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Über dieses Buch

Criteria of orbital symmetry conservation had a profound influence on mechanistic thinking in organic chemistry and are still commonly applied today. The author presents a coherent set of operational rules for the analysis of scope and reliability. It is written from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS). Its advantage lies in its provision of a coherent overview of the relation between symmetry and mechanism. For reasons of consistency, the book remains within the framework of molecular orbital theory.

Inhaltsverzeichnis

Frontmatter

Preliminary Survey

Frontmatter

Chapter 1. The Woodward-Hoffmann Rules in Perspective

Abstract
In order to appreciate properly the impact of Woodward and Hoffmann’s The Conservation of Orbital Symmetry [1] on contemporary chemical thinking, let us look back to the early nineteen-sixties and recall the state of the art of mechanism elucidation at that time.
E. Amitai Halevi

Symmetry and Energy

Frontmatter

Chapter 2. Atoms and Atomic Orbitals

Abstract
Prefatory note: This chapter and the others in Part II have been written in as non-mathematical a style as the author could manage. The theoretical validity of the various statements made without formal proof can be checked in any of the many available texts on Quantum Chemistry [1, 2, 3]. The initiate, who may be tempted to skip this and the following chapter, is urged to skim through them anyway. To paraphrase Shakespeare’s Ulysses [8], “the author’s drift” may be nothing to “strain at” but his “position” may not be altogether “familiar”.
E. Amitai Halevi

Chapter 3. Diatomic Molecules and Their Molecular Orbitals

Abstract
Just as the electronic configuration of an atom is built up by stepwise population — electron by electron — of hydrogen-like atomic orbitals, that of a diatomic molecule is constructed by successively filling the molecular orbitals derived from the hydrogen molecule ion, H+ 2[1].
E. Amitai Halevi

Chapter 4. Formation and Deformation of Polyatomic Molecules

Abstract
Diatomic molecules are necessarily linear, but a triatomic molecule can be either linear like CO2 and HCN or bent like S02 and H20. Mulliken’s correlation diagram procedure was extended to tri- and tetraatomic molecules by Walsh [1], who promulgated a set of simple but remarkably viable [2, 3, 4] rules for predicting whether or not a molecule will remain linear, from the effect of the departure from linearity on the energy of its occupied molecular orbitals.
E. Amitai Halevi

The Classical Thermal Reactions

Frontmatter

Chapter 5. Electrocyclic Reactions and Related Rearrangements

Abstract
In their now classic monograph [1], Wooodward and Hoffmann concentrate on three basic types of “no mechanism” reaction: Electrocyclic reactions — notably polyene cyclizations, cycloadditions, and sigmatropic rearrangements. These three reaction types will be taken up in this and the next two chapters from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) [2, 3, 4], the formalism of which follows naturally from that developed in Chapter 4. The similarities to the original WH-LHA approach [5, 6], and the points at which OCAMS departs from it, will be illustrated. In addition, a few related concepts, such as “ allowedness” and “forbiddenness”, global vs. local symmetry, and “concertedness” and “synchronicity”, will be taken up where appropriate.
E. Amitai Halevi

Chapter 6. Cycloadditions and Cycloreversions I. [2+2]-Cycloaddition

Abstract
It was pointed out in the preceding chapter that correlation diagrams can be read from right to left as easily as from left to right. This is certainly true from the formal viewpoint of orbital symmetry, but does not preclude the need to examine the geometric and energetic consequences of the nuclear motions involved. Thus, for example, the necessity for including a conrotatory (a 2)displacement in the reaction coordinate for the cyclization of s-cis- butadiene (Fig. 5.1) implies that a coordinate with the same irrep has to be incorporated into the reaction coordinate for ring opening of cyclobutene. However the energetic requirements of the nuclear motions involved differ greatly. Internal rotation about the central bond of butadiene — with concomitant conrotation of its terminal methylene groups — is quite facile, whereas the reverse of the same motion in cyclobutene is opposed by a substantial restoring force.
E. Amitai Halevi

Chapter 7. Cycloadditions and Cycloreversions II. Beyond [2+2]

Abstract
There are two ways of going beyond [2+2]-cycloaddition: The straightforward way is to extend the length of one or both of the conjugated π systems of the reactants; the number of electrons involved in [n+m]-cycloaddition is then given by: k = (n + m). It was shown in Chapter 5, however, in connection with the benzvalene-benzene and cubane-cyclooctatetraene interconversions, that just how many electrons are “involved” in a given reaction is a matter of interpretation. For present purposes, a reaction in which the presence of more than four electrons — bonding or non-bonding — cannot be safely ignored in the symmetry analysis will be considered under the heading “beyond [2+2]”, even if it results in closure of a four membered ring.
E. Amitai Halevi

Chapter 8. Degenerate Rearrangements

Abstract
In their discussion of sigmatropic rearrangements, Woodward and Hoffmann state: “For the analysis of these reactions correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements.” [1, p. 114] This is something of an overstatement. For example, they could hardly have meant it to apply to 1,5-hexadiene, which is no less symmetrical than any transition state that can be assumed for its Cope rearrangement.
E. Amitai Halevi

Spin and Photochemistry

Frontmatter

Chapter 9. Electron Spin

Abstract
Up to this point we have managed to sidestep the issue of spin [1,2] entirely, because the reactions treated so far have all involved closed shell singlet reactants and products. Even in those few cases where the transition state is essentially open shell, as in the fluxional isomerization of cyclobutadiene (Section 8.4), its singlet state lies sufficiently far below the corresponding triplet [3, p. 363] that electron spin can be ignored. This is no longer the case in photochemical reactions, several of which will be dealt with in the following chapter, or in the less common — but by no means rare — thermal reactions in which the spin state of the product differs from that of the reactant.
E. Amitai Halevi

Chapter 10. Excited State Reactions

Abstract
The analysis of the fragmentation of tetramethyl-1,2-dioxetane at the end of the preceding chapter, in which three competing processes had to be considered, foreshadows the difficulty of applying the criteria of symmetry conservation to the much more complex reactions that originate in an excited state of the reactant. An attempt will nevertheless be made in the following sections to show how the approach developed in the preceding chapters can be extended to deal with them. Each of the photochemical reactions chosen for discussion was selected in order to illustrate as convincingly as possible a particular point that has to be kept in mind when applying symmetry criteria to excited state reactions. In no case is it claimed that the mechanistic analysis is conclusive.
E. Amitai Halevi

Chapter 11. Into Inorganic Chemistry

Abstract
Even minimally adequate coverage of the application of orbital symmetry criteria to inorganic reactions would increase the scope of this book inordinately and — in any case — is outside the author’s competence. The modest attempt to address them in the final pages of this book does not merit Part status, so the chapter is included as a matter of necessity in Part IV: Spin and Photochemistry. A somewhat labored justification might run as follows: The principal new element that distinguishes inorganic from organic reactions is the ever-present possibility that d orbitals have to be taken into account. As we will see, the need to consider them when dealing with reactions of the main-group elements arises in connection with their photochemistry. They achieve crucial importance in the reactions of transition metal complexes, where they determine one of the essential properties of the reacting molecule or ion: its spin state.
E. Amitai Halevi

Backmatter

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