Antiaromatic pericyclic reactions

Dewar, M. J. S., Ramsden, C. A. Stevens rearrangement. Antiaromatic pericyclic reaction. J. Chem. Soc., Perkin Trans. 11974, 1839-1844. 

Dewar, Michael J. S., 8c Ramsden, Christopher A. 1974. "Stevens Rearrangement. Antiaromatic Pericyclic Reaction." Journal of the Chemical Society, Perkin Transactions 1 1839-1844. 

It is therefore inaccurate and misleading to talk about allowed and forbidden pericyclic reactions. The terms aromatic and antiaromatic pericyclic reaction are much more appropriate. It is also clear that the distinction between them has nothing to do with symmetry. It depends on the topology of overlap of the AOs in pericyclic transition states, not on the symmetries of MOs. If symmetry were involved, the distinction between allowed and forbidden reactions would be attenuated as symmetry was lost. This is not the case. The Woodward-Hoffmann rules, or the equivalent statement embodied in Evans principle, hold just as strongly in systems lacking symmetry as in symmetric systems. Indeed, if this were not the case, they would be far less useful and important. 

Reactions. A reaction is the photochemical analog of an antiaromatic pericyclic reaction (Section 5.28), the BO hole corresponding to the antiaromatic transition state. Since it is not immediately obvious that such a structure will be a BO hole, i.e., a point where the ground-state and excited-state surfaces touch, we must first establish that this is in fact the case. 

FIGURE 6.30. The transition state for an antiaromatic pericyclic reaction is a point where the excited-state and ground-state potential surfaces touch. The dotted line indicates a photochemical route from A to C. 

The transition state for the concerted reaction would be a six-electron Hiickel-type transition state. Such a process should not take place photo-chemically since only antiaromatic pericyclic reactions occur in this way. The reaction moreover seems to involve a triplet excited state since it occurs only in the presence of a ketone as sensitizer. Therefore there seems little doubt that it is, as indicated in equation (6.39), a G j-type process in which the first step is cleavage of a bond py to the excited diene system to give a pair of mesomeric biradicals. These combine on the ground-state surface to form the product. 

A much more interesting case is provided by antiaromatic pericyclic reactions where the transition state is a BO hole at which the excited-state surface and the ground-state surface touch (Section 6.13). Here photoexcitation can lead directly to the ground-state product by passage through the BO hole. This, as we have seen, is the basic requirement for a photochemical reaction. Such a process will lead to products which are qualitatively different from those given by corresponding thermal processes and the formation of these products is often the only evidence that a G reaction is involved. 

The special case of pericyclic reactions is an appropriate means of introducing the subject These reactions are very common, and were extensively studied experimentally and theoretically. They also provide a direct and straightforward connection with aromaticity and antiaromaticity, concepts that mm out to be quite useful in analyzing phase changes in chemical reactions. 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

At this point, it is appropriate to draw a parallel with the straightforward MO explanations for the aromaticity of benzene using approaches based on a single closed-shell Slater determinant, such as HMO and restricted Hartree-Fock (RWF), which also have no equivalent within more advanced multi-configuration MO constructions. The relevance of this comparison follows from the fact that aromaticity is a primary factor in at least one of the popular treatments of pericyclic reactions Within the Dewar-Zimmerman approach [4-6], allowed reactions are shown to pass through aromatic transition structures, and forbidden reactions have to overcome high-energy antiaromatic transition structures. 

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years33, the two concepts are inextricably linked34. This is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states35 and the Huckel/Mobius concept31. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 136. 

The appearance in the previous section of the 4 + 2 and 4r formulas brings to mind the criteria for aromatic and antiaromatic systems discussed in Chapter 1. Furthermore, the HOMO-LUMO interaction patterns discussed in Section 11.2 are reminiscent of those used in Section 10.4 to analyze aromatic stabilization. In this section, we trace the connection between aromaticity and pericyclic reactions, and show how it leads to a third approach to the pericyclic theory. 

Pericyclic reactions that pass through aromatic transition states are allowed in the ground state those that pass through antiaromatic transition states are ground-state forbidden.36... 

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. 

A thermal pericyclic reaction is allowed when its transition state is aromatic and forbidden when it is antiaromatic. 

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

According to Zimmermann [101] and Dewar [102], the allowedness of a concerted pericyclic reaction can be predicted in the following way A cyclic array of orbitals belongs to the Hiickel system if it has zero or an even-number phase inversions. For such a system, a transition state with An+ 2 electrons will be thermally allowed due to aromaticity, while the transition state with An electrons will be thermally forbidden due to antiaromaticity. 

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic Hiickel [4n]annulene or MObius [An + 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

The single-parameter X-model is now extended to a parametric description of complex reactions with an arbitrary number of reaction parameters. Let p( 3) be the number of reaction partn s (reactants, products or intermediates) the reaction lattice is then isomorphic to the lattice Pip + 1) 2 with a diagram of a higher dimensional cube (6.32). Accordin y, the dynamic sublattice is isomorphic to P(p) = 2 and thus contains at least one element of the non-roechanistic dimension A (see Ch. "Generalized reaction lattice"). Ck>nsequently, the choice of the reaction path is no longer unique - in contrast to the sin e-parameter X model for pericyclic reactions with a well defined reaction path (via an aromatic or antiaromatic transition state.). The formal algebraic description of... 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

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