Pericyclic reactions polyenes

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The polyene character of 1 /7-azcpines makes them susceptible not only to a variety of electro-cyclic reactions, but also to cycloaddition with a variety of dienophiles, and to dimerization by [6 + 4] 7i-pericyclic reactions. 

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. 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=24) stereo-isomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two... 

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. 

Of course, this was only a theory—until in 1982 K.C. Nicolaou s group synthesized the proposed endiandric acid precursor polyene—and in one step made both endiandric acids D and E, plus endiandric acid A, which arises from a further pericyclic reaction, an intramolecular Diels-Alder cycloaddition of the acyclic diene on to the cyclohexadiene as dienophile. 

Endiandric acid A has four rings and eight stereogenic centres and yet is formed as a single diastereoisomer in one step from an acyclic polyene And it s all controlled by pericyclic reactions. 

The conjugated polyene systems taking part in pericyclic reactions are linear, so all the tt molecular orbitals of the starting materials have a different energy. The distribution of energy levels is important because it affects the electronic configuration of the starting materials (see Example 6.19) and consequently the course of the reaction. 

A knowledge of the structure and properties of dienes and polyenes is necessary to understand the mechanisms of these processes. Quantum chemical calculations can be very helpful to achieve this goal. Several reviews have discussed the theoretical contributions to different aspects of dienes and polyenes Orlandi and coworkers have reviewed the studies devoted to the ground state structure and spectra of linear polyenes. The molecular electrical properties of several organic molecules, including polyenes, have been considered by Andre and Delhalle. Finally, the mechanism of pericyclic reactions has been discussed by Houk and coworkers and Dewar and Jie. ... 

The dienes and polyenes are compounds which intervene in a large number of organic reactions, as will be seen in different chapters of this book. Several excellent reviews have been devoted to theoretical studies about their reactivity, with special emphasis on the mechanism of pericyclic reactions . As was mentioned in the introduction, this section will only treat, as an example, the Diels-Alder reaction, since it has been the most studied one by theoreticians. Our goal is not to cover all aspects, but instead to show the high potential and usefulness of theoretical methods in order to interpret and rationalize the experimental results. In the rest of the chapter we will concentrate on the last ab initio calculations. 

The Woodward-Hoffmann rules can be rationalized by examining the properties of the frontier MOs of the reactants, i.e., the HOMOs (highest occupied molecular orbitals) and LUMOs (lowest unoccupied molecular orbitals) of the reactants. In order to understand pericyclic reactions, then, you need to be able to construct the MOs of a polyene system from the constituent p orbitals. 

Note that the signs of the terminal orbitals alternate from like, to opposite, to like, to opposite. A physical chemist would say that the symmetries of the MOs alternate between symmetric and antisymmetric. This property is universal among the MOs of polyenes. You do not need to construct the complete MOs of any polyene to determine the signs of the termini of the HOMO and LUMO, on which the properties of pericyclic reactions largely depend. 

Electrocyclic A type of pericyclic reaction in which there is a concerted cyclisation of a polyene, or the reverse, which results in ring opening. 

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