Forbidden pericyclic reactions

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

This lack of mixing of DA and D+A for two ethylenes, in contrast to the situation for ethylene and butadiene, where mixing can take place, will be utilized (Section 3) to analyse the relationship of allowed and forbidden pericyclic reactions in configuration terms. 

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

The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial with singlet carbenes like dichlorocarbene (p. 28). This is clearly a forbidden pericyclic reaction, if it takes place in the sense 3.49 —> 3.50, This is known as the linear approach, in which the carbene, with its two substituents already lined up where they will be in the product, comes straight down into the middle of the double bond. The two reactions above, 3.47 and 3.48, are also linear approaches, but these are both allowed, the former because the total number of electrons (6)... 

The dipolar intermediate 35 is pivotal in further reactions. The formation of 36 requires a 1,2-hydrogen shift and formally at least such a shift would be a thermally forbidden pericyclic reaction. An alternative to the forbidden process would involve a thermally allowed 1,5-shift to give, for example, 37... 

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. 

The global term pericyclic funnel will be used to refer to the funnel or funnels in the S surface that occur at the critically heterosymmetric biradicaloid geometries reached near the halfway point along the path of a thermally forbidden pericyclic reaction, and the minima in S, that are encountered along one-dimensional cuts along reaction paths that miss the conical intersections (in particular, those along high-symmetry paths, which pass... 

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. ...

Forbidden Pericyclic Reactions. The Second-Order Similarity Approach. 

JAI Press, London, 1996, pp. 121-133. Electron Correlation in Allowed and Forbidden Pericyclic Reactions from Geminal Expansion of Pair Densities. 

It is thus possible to expect that the comparison of corresponding pair densities for individual electron states of the antiaromatic reference structure via the similarity index will make it possible to identify the electron state of the reference species which approximates the structure of the transition state the most closely. The values of the second order similarity index calculated for a series of several forbidden pericyclic reactions are collected in Table 10. Let us discuss now at least some conclusions which can be deduced from this Table. The most interesting in this... 

Alternative Derivations of the Woodward-Hoffmann Rules. Allowed and Forbidden Pericyclic Reactions... 

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. 

Pettit and his collaborators found that the rearrangement is very strongly catalyzed by salts of silver or platinum. Thus addition of a solution of silver nitrate to a solution of (222) in alcohol at room temperature leads to complete conversion to (224) in less than 10 sec. This startling observation led to extensive studies of the effects of transition metals on other forbidden pericyclic reactions and many of them have indeed been found to undergo similar accelerations. 

The corresponding thermal reaction would be an antiaromatic ( forbidden ) pericyclic reaction (Section 5.26) since it involves a cis-cis n cycloaddition involving a 4n-membered ring. The transition state for such a reaction is a point at which the BO approximation breaks down (see Fig. 6.29), so the dimerization is not an X-type process but is of type (see Section 6.17). 

This quantity was computed by De Proft et al. along the initial stages of the model reaction coordinate of a series of allowed and forbidden pericyclic reactions the sign of this quantity was shown to predict the allowedness of the reaction in agreement with the famous Woodward-Hoffmann rules for... 

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