Allowed pericyclic reactions

The actual rates of thermally-allowed pericyclic reactions vary vastly, and frontier-orbital theory (14, 15, 16) has proven to be the primary basis for quantitative understanding and correlation of the factors responsible. It is therefore of interest to find the dominant frontier orbital interactions for the group transfer reactions hypothesized to occur. 

Cope himself formulated this transformation as what would now be called a synchronous pericyclic reaction . This interpretation was supported by Woodward-Hoffmann s analysis of pericyclic processes. The Cope rearrangement of 1,5-hexadiene derivatives was regarded therefore for a long time as a classical example of an allowed pericyclic reaction... 

R. C. Dougherty, J. Amer. Chem. Soc., 93, 7187 (1971) has argued that systems whose ground states are aromatic have antiaromatic excited states and vice versa, and that therefore the universal criterion for allowed pericyclic reactions, both ground and excited-state, is that the transition state be aromatic. The uncertainty of our present knowledge of excited states nevertheless indicates that the more restricted statement given here is to be preferred. 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

Although such a definition is seemingly quite clear and unique, the practical exploitation of the above criterion is complicated by the fact that the scission and formation of bonds is a microscopic process, inaccessible to direct experimental observation. This, of course, suggests the necessity of searching other, more easily exploitable, criteria of concert. One such criterion is the remarkable stereospecificity accompanying the formation of products in allowed pericyclic reactions [60,61]. The fact that the origin of the synchronisation in the process of scission and the formation of the bonds was always intuitively related to a certain energetic stabilisation led to another widespread opinion that all allowed reactions are automatically concerted. On the other hand nonconcertedness, advocated by frequently observed stereo-randomization [60] was practically always expected in forbidden reactions. 

An allowed pericyclic reaction has an aromatic transition state whereas a forbidden reaction has an antiaromatic transition state (p. 40). However, the aromaticity or... 

For many compounds there are several possible allowed pericyclic reactions. On the basis of your limited experience in organic chemistry, it is often difficult to decide which of the allowed reactions will occur for such compounds. In addition, these reactions are allowed in both directions, so the position of the equilibrium may also be important. Even an organic chemist with considerable experience in this area may have difficulty predicting exactly what will happen in every case. Elowever, some pericyclic reactions are more common than others. Table 22.1 summarizes those that are encountered most often. 

This chapter is subdivided into [3 + 2], [4 + 2] and [4+4] cycloaddition reactions. According to the rules of conservation of orbital symmetry, the first two cycloaddition reactions, [3 + 2] and [4 + 2], are of the same electronic type, namely [W4S + W2S], and are synchronous, thermally allowed, pericyclic reactions (69AG(E)78l). 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

The stereochemical and kinetic data for the thermal ring contraction which proceeds with inversion of configuration at the halogen-receiving carbon atom are consistent with an intermolecular halogen transfer and exclude the occurrence of a thermally allowed pericyclic reaction ... 

As pointed out in a 1996 review of the importance of dynamic electron correlation [le], in general, addition of correlation between the active and inactive electrons stabihzes the ionic terms in the CASSCF wave functions for the active electrons. Not only in the Cope TS, but also in the TSs for other Woodward-Hoffmann-allowed, pericyclic reactions [3], the ionic terms in the RHF configuration benefit from this type of stabilization. Consequently, with the inclusion of dynamic electron correlation, the weight of the RHF configuration in the wave function for the TS increases and the energy of the TS is lowered, relative to that of the reactants and products. It is for this reason that calculations which include dynamic electron correlation usually give much lower activation enthalpies for allowed pericychc reactions than either RHF or CASSCF calculations do [le,21]. 

The view that the transition structures of allowed pericyclic reactions are aromatic is supported by a more detailed analysis. For example, both the suprafacial [1,5] hydrogen shift of 1,3-pentadiene and the disrotatory... 

Describe each reaction below as the result of one or more allowed pericyclic reactions. 

Dipolar cycloadditions are a class of chemical reactions of signiflcant experimental interest. In fact, they provide a versatile method of preparing five-membered heterocyclic compounds. But they are also a class of reactions of theoretical interest, since they represent an example of thermally allowed pericyclic reactions. ... 

Give series of allowed pericyclic reactions that explain these overall transformations. 

A further criticism of this approach is its purely qualitative nature. There is no way of telling how large or how small will be the factors preventing violations of the rules derived from it. This has led to an exaggerated estimate of the potency of the factors distinguishing allowed pericyclic reactions from forbidden ones and the consequent belief that forbidden reactions cannot occur as concerted pericyclic processes. 

The choice between these two possibilities (—G or —Gr) is still uncertain. Note that if the reaction is concerted, then it must take place by the reverse of a face-to-face n cycloaddition. If the reaction took place by the reverse of an allowed cis-rrans n cycloaddition (see p. 350), it could not be chemiluminescent because in an aromatic ( allowed ) pericyclic reaction, the ground-state and excited-state surfaces remain far apart throughout. Similar chemiluminescent reactions are also observed in the case of bicyclic oxetanes, a good example being the chemiluminescent oxidation " of lophine (115) by alkaline hydrogen peroxide. Here again it is uncertain whether the final step is a concerted — G -type process or a — G -type process involving an intermediate biradical. 

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