Aromaticity transition states

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

Another concept that has recently been introduced is the aromaticity of the transition state (119). It was suggested that this aromaticity is compatible with a ring current circulating within the molecular plane of the hve atoms in the transition state for cycloaddition (119). According to these calculations, such transition state aromaticity does not, however, impact upon the regioselectivity of the cycloaddition (119). 

In the aromatic transition state approach, the basic criterion was that a reaction is allowed in the ground state if and only if there occurs in the transition state aromatic stabilization. This criterion led to the Dewar-Zimmerman selection rule (Equation 11.36), where p. i. = 0 signifies an even number of phase inversions, p. i. = 1 signifies an odd number of phase inversions, and N is the total number of electrons. 

Possible reasons why transition state aromaticity is able to develop early while resonance development lags behind proton transfer at the transition state, and why anti-aromaticity lags behind proton transfer, will be discussed in the section on ab initio calculations. These calculations have provided important additional insights because they allow a direct probe of transition state aromaticity or anti-aromaticity. 

Further evidence showing disproportionately high transition state aromaticity comes form NICS values,144,145 Bird indices,148,149 and HOMA142,143... 

NICS, HOMA, and Bird indices were also calculated for the transition states of the reactions of 61H-0 and 61H-S with a series of carbanions. The results are reported in Table 23. The trends in these parameters show a clear increase as the transition state becomes more product-like with increasing endothermicity, indicating an increase in transition state aromaticity. Even more revealing is the % progress at the transition state which indicates that this progress is >50% not only for the endothermic reactions (product-like transition states) but even for most of the exothermic reactions (reactant-like transition states) except those with strongly negative AH° values. 

Table 23 Transition state aromaticity indices for the reactions of 61H-0 and 61H-S with carbanions in the gas phase3... 

What the PNS cannot deal with is the effect on reactivity by factors that only operate at the transition state level but are not present in either reactant or product. Examples mentioned in this chapter include transition state aromaticity in Diels Alder reactions, steric effects on reactions of the type A + B ty C + D, or hydrogen bonding/electrostatic effects that stabilize the... 

The most noticable feature of these results is the very high reactivity of the compounds. Indeed, A -methylisoindole is the most reactive aromatic yet known. The high reactivity derives from the fact that, on going to the transition state, aromaticity is created whereas for a normal elec-... 

Although Otto Diels and Kurt Alder won the 1950 Nobel Prize in Chemistry for the Diels-Alder reaction, almost 20 years later R. Hoffmann and R. B. Woodward gave the explanation of this reaction. They published a classical textbook, The Conservation of Orbital Symmetry. K. Fukui (the co-recipient with R. Hoffmann of the 1981 Nobel Prize in Chemistry) gave the Frontier molecular orbital (FMO) theory, which also explains pericyclic reactions. Both theories allow us to predict the conditions under which a pericyclic reaction will occur and what the stereochemical outcome will be. Between these two fundamental approaches to pericyclic reactions, the FMO approach is simpler because it is based on a pictorial approach. Another method similar to the FMO approach of analyzing pericyclic reactions is the transition state aromaticity approach. 

Transition state aromaticity (Huckel and Mobius topologies)... 

This chapter explores the reasons why some molecular reactions take place whereas others do not, and introduces the concepts of frontier orbitals and transition state aromaticity. 

Generalization of either the frontier orbital, the orbital symmetry, or the transition-state aromaticity analysis leads to the same conclusion about the preferred stereochemistry for concerted thermal electrocyclic reactions The stereochemistry is a function of the number of electrons involved. Processes involving 4n + 2 electrons will be disrotatory those involving 4n electrons will be conrotatory for Hiickel transition states. The converse holds for Mobius transition states. 

The conclusions reached are the same as for the frontier orbital approach. The suprafacial 1,3-shift of hydrogen is forbidden but the suprafacial 1,5-shift is allowed. Proceeding to a 1,7-shift of hydrogen, it is found that the antarafacial shift is allowed. These conclusions based on transition state aromaticity are supported by MO calculations at the 6-31 level, which conclude that 1,5-shifts should be suprafacial while 1,7-shifts should be antarafacial. Theoretical calculations also find that the 1,3-shift of hydrogen should be antarafacial, but, in agreement with expectations based on molecular geometry, the transition state which is found is so energetic that it is close to a stepwise bond dissocation process. ... 

Table 20.2 summarizes all An and 4m + 2 reactions. Other 4m processes will follow the rules for the 2 + 2 dimerization of a pair of alkenes, and 4m + 2 processes will resemble the 4 + 2 cycloaddition we know as the Diels-Alder reaction. Perhaps you can see the relationship to aromatidty (4m + 2) that plays a role in this analysis. The transition state for these cycloaddition reactions is cyclic and will be allowed only in the cases where the number of electrons makes the transition state aromatic, 4m + 2 electrons for thermal processes and 4m for photochemical reactions. 

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