Transition state structure, secondary

Tetrahedral intermediates, derived from carboxylic acids, spectroscopic detection and the investigation of their properties, 21, 37 Topochemical phenomena in solid-state chemistry, 15, 63 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition state structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1 Transition states, theory revisited, 28, 139... 

Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition state structure, secondary deuterium isotope effects and,... 

Deuterium kinetic isotope effects, secondary, and transition state structure, 31,143 Diazo compounds, aliphatic, reactions with acids, 5, 331... 

Transition stale structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1 Transition states, theory revisited, 28, 139... 

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

Secondary Deuterium Kinetic Isotope Effects and Transition State Structure... 

The effect of a change in solvent on the secondary a-deuterium KIEs 195 Secondary a-deuterium KIEs and the effect of ionic strength on transition state structure 197... 

Wolfe and Kim (1991) also reported that the magnitude of a secondary a-deuterium KIE is primarily determined by the changes that occur in the Ca—H(D) stretching vibrations when the reactant is converted into the transition state. Wolfe and Kim calculated the transition state structures and the secondary a-deuterium KIEs for a series of identity SN2 reactions of methyl substrates [reaction (8)] at various levels of theory ranging from 4-31G to MP4/6-31 + G //6-31 + G. The KIEs were partitioned into two contributions, those from the Ca—H(D) stretching vibrations and those from the Ca—H(D) bending vibrations. 

Another surprising result of these calculations was that they suggested the relationship between the magnitude of the secondary a-deuterium KIE and transition state structure that had been based on experimental results (Streitwieser et al., 1958 Bartell, 1961 Kaplan and Thornton, 1967) was incorrect. Wolfe and Kim plotted the calculated secondary a-deuterium KIE at various levels of theory versus a looseness parameter, L, for the transition state. The L parameter was defined as the sum of the percentage extension of the C—X and the C—X bonds on going from the reactant (product) to... 

Wolfe and Kim s view of the origin of secondary a-deuterium KIEs has been challenged by two different groups. Barnes and Williams (1993) calculated the transition state structures and the secondary a-deuterium KIEs for the identity SN2 reactions between chloride ion and several substituted methyl chlorides (reaction (11)). 

Poirier, Wang and Westaway (1994) also investigated the relationship between transition state structure and the magnitude of the secondary a-deuterium KIE in a theoretical study of the SN2 reactions between methyl and ethyl chlorides and fluorides with several different nucleophiles (reaction (12)). 

Although the relationship between the secondary a-deuterium KIEs and transition state structure is different for the two types of transition state and interpreting secondary a-deuterium KIEs is, therefore, more difficult, it appears that the change in the KIE with substituent should be a good indicator for determining whether an SN2 transition state is symmetrical or unsymmetrical. 

Secondary a-tritium KIEs are much larger and more sensitive to a change in transition state structure than secondary a-deuterium KIEs. 

SECONDARY a-DEUTERIUM KIEs AND THE EFFECT OF IONIC STRENGTH ON TRANSITION STATE STRUCTURE... 

Table 29 The secondary a-deuterium and primary nitrogen KIEs and the relative transition state structures for the ion-pair SN2 reactions between sodium thiophenoxide and benzyldimethylphenylammonium nitrate in DMF at different ionic strengths at 0°C."...

Finally, as is the case for the secondary a-deuterium KIEs, the /3-deuterium KIE is assumed to vary in magnitude from near unity for a reactant-like transition state to a maximal value for a transition state resembling the carbocation formed in an SN1 reaction. The experimentally determined KIE may, therefore, be used as a measure of transition state structure provided that the maximum value of the KIE, i.e. the EIE for the formation of the carbocation, is known. 

This corresponds to an isotope effect of approximately 3.5% per deuterium. In comparison, the secondary /3-deuterium KIEs in SN1 reactions are all normal and range from 5% to 15% per deuterium. Because the normal KIEs in SN1 reactions result from the weakening of the C,—L bond by a hyperconjugative interaction with the incipient carbocation in the transition state, the authors concluded that hyperconjugative interactions are present also in the transition state for the insertion reaction. The normal secondary /3-deuterium KIE observed for the insertion reaction is consistent with the dipolar three-centre transition state structure [15] proposed by Seyferth et al. (1970a,b) because the partial positive charge on the a-carbon is stabilized by hyperconjugation. 

Fig. 10.12 (a) Transition state structures (C—C bond lengths) calculated at two levels for the concerted and step-wise Diels-Alder reaction shown in Fig. 10.11 (Houk, K. N., Gonzalez, J., and Li, Y,Accts. Chem. Res. 28, 81 (1995). The parenthesized values show results for calculations at a much higher (and much more expensive) level, (b) Calculated secondary deuterium isotope effects, kH/kD (per D) for the concerted and stepwise Diels-Alder reactions shown in Fig. 10.11 (Houk, K. N., Gonzalez, J., and Li, Y,Accts. Chem. Res. 28, 81 (1995). The parenthesized values show results for calculations at a much higher (and much more expensive) level)...

Secondary isotope effects measure transition-state structure 37 Quantum tunneling in enzyme-catalyzed reactions breakthroughs 42 Experimental phenomenology of quantum tunneling in enzyme-catalyzed... 

T-secondary isotope effect can be determined. As recounted in the last item of Chart 3, such effects are expected to be measures of transition-state structure. If the transition state closely resembled reactants, then no change in the force field at the isotopic center would occur as the reactant state is converted to the transition state and the -secondary kinetic isotope effect should be 1.00. If the transition state closely resembled products, then the transition-state force field at the isotopic center would be very similar to that in the product state, and the a-secondary kinetic isotope effect should be equal to the equilibrium isotope effect, shown by Cook, Blanchard, and Cleland to be 1.13. Between these limits, the kinetic isotope effect should change monotonically from 1.00 to 1.13. 

SECONDARY ISOTOPE EEEECTS MEASURE TRANSITION-STATE STRUCTURE... 

---