Pericyclic reactions mechanisms

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

Woodward and Hoffmann provided an understanding of pericyclic reaction mechanisms based on conservation of orbital symmetry. A few years later, Ross et al. [118] coined the term pseudopericyclic for a set of reactions they discovered, which were not explained by the Woodward-Hoffmann rules (like the oxidation of tricyclic... 

Wiest O, Montiel DC, Houk KN. Quantum mechanical methods and the interpretation and prediction of pericyclic reaction mechanisms. J Phys Chem A 1997 101 8378-8388. 

Wiest, O., Montiel, D. C., and Houk, K. N. "Quantum Mechanical Methods and the Interpretation and Prediction of Pericyclic Reaction Mechanisms." J. Phys. Chem. A, 101,8378 (1997). 

Under ordinary conditions, i.e., in the absence of alkaline-earth metal ions, 3.4-threo configurated products, particularly arabino-2-hexulose (fructose) and xylo-2-hexulosefsorboseK prevail. Typically, the product composition is arabino xylo ribo lyxo = 51 38 7 4. This diastereoselectivity is in accordance with a pericyclic reaction mechanism and the intermediacy of the transition state (3). In the presence of high concentrations of Sr or ca, increased proportions of hexuloses with the 3,4-erythro-confiauration. especially the lyxo isomer, are formed at the expense of fructose e.a.. arabino xylo ribo lyxo = 22 34 12 32) indicating a-chelation and attack of the complex (4) by the dihydroxyacetone enolate from the least hindered side. CH OH... 

Theoretical calculations have been particularly successful in the study of pericyclic reaction mechanisms, where they have contributed to the settling of a number of controversies ... 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

So we shall be using discoimections corresponding to ionic and pericyclic reactions, and we shah be looking all the time for a good mechanism to guide us. You should now see what a disconnection means and be ready for the next stage. In the next few chapters we... 

The key to understanding the mechanism of the concerted pericyclic reactions was the recognition by Woodward and Hoffmann that the pathways of such reactions were determined by the symmetry properties of the orbitals that were directly involved. Their recognition that the symmetry of each participating orbital must be conserved during the... 

The mechanism of the Diels-Alder cycloaddition is different from that of other reactions we ve studied because it is neither polar nor radical. Rather, the Diels-Alder reaction is a pericyclic process. Pericyclic reactions, which we ll discuss in more detail in Chapter 30, take place in a single step by a cyclic redistribution of bonding electrons. The two reactants simply join together through a cyclic transition state in which the two new carbon-carbon bonds form at the same time. 

Most organic reactions take place by polar mechanisms, in which a nucleophile donates two electrons to an electrophile in forming a new bond. Other reactions take place by radical mechanisms, in which each of two reactants donates one electron in forming a new bond. Both kinds of reactions occur frequently in the laboratory and in living organisms. Less common, however, is the third major class of organic reaction mechanisms—pericyclic reactions. 

The following thermal rearrangement involves two pericyclic reactions in sequence. Identify them, and propose a mechanism to account for the observed result. 

The following synthesis of dienones occurs readily. Propose a mechanism to account for the results, and identify the kind of pericyclic reaction involved. 

The sex hormone estrone has been synthesized by a route that involves the following step. Identify the pericyclic reactions involved, and propose a mechanism. 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

Antisymmetric matrix, non-adiabatic coupling, vector potential, Yang-Mills field, 94-95 Aromaticity, phase-change rule, chemical reaction, 446-453 pericyclic reactions, 447-450 pi-bond reactions, 452-453 sigma bond reactions, 452 Aromatic transition state (ATS), phase-change rule, permutational mechanism, 451-453... 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

Chorismate mutase catalyzes the Claisen rearrangement of chorismate to prephenate at a rate 106 times greater than that in solution (Fig. 5.5). This enzyme reaction has attracted the attention of computational (bio)chemists, because it is a rare example of an enzyme-catalyzed pericyclic reaction. Several research groups have studied the mechanism of this enzyme by use of QM/MM methods [76-78], It has also been studied with the effective fragment potential (EFP) method [79, 80]. In this method the chemically active part of an enzyme is treated by use of the ab initio QM method and the rest of the system (protein environment) by effective fragment potentials. These potentials account... 

The partitioning of the system in a QM/MM calculation is simpler if it is possible to avoid separating covalently bonded atoms at the border between the QM and the MM regions. An example is the enzyme chorismate mutase [39] for which the QM region could include only the substrate, because the enzyme does not chemically catalyze this pericyclic reaction. In studies of enzyme mechanisms, however, this situation is exceptional, and usually it will be essential, or desirable, to include parts of the protein (for example catalytic residues) in the QM region of a QM/MM calculation, i.e. the boundary between the QM and MM regions will separate covalently bonded atoms (Fig. 6.1). 

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. 

Make C1-C4, C3-C8. Break C1-02, C8-Br. The light suggests a free-radical or pericyclic reaction is operative in at least part of the mechanism. 

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