between the lone pair and the C—C double... 

On the other hand, Lemal and his coworkers prepared Dewar thiophene sulfoxide. A study of the NMR spectrum reveals that the sulfoxide group migrates around the cyclobutene ring 130). They called this migration a pseudopericyclic reaction. An investigation of the temperature-dependent NMR spectrum shows that this migration... 

In their study on the aromatic character of electrocyclic and pseudopericyclic reactions de Lera et examined the thermal cyclization of (2Z)-hexa-2,4-5-trienals and their Schiff bases. They used the NICS at the RCP to detect aromaticity, since a large negative NICS value is associated with aromatic character. 

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. ... 

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