Concerted reactions orbital symmetry control

Chlorination of olefins has also been achieved with SbCls in chlorinated solvents, which gives with mono-olefins vicinal dichloroalkanes by a syn addition. A concerted mechanism was initially proposed68 to rationalize this stereochemical behavior and the unexpectedly large amount of c -l,4-dichloro-2-butene found in the reaction of butadiene. In this case, however, because of orbital symmetry control it has been suggested that the addition occurs in an antarafacial direction69. 

Dyotropic rearrangements are uncatalyzed concerted dihydrogen exchange reactions, another class of orbital symmetry controlled processes, which involve the simultaneous migration of two cr-bonds. These conversions can be both thermal and photochemical. They can be subdivided into two types (1) reactions in which two migrating cr-bonds interchange their positions (equation 78), and (2) reactions without such positional interchange (equation 79)91,92. 

A further example of a concerted thermal elimination reaction of a thiepane derivative was the formation of c/s-hexatriene and the extrusion of sulfur dioxide from heating 2,7-dihydrothiepin 1,1-dioxide (116) (67JA1281). That this reaction was under orbital symmetry control was deduced from the results obtained by heating cis- (117) and trans-... 

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

Many chemical processes are initiated simply by mixing the appropriate reagents, and (usually) the higher the temperature, the faster the reaction rate such reactions are classified as thermally activated or thermal reactions. Sometimes, thermal activation is not enough to initiate the reaction or, in orbital-symmetry-controlled concerted processes, initiates the wrong reaction, and photochemical activation is necessary. Although the procedure to obtain a mechanistic rate law also applies for photochemical reactions, we shall not consider them specifically in this chapter. 

For the allene-olefin system, orbital symmetry controls the stereochemistry leading to the 2-(dimethylene) allylene intermediate as though the reaction were to be an energetically concerted Jt2s + ji2s + process,... 

The 1,4- and 3,2-rearrangements are thermal, concerted, orbital symmetry allowed processes possessing suprafacial-suprafacial characteristics. They are facile reactions, occurring readily and under mild conditions. The 3,2-rearrangement will take place with allylic inversion, as demanded by orbital symmetry control. Thus, the sulfonium salt (30) gave the allylic sulfide (31), whereas (32) gave the isomeri-cally pure (33 Scheme 9). ... 

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,...) ... 

The hypothesis of dynamical control has been pursued in biradical reactions, vide infra, but the notion that temperature independence of reaction pathways is a criterion for such behavior seems inappropriate since the parent bicyclo[2.I.I]-hexene does not show such behavior despite the fact that it, having many fewer vibrational modes, should be most likely to do so. The question of whether or not the stereochemistry of the rearrangement of the parent compound is under orbital symmetry control requires that the reaction be concerted. However, the heat of formation of the parent material and the heat of formation of the methyl cyclopentene diyl suggests a BDE of less than roughly 22 kcal/mol for the non-concerted cleavage of the C1-C5 bond which is 12 kcal/mol less than that observed. However, these types of BDE estimates are invariably 8-10 kcal/mol less. So, it is unclear whether or not the reaction in the parent case is perhaps partially concerted. 

Many of the reactions to be discussed are concerted processes and may be treated mechanistically in light of the concepts of orbital-symmetry control introduced in Part A, Chapter 10. Some others are stoichiometrically similar, but on mechanistic scrutiny have been found to proceed through discrete, short-lived intermediates. 

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... 

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... 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

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.- ... 

R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Academic Press, New York, 1970. This is the principal treatise on the controlling factors in concerted reactions, but you may not find it simple reading. 

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. 

---