Other Concerted Reactions

Molecular orbital correlation diagram for ( 4s -I- cycloaddition of ozone and ethene. (Adapted from reference 122.) 

Retro-[ 4, + and [ 4,+ 2J reactions in the conversion of the initial ozonide to final ozonide. 

Nonlinear (left) and linear (right) cheletropic reaction. 

Allowed Thermal Reactions Allowed Photochemical Reactions  

4n Disrotatory 4n + 2 Conrotatory Conrotatory Disrotatory Conrotatory Disrotatory Disrotatory Conrotatory 

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The lower part of the figure demonstrates that Fukui frontier molecular orbital theory also provides a good rationalization of the Diels-Alder (and other) concerted reactions. In 1981, Roald Hofffnann and Kenichi Fukui shared the Nobel Prize in chemistry (Woodward had died in 1979). 

We have developed a different set of selection rules for electrocyclic, sigma-tropic, cycloaddition, and other concerted reactions, but the fundamental principle—the conservation of orbital symmetry—is the same in all cases. Now we will see that all of these reactions can be considered to be variants of cycloaddition reactions. To do so, we must first note that a cr bond can participate in a cycloaddition process, just as can a n bond, with the following provisions ... 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

The concept, quasi-intennediate [70], was introduced in 1975 to symbolize a boundary between concerted and stepwise mechanisms. Recent advances in computer chemistry are allowing us to investigate subtle problems more clearly in the years since 2000. Concerted/stepwise boundary mechanisms were proposed for other diverse reactions than those of singlet molecular oxygen ( A. ... 

In other ester reactions, there may be concern that the reaction might be concerted, bypassing the tetrahedral intermediate. We will return to this question later. If the properties of Nu or Lv can be varied so that the relative leaving group... 

Rates of addition to carbonyls (or expulsion to regenerate a carbonyl) can be estimated by appropriate forms of Marcus Theory. " These reactions are often subject to general acid/base catalysis, so that it is commonly necessary to use Multidimensional Marcus Theory (MMT) - to allow for the variable importance of different proton transfer modes. This approach treats a concerted reaction as the result of several orthogonal processes, each of which has its own reaction coordinate and its own intrinsic barrier independent of the other coordinates. If an intrinsic barrier for the simple addition process is available then this is a satisfactory procedure. Intrinsic barriers are generally insensitive to the reactivity of the species, although for very reactive carbonyl compounds one finds that the intrinsic barrier becomes variable. ... 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

In a concerted reaction, orbital and state symmetry is conserved throughout the course of the reaction. Thus a symmetric orbital in butadiene must transform into a symmetric orbital in cyclobutene and an antisymmetric orbital must transform into an antisymmetric orbital. In drawing the correlation diagram, molecular orbitals of one symmetry on one side of the diagram are connected to orbitals of the same symmetry on the other side, while observing the noncrossing rule. 

Apart from their intrinsic interest, these electrocyclic reactions have considerable synthetic carbon-carbon bond-forming importance because of their rigid stereospecificity, which is much greater than in the vast majority of other, non-concerted reactions involving biradical or bipolar intermediates. 

Particular interactions between energy levels could stabilize supra-facial-suprafacial [2+2] concerted reactions, [2.ns- - ig, 57) or supra-facial-antarafacial modes, [2Jts-hn a]- Other interactions might be favorable toward biradical reactions. These diagrams will be discussed in detail in Section VI. 

The first-order interaction of the two bonding levels should be the controlling interaction. The calculated PMO energies for concerted reactions are 0.61/ for the observed orientation and 0.53y for the other orientation. Calculated energies for biradical reactions are much smaller. 

The trans compound cannot be formed by a concerted reaction from the 33t-7t state because the predominant level perturbation is the almost degenerate interaction of K(jr) and 0(jr), that would give cis geometry of product. Since the trans adduct must be formed via biradical intermediate (the only other possibility), it has usually been convenient to suppose that the cis adduct is also formed from the same biradical intermediate. Another choice of mechanism is that cis compound is formed via a concerted reaction, and the trans compound arises from a biradical pathway. In this case, the spin prohibition could be outweighed by two factors, the favorable geometry and the stabilizing first-order perturbation. 

Song and Beak found intramolecular and intermolecular hydrogen-deuterium kinetic isotope effects of 1.1 0.2 and 1.2 0.1, respectively, for the tin tetrachloride catalysed ene reaction. Since significant intramolecular and intermolecular primary deuterium kinetic isotope effects of between two and three have been found for other concerted ene addition reactions161, the tin-catalysed reaction must proceed by the stepwise pathway with the k rate determining step (equation 107). 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

As a result of these heavy-atom KIE experiments the principal features of the benzidine rearrangements have now been firmly established. The two main products arise from two parallel reactions one of which is concerted and the other is not. Other concerted processes have been identified and all of the concerted processes can be readily classified in the terminology of sigmatropic rearrangements within the general class of percyclic reactions. 

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