Symmetry and Reactivity

Our understanding of organic chemistry has improved dramatically over the past few decades with the development of molecular orbital theory. In particular, the contributions of Woodward and Hoffmann have emphasised the importance of orbital symmetry in dictating the course of organic reactions. More recently, the symmetry properties of the orbitals close to the potential energy surface, the so-called frontier orbitals, have been shown to be of paramount importance. The basics of frontier orbital theory are 

Let us consider a real case - the hydrolysis of an imine, R2C=NR The Tt-molecular orbitals of an imine are shown in Fig. 2-27. 

The 7t-orbital is the HOMO and the 71 the LUMO. Notice that the coefficients of the orbitals are unequal, since nitrogen is more electronegative than carbon, and that the magnitude of the coefficients alternates from HOMO to LUMO. We may now imagine a water molecule approaching the imine. On the basis of orbital symmetry rules, the important interactions could be the LUMO of the water with the HOMO of the imine, or the HOMO of the water with the LUMO of the imine. This selectivity is on the basis of better matching of orbital energies. It is commonly found that the important interaction is that of the HOMO of the nucleophile with the LUMO of the electrophile (Fig. 2-28). The 

HOMO of the water molecule will approximate to one of the sp3 lone-pairs on the oxygen atom. This will interact with the LUMO of the imine in such a way that the overlap of orbitals is maximised, i.e., at the carbon atom, which has the larger coefficient, rather than at the nitrogen atom. 

If the metal possesses two electrons in the appropriate d orbital, the molecular orbitals XF1 and 4 are filled. The important orbital is which is derived (in part) from the old 7t -level of the ligand. Placing electron density within this orbital results in a build-up of electron density in the tt-symmetry orbitals on the carbon atom of the imine. This will result in a repulsion being experienced by any incoming nucleophile, and a deactivation of the imine towards nucleophilic attack. 

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The interrelation of initial molecular symmetry and reactivity with final structure is an important issue in fossil fuels, as in the pyrolysis of coal or petroleum to make coke. In the general area of pyrolysis of aromatic hydrocarbons, several reviews have been published (22-24), and in this book, Lewis and Singer discuss recent advances in this field (25). 

Although the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the n orbitals of both linear and cyclic conjugated polyene systems that it provides are correct as to symmetry and the relative energy of the orbitals. In many reactions where the n system is the primary site of reactivity, these orbitals correctly describe the behavior of the systems. For that reason, the reader should develop a familiarity with the qualitative description of the n orbitals of typical linear polyenes and conjugated cyclic hydrocarbons. These orbitals will be the basis for further discussion in Chapters 9 and 11. 

The examples that have been presented in this section illustrate the approach that is used to describe structure and reactivity effects within the framework of MO description of structure. In the chapters that follow, both valence bond theory and MO theory will be used in the discussion of structure and reactivity. Qualitative valence bond terminology is normally most straightforward for saturated systems. MO theory provides useful insights into conjugated systems and into effects that depend upon the symmetry of the molecules under discussion. 

The aim of the series is to present the latest fundamental material for research chemists, lecturers and students across the breadth of the subject, reaching into the various applications of theoretical techniques and modelling. The series concentrates on teaching the fundamentals of chemical structure, symmetry, bonding, reactivity, reaction mechanism, solid-state chemistry and applications in molecular modelling. It will emphasize the transfer of theoretical ideas and results to practical situations so as to demonstrate the role of theory in the solution of chemical problems in the laboratory and in industry. 

This mesomerism (or resonance )59 between equivalent Kekule structures was recognized as the quintessential feature underlying the aromaticity of benzene, conferring highly distinctive symmetry, stability, and reactivity patterns. 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achievable. If a diphosphine ligand with C2 symmetry is used, two diastereomers for the enamide-coordinated complex can be formed because the olefin can interact with the metal from either the Re- or Sf-face. Therefore, enantioselectivity is determined by the relative concentrations and reactivities of the diastereomeric substrate-Rh complexes. It should be mentioned that in most cases it is not the preferred mode of initial binding of the prochiral olefinic substrate to the catalyst that dictates the final stereoselectivity of these catalyst systems. The determining factor is the differ-... 

A ruthenium(VI) nitrido complex containing a tetrapyrrolic macrocycle ligand, [Ru (N)(L)] (82) (H4L = mcio-octamethylporphyrinogen), has been synthesized by the reaction of diphewldiazomethane with [Ru"(L)] , which is prepared from [Ru(cod)(Cl)2] and Na4(L) 4THF. X-ray crystal studies reveal that [Ru (N)(L)] has a Cjv symmetry, and it has the usual saddle conformation, with the metal atom displaced 0.482 A out of the N4 mean plane. The reactivities and redox chemistry of this complex are summarized in Scheme 12 (see also Section 5.6.5.2). 

The Hartree-Fock or self-consistent-field approximation is a simplification useful in the treatment of systems containing more than one electron. It is motivated partly by the fact that the results of Hartree-Fock calculations are the most precise that still allow the notion of an orbital, or a state of a single electron. The results of a Hartree-Fock calculation are interpretable in terms of individual probability distributions for each electron, distinguished by characteristic sizes, shapes and symmetry properties. This pictorial analysis of atomic and molecular wave functions makes possible the understanding and prediction of structures, spectra and reactivities. 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

Low Miller index surfaces of metallic single crystals are the most commonly used substrates in LEED investigations. The reasons for their widespread use are that they have the lowest surface free energy and therefore are the most stable, have the highest rotational symmetry and are the most densely packed. Also, in the case of transition metals and semiconductors they are chemically less reactive than the higher Miller index crystal faces. 

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