Transition structure antiaromatic

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. 

Antiaromatic transition structures having four electrons. 

Delocalization energy (this is related to aromaticity and antiaromaticity) Transition state structures and energies (see the hedge below)... 

Quadrelli, P Romano, S. Toma, L. Caramella, P. A bispericyclic transition structure allows for efficient relief of antiaromaticity enhancing reactivity and endo stereoselectivity in the dimerization of the fleeting cyclopentadienone, 7. Org. Chem. 2003, 68, 6035-6038. 

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. 

To explain the increase in the rate of an electrocyclic ring opening like 6.443 > 6.444, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the antiaromatic Hiickel cyclobutadiene that we saw in Fig. 1.46. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.55, where we can see that a donor, a withdrawing group, and a C-substituent on C-3 can each accelerate the reaction—the numbers on the right, —4.29 and —4.06, are more negative than for the unsubstituted system, 3.66. 

In msmy cases the information derived from these loops may be sufficient to estimate a starting geometry for a computational location of the conical intersection. However, more detailed information is available. If phase inversion between B and C (antiaromatic transition state with wave function I B-C>) is assumed, and if the position vectors of the three structures A, B, and C with wave functions A>, B>, and I C>, respectively, are denoted by r, rg, and r, the phasepreserving coordinate Q, = 2r - b - d for in-phase) connecting A with B - C and the phase-inverting coordinate Qo = - fc (O for out-of-phase) connecting B with C are... 

Electrocyclic reactions can also be analyzed on the basis of the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground state molecules. A stabilized aromatic TS results in a low activation energy, i.e., an allowed reaction. An antiaromatic TS has a high energy barrier and corresponds to a forbidden process. The analysis of electrocyclizations by this process consists of examining the array of basis set orbitals that is present in the transition structure and classifying the system as aromatic or antiaromatic. For the butadiene-cyclobutene interconversion, the TSs for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, that is, the complete set of 2p orbitals involved in the reaction process, not the individual molecular orbitals. The tilt at C(l) and C(4) as the butadiene system rotates toward the TS is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

Aromatic (top) and antiaromatic (bottom) Mobius transition structures. 

The calculated values of these indices for several selected reactions are given in Table 9, into which the values corresponding to antiaromatic reference structures, which represent a natural counterpart to ideally aromatic standards, were included for comparison. As can be seen from this Table, the predictions of Dewar classification are indeed confirmed since for allowed reactions the similarity to aromatic reference structures are systematically higher than to the antiaromatic ones. On the other hand, for forbidden reactions the similarity to antiaromatic structures dominates so that these structures can be expected to play the role of transition states in this case. The above conclusions suggesting the important role of antiaromatic reference species as the eventual transition states in forbidden reactions was investigated in the study [158], in which the more detailed specification of the structure of antiaromatic transition states was attempted. The basis of this approach is a straightforward reformulation of the above procedure in terms of second order or pair density matrices. These matrices are generally defined by eq. (105),... 

Table 9 Calculated values of similarity indices of transition states with aromatic and antiaromatic reference structures for several selected pericyclic reactions.

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... 

We can thus expect that correct reaction path is the one for which the antiaromatic transition state will be best approximated just by Zj - Z2 state of the biradicaloid reference structure. Since, however, just such kind of correspondence is observed in Table 11, it is possible to expect that the transition state in forbidden reactions should be better approximated by the structure X(-ti/4), characterized by the wave function (107a) than by the original structure X(7t/4) which thus describes rather the excited state of the transition state.(Eq. 107b)... 

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. 

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. 

Several phosphete-containing transition metal complexes have been structurally determined. In their crystallographic structures, phosphete rings indicated their delocalized structures. Therefore, the aromaticity and antiaromaticity of these classes of compounds attract special attention, and encourages comparison to the highly antiaromatic cyclobutadienes. 

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