Transition structure aromatic

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. 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. 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. 6.42, where we can see that a... 

Whereas methylenecyclopropanes only react with highly electron-deficient dienophiles in a [ 2n + 2fT) + 2n] fashion, alkenylidenecyclopropanes 1 readily undergo this cycloaddition type. A number of comprehensive and elaborate investigations with various alkenylidenecyclopropanes and 4-phenyl-l,2,4-triazoline-3,5-dione indicate that these reactions are concerted and proceed via [( 2j+,25+ 2 J -I-, 2 J transition states, involving the terminal double bond in an eight-electron Mobius aromatic transition structure 4. 

Curved arrow notation for benzene resonance and for aromatic transition structures in pericyclic reactions invoiving six eiectrons. (Adapted from reference 19.)... 

Huckel aromatic transition structure, while systems with 4 electrons prefer... 

The orbital phase theory can be applied to cyclically interacting systems which may be molecules at the equilibrium geometries or transition structures of reactions. The orbital phase continuity underlies the Hueckel rule for the aromaticity and the Woodward-Hoffmann rule for the stereoselection of organic reactions. 

Scheme 17 Cyclic orbital interactions at the transition structures of electrophilic aromatic substitutions...

Orbitals interact in cyclic manners in cyclic molecules and at cyclic transition structures of chemical reactions. The orbital phase theory is readily seen to contain the Hueckel 4n h- 2 ti electron rule for aromaticity and the Woodward-Hof nann mle for the pericyclic reactions. Both rules have been well documented. Here we review the advances in the cyclic conjugation, which cannot be made either by the Hueckel rule or by the Woodward-Hoffmann rule but only by the orbital phase theory. 

We have carried out DFT level investigations to explain the observed stereo and regioselectivities. Concerted nature of the mechanism has been confirmed by the involvement of aromatic but asynchronous transition structures as confirmed by their NICS values in each case [101, 102],... 

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. 

Using a valence bond scheme parametrized with an effective Hamiltonian technique, it was shown that the mechanistic preference for a synchronous pathway with an aromatic transition state versus a non-synchronous mechanism via biradicaloid intermediate can be controlled by two factors (1) the stability of the long bond in the Dewar valence bond structure, and (2) the softness of the Coulomb interaction between the end methylene groups in the 1,5-diene chain. This means that the mechanism of rearrangement (equation 153) can strongly depend on substituents218. 

The three aromatic amino acids (Phe, Tyr, Trp) have side-chain groups corresponding to the benzene, phenol, and indole chromophores, respectively. The spectroscopic properties of the rat transitions in these chromophores have been reviewed.136-381 Coupling of aromatic transitions among themselves and with peptide transitions can give rise to CD bands in the near and far (k < 250 nm) UV. Near-UV CD bands are useful indicators of the environment of the aromatic chromophores and can frequently be assigned to specific types of side chain, based upon band position, presence of vibrational fine structure, etc. Far-UV CD bands due to aromatic side chains, except for the La band of Tyr (-230 nm) and the Bb band of Trp (-225 nm), are generally difficult to resolve from peptide CD bands and can complicate the conformational analysis of peptides. 

Olefins react directly at the electron-rich and rather electron-deficient oxygens. If the dimer is much more reactive toward olefins than the monomer, only a small fraction of the alkaloid-Os04 complex need be present as a dimer (94a). Houk developed a symmetrical five-membered transition-structure model on the basis of X-ray crystal structures of Os04-amine complexes and osmate ester products and ab initio transition structures of analogous reactions (Scheme 40). The MM2 calculations based on this [3 + 2] reaction model reproduce the stereoselectivities of the stoichiometric reactions observed with several chiral diamines (94b). The transition state may be stabilized by tt-tt interaction of the alkene substrate and the ligand aromatic ring (95). 

All the reactions described so far have mobilized six electrons in the transition structure. Other numbers are possible, notably a few [8+2] and [6+4] cycloadditions involving ten electrons in the cyclic transition structure. It is no accident, as we shall see in the next chapter, that these reactions have the same number of electrons (4n+2) as aromatic rings. 

The first and the most simple is the observation that the common thermally induced reactions have transition structures involving a total of (4n+2) electrons. We saw in the last chapter that [4+2], [8+2], and [6+4] thermal cycloadditions are common, and that [2+2], [4+4], and [6+6] cycloadditions are almost only found in photochemically induced reactions. The total number of electrons in the former group are (4n+2) numbers, analogous to the number of electrons in aromatic rings. 

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