Asymmetric transition state

Were we to invoke an asymmetric situation, a pair of such transition states with identical fluxes would be required. Under special circumstances that lie outside this treatment, however, it is possible for a reaction to proceed by a pair of asymmetric transition states.2425... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of 3-formylamino-l,2,4-oxadiazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out7 The treatment has shown the participation of asymmetric transition states and non-concerted processes via symmetrical intermediates. A detailed ab initio and density functional study of the Boulton-Katritzky rearrangement of 4-nitrobenzofuroxan has indicated a one-step mechanism for the process. 

The differences in deuterium and carbon isotope effects indicate the asymmetric transition state with more advanced carbon-carbon bond formation to the terminal Cl atom. The difference between deuterium isotope effects for HCI-S and Htmns hydrogens probably originates from experimental uncertainty. Theoretical calculations (B3LYP/6-31G, B3LYP/6-311+G ) for carbene addition to 1-butene were carried out for two modes with carbene approaching carbon atom Cl or C2. The best agreement for experimental isotope effects is for carbene attack on terminal carbon atom and the carbene-alkene separation in the transition state of 2.5 A. 

Pai and Doren97 using DFT and cluster models found only the asymmetric transition state. They predicted the desorption activation energy to be 55 kcalmol-1 using LSD and... 

Similar results have been obtained by Baciocchi for the deprotonation of a-substituted 4-methoxytoluenes by 2,6-lutidine and NOs in acetonitrile [145]. In this study, the same values of the Bronsted coefficient (a = 0.24), and of the deuterium kinetic isotope effect (kn/kD = 2.0 for 4-methoxytoluene radical cation) have been obtained with the two bases these results point again towards a highly asymmetric transition state with a very small amount of C-H bond cleavage. Moreover, values of 0.53 and 0.66 eV have been calculated for the intrinsic barrier of the reactions of the radical cations with NO3" and 2,6-lutidine, respectively, again comparable with those observed for acid-base reactions involving carbon acids [140, 141]. 

Going from sila- to stannaethenes the polarity of the double bond remains nearly constant. But the formation of an asymmetrical transition state with stannaethenes is disadvantaged for two reasons the tin atomic radius is much greater than the silicon or germanium radius and Sn-C single bonds (which have to be newly formed) are much weaker than Si-C and Ge-C bonds, respectively. Therefore, stannaethenes are poorer enophiles and [2+2] cycloadduct partners in comparison to sila- or germaethenes. 

Gung, B. W., Xue, X. Asymmetric transition states of allylation reaction an ab initio molecular orbital study. Tetrahedron Asymmetry 2001, 12, 2955-2959. 

All this information about the reactivity of pyrrole and its derivatives as dienes for Diels-Alder reactions was obtained from the values computed on two separated reactants. It is much more appropriate to compare changes of the FMO energies for reactants to reach transition state structures. Some representative transition state structures for acetylene addition to pyrrole derivatives are presented in Figure 3. The transition state structures are for synchronous formation of both CC bonds with slightly asymmetric transition state structures, although both diene and dienophiles have a plane of symmetry that coincides with a plane of symmetry for the transition state structure. All of these transition state structures have similar bond distances for CC bonds, which is true for almost all Diels-Alder reactions. The FMO orbital changes for transformation of reactants into transition state structures are presented in Table 16. 

A new approach that we would like to introduce here to evaluate the reactivity of aromatic heterocycles that have asymmetric transition state structures is a comparison of the changes of the ring bond order between the heterocycle and... 

Since the activation parameters are quite similar to those of the silyl-silyl exchange in silyl silylmethyl ethers (Section II), the authors suggest that the rearrangements in the hydroxylamine series may also occur via a dyotropic mechanism (51). Furthermore, insensitivity to solvent polarity appears to rule out the intermediacy of ions or zwitterions. The possibility of an asymmetric transition state was discussed (51). 

Fig. 15. The potential energy profile along two reaction coordinates for the desorption of H2 from a Si surface dihydride via a symmetric and an asymmetric traunsition state. The experimental results are consistent with a lower barrier, suggesting that desorption occurs via the asymmetric transition state. Adapted from Ref. 288. See also Ref. 226.

The quantity AG " in Equations (119)-(121) can be related qualitatively to the asymmetry of the transition state, and terms such as the degree of proton transfer have been widely used highly asymmetric transition states are often described as being reactant-like or product-like. These concepts could also be expressed quantitatively in terms of the bond lengths, bond orders, or force constants of the two bonds in a transition state However, the relation between these quantities is a matter for speculation, and AG° is the only one of them which can (in favourable cases) be... 

Bettinger, H. R, Yakobson, B. L, Scuseria, G. E. (2003). Scratching the surface of Buckminster-fullerene The barriers for Stone-Wales transformation through symmetric and asymmetric transition states. Journal of the American Chemical Society, 125, 5572-5580. and references therein. 

One of the simplest examples of electrostatic catalysis is the acceleration of the 1,5-hydride shift in cyclopentadiene by the influence of Li" " cations. The reaction proceeds via an asymmetric transition state that is 34 kJ mol more stable than the symmetric ground state. It could be shown that this extra stabilization (catalytic rate acceleration) is completely due to the effect of the electrostatically bound cation. Difference in cation complexation energies could be adequately illustrated by molecular electrostatic potential (MEP) maps for the ground and transition states (see Electrostatic Potentials Chemical Applications). The maps are symmetric and asymmetric for the ground and transition states, respectively, and indicate larger cation attraction for the latter. 

The analyses of statistical factors may provide important information on the symmetry of the transition state. For example, the isomerisation of cyclopropane to propene may proceed via a symmetrical or an asymmetrical transition state (Figure 6.4). In the first case, (Tf = 2, whereas in the second case cif = 1. The symmetrical transition state corresponds to... 

Figure 6.4 Symmetrical and asymmetrical transition states in the isomerisation of cyclopropane to propene. The symmetrical transition state does not have any physical meaning because it requires the splitting of the reaction path into two paths at the transition state.

A remarkable feature of the cyclizations of linaloyl derivatives is the high stereoselectivity in the formation of a-terpineol. Thus, the hydrolysis of ( —) linaloyl phosphate (33-OP) affords (-h )-a-terpineol with an optical purity of 40% (79). The cyclization of linalool to optically active a-terpineol may also be carried out with sulfuric acid in acetic acid (81) and aqueous sulfuric acid (82). The a-terpinyl j9-nitrobenzoate produced in the hydrolysis of linaloyl /7-nitrobenzoate is obtained with an optical purity as high as 87% (80). These results require that cyclization occurs, at least in part, from an asymmetric transition state. 

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