Symmetry controlled reactions cycloaddition

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

A cycloaddition reaction is one in which two unsaturated molecules add to one another, yielding a cyclic product. As with electrocyclic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

A great number of olefinic compounds are known to photodimerize in the crystalline state (1,2). Formation of a-truxillic and / -truxinic acids from two types of cinnamic acid crystals was interpreted by Bernstein and Quimby in 1943 to be a crystal lattice controlled reaction (5). In 1964 their hypothesis on cinnamic acid crystals was visualized by Schmidt and co-workers, who correlated the crystal structure of several olefin derivatives with photoreactivity and configuration of the products (4). In these olefinic crystals the potentially reactive double bonds are oriented in parallel to each other and are separated by approximately 4 A, favorable for [2+2] cycloaddition with minimal atomic and molecular motion. In general, the environment of olefinic double bonds in these crystals conforms to one of three principal types (a) the -type crystal, in which the double bonds of neighboring molecules make contact at a distance of -3.7 A across a center of symmetry to give a centrosymmetric dimer (1-dimer) (b) the / -type crystal, characterized by a lattice having one axial length of... 

Systems with more than two conjugated double bonds can react by [6 + 2] processes, which in azepines can compete with the [4 + 2] reaction (Scheme 73). Oxepins prefer to react as 4-components, through their oxanorcaradiene isomer, in which the 4-system is nearly planar (Scheme 74). Thiepins behave similarly. Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions. 

A cycloaddUion reaction lA one in which t wo unsaturated tnoleruies add to one another, yieldintr o r jc product. A wfth electrocydw reaction, cycloadditions are controlled t>y the orbital symmetry of the reactants. Symmetry allowed processes often lahe place readily, but symsnetry-disallowod processes take place with great dilTiculty, if at all, and then only by non-concerted pathways. 1 a look at two examples to see how they differ. 

It has become clear from the Woodward-Hoffmann-rules how orbital symmetry controles in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction 239>. For cycloaddition reactions of a m-ji-electron system to a M-jr-electron molecule the following stereochemical selection rules have been established (q = 0,1,2,...) ... 

Since a cycloreversion, the reverse of a cycloaddition, travels the same reaction path as the forward reaction, the considerations of stereochemistry and orbital symmetry that govern concerted cycloadditions are equally applicable to cycloreversions. The number of such reactions that have been studied in detail is not large, but there is sufficient information to establish that orbital-symmetry controls are indeed operating. The principles of orbital-symmetry conservation specify which processes can occur in concerted fashion and the stereochemical restrictions that are imposed by a concerted mechanism. We will first discuss some reactions that do occur by concerted mechanisms, and then turn to some of the elimination processes that involve discrete intermediates. 

The thermal cycloaddition of two ethylenes is one of the textbook examples used in the illustration of the Woodward-Hoffmann rules of orbital symmetry control in concerted reactions. Therefore, the related potential energy surface can provide various types of information of chemical and theoretical interest. A first question is associated with the mechanistic question of whether this reaction proceeds via diradical or concerted pathways. Since this reaction is an example of a concerted thermally forbidden process, it can be expected that the flavoured path be the diradical one. However, it is important to have a detailed description of the structural and energetic features of these different pathways. A second question is associated... 

The next four items (examples 2-5, inclusive) in Table 6.6 are all electrocyclic reactions, clearly related to the cycloadditions and others already discussed earlier in this chapter and the symmetry controlled processes of Chapter 4. Example 2, a conrotatory four-electron 2% + 27t = 27t + 2d) process relating trans or ( )-3,4-dimethylcyclobutene to trans, trans or (2 ,4 )-hexadiene conserves C2 symmetry as shown in Figure 4.41 and again here in Equation 6.59. Examples 3,4, and 5 are six-electron disrotatory processes. 

Cycloaddition reactions are also orbital symmetry-controlled, pericyclic reactions. We have seen one example already, the Diels-v lder reaction, and we will use it as our prototype. We found the Diels-Alder cycloaddition to be a thermal process that takes place in a concerted (one-step) fashion, passing over a cyclic transition state. Several stereochemical labeling experiments were described in Chapter 12 (p. 549), all of which showed that the reaction involved neither diradical nor polar intermediates. This stereospecificity is important because orbital symmetry considerations apply only to concerted reactions. Of course, all reactions can be subdivided into series of single-step, single-barrier processes, and each of these steps could be... 

Therefore, sigmatropic rearrangements are controlled by the same orbital symmetry considerations as cycloaddition reactions. Table 25.3 hsts the rules for thermal and photochemical sigmatropic rearrangements. 

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

Much of what we have said about the electronic factors controlling whether a cycloaddition reaction can be concerted or not originally was formulated by the American chemists R. B. Woodward and R. Hoffmann several years ago, in terms of what came to be called the orbital symmetry principles, or the Woodward-Hoffmann rules. Orbital symmetry arguments are too complicated for this book, and we shall, instead, use the 4n + 2 electron rule for-normal Hiickel arrangements of tt systems and the An electron rule for Mobius arrangements. This is a particularly simple approach among several available to account for the phenomena to which Woodward and Hoffmann drew special attention and explained by what they call conservation of orbital symmetry.- ... 

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