Repulsion in the transition state

The difference between the reactions with a high energy of the X—Y (C—C and C—N) bond for which Ee0 75-80 kJ mol-1 and the reactions with a very low energy of this bond (O—O) for which Ee0 42-46 kJ mol-1 can be clearly traced. Within the limits of the error of measurement, the parameter re for the last four reactions is constant rc = (2.86 + 0.05) x 10 41 m. This value is characteristic of reactions with zero triplet repulsion in the transition state. On substituting this quantity in Equation (6.10), we obtain the following equation for the estimation of the contribution of triplet repulsion AET to the activation energy... 

Evidently, the dissociation energies of the H—H and Cl—H bonds are very close and the triplet repulsion in the transition states of these reactions is, therefore, almost identical. Nevertheless, the quantities Eeo and re in these two reactions differ very considerably. The reason for this is that the H—H bond is nonpolar, while the Cl—H bond is polarized its AEA 92.3 kJ mol 1 (Equation [6.29]). As in the HC1 molecule, in the transition state there is evidently a strong attraction between Cl and H, which in fact induces a decrease in re and Ee0. If the Cl + H2 reaction was characterized by the same parameter re = 3.69 x 10-11m as the H + H2 reaction, an activation energy of Ee0 = 56.5 kJ mol 1 would be obtained for that reaction. The difference between the observed and expected activation energies (A ,ea = 36.7—56.5 = —19.8 kJ mol 1) must be attributed to the influence of the unequal electronegativities of the hydrogen and the chlorine atoms on Ec(, in the Cl + H2 reaction. 

The activation energy of radical abstraction is influenced by the so-called triplet repulsion in the transition state. This influence is manifested by the fact that the stronger the X—R bond towards which the hydrogen atom moves in the thermally neutral reaction X + RH, the higher the activation energy of this reaction. The triplet repulsion is due to the fact that three electrons cannot be accommodated in the bonding orbital of X—C therefore, one electron... 

Thus, the radius of the atom carrying the free valence has a substantial influence on the activation barrier to the addition reaction the greater the radius of this atom, the higher the activation energy. Apparently, this effect is due to the repulsion in the transition state, which is due to the interaction between the electron shells of the attacked double bond and the atom that attacks this bond. 

Many theories have been put forward to explain the mechanism of inversion. According to the accepted Hugles, Ingold theory aliphatic nucleophilic substitution reactions occur eigher by SN2 or SN1 mechanism. In the SN2 mechanism the backside attack reduces electrostatic repulsion in the transition state to a minimum when the leaving meleophile leaves the asymmetric carbon, naturally an inversion of configuration occurs at the central carbon atom. 

For a series of geminal dimethyl trisubstituted alkenes, the same trend of regioselectivity was earlier recognized by Thomas and Pawlak as well as Rautenstrauch and coworkers . By examining molecular models and assuming that the reaction is concerted, Thomas and coworkers proposed that the difference in regioselectivity could be due to the different conformations and steric repulsions in the transition states. 

The faster rate of the conversion of ds-2-butene to / -methoxy ester relative to trans-2-butene is consistent with this mechanism which requires formation of a more stable tt complex for the cis isomer (22) and greater relief of steric repulsion in the transition state for the trans addition to a cis olefin. By contrast, cis addition of coordinated carboxylate... 

In the discussion on 1,1,4,4-tetrafluorocyclooctane the argument was made that substitution of a methylene group in cyclooctane by a difluoro-methylene group increases the barrier to pseudorotation by about 1.2 kcal/ mole, owing to the presence of additional non-bonded repulsions in the transition state for the boat-chair to twist-boat-chair interconversion. Supporting evidence for this view comes from proton nmr studies on 1,1-dimethylcyclooctane (VIII) and on the ethylene ketal (IX) and the ethylene dithioketal (X) of specifically deuterated cyclooctanone. 

DFT calculations indicate that the oxidative addition reactions of a G-F bond in GeFe at Ni(H2PGH2GH2PH2) and Pt(H2PGH2GH2PH2) proceed initially via exothermic formation of an 77 -arene complex. The G-F oxidative addition reaction is more exothermic at nickel than at platinum. The barrier for exothermic oxidative addition is higher at Pt than at Ni because of strong d-p repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt-F bond with a considerably lower stretching frequency in the oxidative addition product than for... 

Third, to the extent that the catalyst system preferentially utilizes the more stereoselective site for monomer insertion, enhancement of the stereoselectivity at that site will lead to higher isotacticity. Thus, efforts to increase the size of the fluorenyl ring have led to enhanced isoselectivity, presumably because a substituted benzo moiety is more repulsive in the transition state than the benzo group itself. Catalysts derived from fert-butylated fluorenes such as f-16 ([mmmm] = 95.7%, = 153 °C)... 

A cyclic transition state model, that differs from the Zimmerman-Traxler and the related cyclic models inasmuch as it does not incorporate the metal in a chelate, has been proposed by Mulzer and coworkers [78] It has been developed as a rationale for the observation that, in the aldol addition of the dianion of phenylacetic acid 152, the high ti-selectivity is reached with naked enolate anions (e.g., with the additive 18-crown-6). Thus, it was postulated that the approach of the enolate to the aldehyde is dominated by an interaction of the enolate HOMO and the n orbital of the aldehyde that functions as the LUMO (Scheme 4.31), the phenyl substituents of the enolate (phenyl) and the residue R of the aldehyde being oriented in anti position at the forming carbon bond, so that the steric repulsion in the transition state 153 is minimized. Mulzer s frontier molecular orbital-inspired approach reminds of a 1,3-dipolar cycloaddition. However, the corresponding cycloadduct 154 does not form, because of the weakness of the oxygen-oxygen bond. Instead, the doubly metallated aldol adduct 155 results. Anh and coworkers also emphasized the frontier orbital interactions as being essential for the stereochemical outcome of the aldol reaction [79]. 

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