A pericyclic reactions

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

The Diels-Alder cycloaddition is one exanple of a pericyclic reaction, which is a one-step reaction that proceeds through a cyclic transition state. Bond formation occurs at both ends of the diene system, and the Diels-Alder transition state involves a cyclic ariay of six carbons and six tt electrons. The diene must adopt the 5-cis conformation in the transition state. 

In addition to polar and radical reactions, there is a third, less commonly encountered process called a pericyclic reaction. Rather than explain pericyclic reactions now, though, we ll look at them more carefully in Chapter 30. 

What do molecular orbitals and their nodes have to do with pericyclic reactions The answer is, everything. According to a series of rules formulated in the mid-1960s by JR. B. Woodward and Roald Hoffmann, a pericyclic reaction can take place only if the symmetries of the reactant MOs are the same as the symmetries of the product MOs. In other words, the lobes of reactant MOs must be of the correct algebraic sign for bonding to occur in the transition state leading to product. 

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. 

Antarafacial (Section 30.6) A pericyclic reaction that takes place on opposite faces of the two ends of a tt electron system. 

Sigmatropic reaction (Section 30.8) A pericyclic reaction that involves the migration of a group from one end of a tt electron system to the other. 

Besides the obvious biological interest, chorismate mutase is important for being a rare example of an enzyme that catalyses a pericyclic reaction (the Claisen rearrangement), which also occurs in solution without the enzyme, providing a unique... 

In this section are described those domino reactions which start with a retro-pericy-clic reaction. This may be a retro-Diels-Alder reaction, a retro-l,3-dipolar cycloaddition, or a retro-ene reaction, which is then usually followed by a pericyclic reaction as the second step. However, a combination is also possible with another type of transformation as, for example, an aldol reaction. 

The main reason for the rapid development of metathesis reactions on a laboratory scale (the reaction itself had been known for quite a long time) has been the development of active and robust second-generation ruthenium catalysts (6/3-14 to 6/3-16), which usually provide better yields than the first-generation Grubbs catalysts (6/3-9 or 6/3-13) (Scheme 6/3.2). This also reflects the huge number of domino processes based on ruthenium-catalyzed metathesis, which is usually followed by a second or even a third metathesis reaction. However, examples also exist where, after a metathesis, a second transition metal-catalyzed transformation or a pericyclic reaction takes place. 

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. 

Let us consider a pericyclic reaction in which the electrons of a n system are used in the transition state and new bonds are being formed. The question arises in how many ways the orbitals can react There are only two ways in which the orbitals will react. 

A pericyclic reaction is allowed if orbital symmetry is conserved. In such reactions there is conversion of the ground (electronic) state of reactant into the ground state of the product. Such reactions are said to be thermally allowed, or there is the conversion of the first excited state of the reactant into the first excited state of the product. There are photo-chemically allowed reactions. 

A pericyclic reaction is forbidden if its orbital symmetry is not conserved. In such reactions the occupied orbitals of the reactant do not transform into the occupied orbitals of the product with the same symmetry. 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

In reality, the reaction could not be persuaded to go exactly as shown in Scheme 21.1, because the Cl—C3 bond would certainly break at very nearly the same rate as Cl—C2. In the experiments actually conducted by Baldwin et al., this problem was resolved by deuterium labeling both C2 and C3—creahng diastereomericaUy pure, but achiral molecules. Even then, there remained a large number of technical difficulties, which in the end the researchers were able to overcome. Their results indicated that the four stereochemical courses for the reaction run at 300 °C were sr 23%, si 40%, ar 13%, and ai 24%. These numbers do not ht the expectations from either mechanism. Clearly, the Woodward-Hoffmann forbidden and allowed products are formed in nearly equal amounts ([sr] + [ai] =47% [si] + [ar] = 53%)— hardly what one would expect for a pericyclic reaction. On the other hand, the stereochemical paths do not show the pairwise equalities expected from the stepwise mechanism. 

We have discussed in Section 10.3 the application of perturbation theory to processes in which two molecules come together. We saw there that the most important interactions will be between the HOMO of one molecule and the LUMO of the other. This method can serve as a useful guide in deciding whether there will be a stabilization as a pericyclic reaction begins to occur. The HOMO-LUMO approach was the first one that Woodward and Hoffmann used for ana-... 

A pericyclic reaction is allowed in the electronic ground state if the total number of (4qr + 2)s and (4r)a components is odd. 

The curly arrows in a pericyclic reaction share the capacity that they have in ionic reactions to show which bonds are breaking and where new bonds are forming, but they do not show the direction of electron flow. 

The four-electron system including an alkene Jt-bond and an allylic C-H o-bond can participate in a pericyclic reaction in which the double bond shifts and new C-H and C-C o-bonds are formed. This allylic system reacts similarly to a diene in a Diels-Alder Reaction, while in this case the other partner is called an enophile, analogous to the dienophile in the Diels-Alder. The Alder-Ene Reaction requires higher temperatures because of the higher... 

The Diels-Alder reaction,1 e.g. 1 + 2, is one of the most important reactions in organic synthesis because it makes two C-C bonds in one step and because it is regio- and stereoselective. It is a pericyclic reaction between a conjugated diene 1 and an alkene 2 or 4 (the dienophile) conjugated with, usually, an electron-withdrawing group Z forming a cyclohexene 3 or 5. 

Thus hydroboration is not a pericyclic reaction, because the boron atom makes use of two AOs. Similarly, the reaction between a carbene and a double bond is not pericyclic because the carbon atom uses two AOs. Cheletropic reactions are not pericyclic either. Consider, for example, the fragmentation of 3-cyclopentenone to give CO and butadiene. Not only does the expelled carbon atom use two AOs to bond with its neighbours, but also the oxygen atom, which is an intervening atom (it was initially linked to the carbon atom by a double bond, which becomes a triple bond in CO) is exocyclic in the transition state. 

In a pericyclic reaction, electron counting can be effected in several ways, all equivalent. For example, in the Diels-Alder reaction, one can count the number of conjugated atoms in butadiene and in ethylene, or the number of bonds made (two o and one n bonds) or broken (three n bonds) in the process. In all cases, a total of six intervening electrons are obtained. 

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