Cycloadditions allowed geometry

The symmetries of the terminal lobes of the HOMO and LUMO of the reactants in a [4 + 2] thermal cycloaddition allow the reaction to proceed with suprafacial geometry. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

But if the approach of the molecular planes is perpendicular, i.e., when one surface of the n orbital of one molecule interacts with both surfaces of the n orbital of the other molecule, then cycloaddition will be allowed here but the geometry of the approach will be the deciding factor and geometrically it is inaccessible. 

The thermally allowed [8 + 2] cycloaddition reactions may be considered as the 10tt analogs of the Diels-Alder reaction in which the diene component has been replaced by a tetraene component. Like trienes in the [6 + 4] cycloaddition reactions, the 87t tetraenes must satisfy certain requirements concerning geometry in order to be able to participate in an [8 + 2] cycloaddition. For example, tetraenes 518 and 519 can undergo an [8 + 2] cycloaddition, whereas an [8 + 2] cycloaddition with 520 is virtually impossible. Due to its fixed -system, 519 is more reactive in cycloaddition reactions than 518 and is therefore more often encountered in the literature. [8 + 2] Cycloadditions have been applied only... 

A number of cyclic nitrones have been developed that avoid the issue of nitrone ( /Z) isomerization by permitting only a single geometry about the C=N double bond and so reduce the number of possible cycloaddition products. Cyclic nitrones have also become popular as facially differentiated reagents, allowing predictable... 

There is evidence that the reactions can take place by all three mechanisms, depending on the structure of the reactants. A thermal [,2S +, 2,] mechanism is ruled out for most of these substrates by the orbital symmetry rules, but a [ 2S + 2a) mechanism is allowed (p. 851), and there is much evidence that ketenes and certain other linear molecules939 in which the steric hindrance to such an approach is minimal can and often do react by this mechanism. In a [ 2S + a2a] cycloaddition the molecules must approach each other in such a way (Figure 15.12a) that the + lobe of the HOMO of one molecule (I) overlaps with both + lobes of the LUMO of the other (II), even though these lobes are on opposite sides of the nodal plane of II. The geometry of this approach requires that the groups S and U of molecule II project into the plane of molecule I. This has not been found to happen for ordinary... 

In all of the above discussion we have assumed that a given molecule forms both the new ct bonds from the same face of the n system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [ 2s + 4J-cycloaddition (the subscript 71 indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2 + 4a]-cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photoche-mically allowed. Thus in order for a [fZs + -reaction to proceed, overlap between the highest occupied n orbital of the alkene and the lowest unoccupied 71 orbital of the diene would have to occur as shown in Fig. 15.10, with a + lobe... 

Under thermal conditions, the [2+2]-cycloaddition of olefins is symmetrically forbidden, according to the Woodward-Hoffmann rules. However, under photochemical conditions, [2+2]-cycloadditions become a suprafacial process for both components The orbital geometry of the interacting orbitals is equal and therefore the entire reaction is symmetrically allowed. 

The di- r-methane rearrangement is not confined to acyclic and mono-cyclic systems. Bicyclic 1,4-dienes such as barrelene (123) also rearrange, but they do so upon triplet sensitization (Zimmerman and Grunewald, 1966). In all probability, the built-in geometry constraint of a bicyclic molecule does not allow for the loose geometries otherwise characteristic of the triplet state, while in the singlet state other reactions such as [2 + 2] cycloaddition predominate. 

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