Pericyclic reactions ground-state forbidden

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... 

Transition metal ions catalyze a number of ground-state forbidden pericyclic reactions. See (a)... 

Whereas a [2 + 2] pericyclic reaction is essentially forbidden in the ground state, a [2+1] open-shell reaction is feasible. In this respect, the radical cations detected in this context represent distinct stages of pericyclic, radical-cation catalyzed cycloaddi-tions/cycloreversions. In Fig. 7.11, three distinct stages, a tight (cyclobutane-like), an extended (bis ethene), and a trapezoid, of a hole- (or radical-cation) catalyzed cycloaddition/cycloreversion are presented in a schematic way. °... 

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. 

Figure 6.11. Schematic correlation diagrams for ground-state-forbidden pericyclic reactions a) HMO model of Zimmerman (1966), b) PPP model of van der Lugt and Oosterhoff (1969), and c) real conical intersection resulting from diagonal interactions. The two planes shown correspond to the homosymmetric (y) and heterosym-metric (6) case. Cf. Figure 4.20.

Figure 7.27. Schematic representation of the state correlation diagram for a ground-state-forbidden pericyclic reaction with an excimer minimum E a) at geometries well before the pericyclic funnel P is reached, and b) at geometries similar to those of P. ...

Bemardi, F., De, S., Olivucci, M., Robb, M. A., Mechanism of Ground state Forbidden Photochemical Pericyclic Reactions Evidence for Real Conical Intersections, J. Am. Chem. Soc. 1990, 112, 1737 1744. 

Bemardi F, De S, Ohvucci M, Robb MA. Mechanism of Ground-state-forbidden photochemical pericyclic-reactions - evidence for real conical intersections. J Am Chem Soc. 1990 112 1737-1744. 

Fig. 7. Orbital (a), configuration (b), and state (c, d) correlation diagrams for a typical ground-state symmetry-forbidden pericyclic reaction...

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 photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

Application of this method to pericyclic reactions led to the generalization that thermal reactions take place via aromatic or stable transition states whereas photochemical reactions proceed via antiaromatic or unstable transition states. This is the case because a controlling factor in photochemical processes is conversion of excited state reactants into ground state products. Thus, the photochemical reactions convert the reactants into the antiaromatic transition states that correspond to forbidden thermal pericyclic reactions and so lead to corresponding products. 

Another approach to analyzing concerted pericyclic reactions is based on the observation that the forbidden [2+2] cycloaddition involves a cyclic array of four electrons in the transition state, while the allowed [4+2] cycloaddition involves a cyclic array of six electrons. This is a familiar pattern that immediately calls to mind aromaticity, in which the ground states of molecules with four ir electrons in a cycle are destabilized and termed antiaromatic, while molecules with six tt electrons are stabilized and aromatic. Building off an earlier analysis by Evans, Zimmerman developed aromatic transition state theory. Simply put, reactions with a simple cyclic array of 4h + 2 electrons (commonly six) in a pericyclic transition state will be stabilized by aromaticity, making the reactions favorable. Note that the relevant electrons need not be exclusively in it orbitals a mixture of cr and it bonds in a cyclic array is acceptable. 

Alternative models for the description and analysis of pericyclic reactions continue to appear. Multicentre pericyclic reactions in ground states have been treated as a sequence of one-centre reactions analogous to 5 2 nucleophilic substitution reactions the procedure is reminiscent of the FMO approach and can, with modification, analyse excited state processes. A second approach is presented in which multicentre reactions that retain at least one common element of symmetry in the reaction process connecting reactants to products are divided into individual steps. If none of these steps is symmetry forbidden, then the total process is allowed. Directions for deciding the individual steps are given. 

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