Steric Effects on Reaction Rates. Part IX. Force‐Field Parameters for Bridgehead and Rigid Tertiary Carbenium Ions

Müller, Paul; Mareda, Jiri

doi:10.1002/hlca.19870700414

Cited by 18 publications

(2 citation statements)

References 32 publications

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“…The 7-methyl-7-norbornyl derivatives exhibited significant deviations from the strain-reactivity plot, the cation being apparently too strained [9]. More detailed inspection of the calculated structures revealed that the force-field calculations attributed an unrealistic bond angle of 110.7" to the cationic centre, while semi-empirical methods (MIND0/3) and ah initio calculations (STO-3G) resulted in bond angles of only 98" and 99.5", respectively [9] [lo]. This indicates, that the parametrization is not valid, when large deviations from the ideal bond angles are involved.…”

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confidence: 98%

Steric Effects on Reaction Rates. Part XII. Force‐Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives

Müller

Milin

1991

Helvetica Chimica Acta

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(1 6.VIII.91) Force-field parameters have been developed for the molecular-mechanics calculation of tertiary carbenium ions with tricyclane structure, for tertiary cyclobutyl and cubyl cations. The cyclobutyl parameters are also applicable to tertiary 7-norbornyl cations. Satisfactory plots are obtained for correlation of the rates of solvolysis with the differences in steric energies between carbenium ions and the corresponding bromides.Introduction. -Bridgehead derivatives constitute a mechanistically homogeneous series in solvolysis reactions. Their solvolytic reactivity has been investigated systematically by Schleyer and coworkers [I-31. The reactions were found to be dominated by strain changes upon change of hybridization of the reacting C-atom from sp3 to sp2 in going to the transition state. These strain changes were calculated by molecular mechanics [3], and they were successfully correlated with the experimental rate constants (log k ) for solvolysis of bridgehead chlorides, bromides, p -toluenesulfonates, and trifluoromethane sulfonates. The solvolytic reactivity of bridgehead derivatives varies enormously with the structure of the substrate, and it is impossible to measure the rate constants with one and the same leaving group and under identical experimental conditions. For quantitative strain-reactivity correlations, a uniform reactivity scale is a prerequisite. Such a reactivity scale has been set up. Bentley and Roberts [4] have critically established rate constants for several bridgehead derivatives with different leaving groups. It was found that relative rate constants in the series are almost independent from the leaving group and the solvent, and subsequently to this observation, correction factors for solvent and leaving group changes (chlorides, bromides, p-toluenesulfonates, and trifluoromethane sulfonates) have been proposed. A reactivity scale for standard conditions (TsO leaving group solvolyzing at 70°in 80 % aq. EtOH) spanning a rate range of 22 log units was established. Similarly, conversion factors for p -nitrobenzoate solvolysis and for acid-catalyzed dehydration of tertiary alcohols in the bridgehead and nonbridgehead series have been determined by using linear free-energy relationships [5]. The extrapolated rate constants, when taken from different conditions, agree in general within 0.5 log units.The original strain-reactivity calculations for bridgehead derivatives were carried out with a preliminary force field which was later abandoned. More recently the strain calculations of Schleyer have been repeated with a carbenium ion force field similar

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Steric Effects on Reaction Rates. Part XII. Force‐Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives

Müller

Milin

1991

Helvetica Chimica Acta

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“…In the case of 35 a correction of -6.93, based on comparison of calculations with 34 was used. The strain-reactivity correlations are of log kCalc = -AAE,t + B (3)(4)(5)(6) The quality of the fit is characterized by the correlation coefficient, r, and by the standard deviation of the experimental points from the regression line, c. The parameters for t h e correlations a r e summarized i n Table 11. The d a t a for t h e nonbridgehead compounds are shown in Figure I, where the MM2 results are indicated by crosses, and the BIGSTRN calculations by circles.…”

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Steric effects on reaction rates. XI. Solvolysis of tertiary carbon substrates rationalized by molecular mechanics calculations

Müller

Mareda

1989

J Comput Chem

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The steric energy difference (Est) between tertiary carbenium ions (R+) and the corresponding alcohols has been calculated by MM2 for a series of tertiary nonbridgehead substrates and correlated with their rate of solvolytic reactivity. Satisfactory correlation is obtained, except for p-nitrobenzoates of highly congested substrates. The slope and intercept of the correlations remain almost unchanged if bridge-head substrates are included in the plot. However, the quality of the fit is better for bridgehead substrates alone

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A new method for empirical force field calculations on localized and delocalized carbocations

1996

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A new empirical force field method for localized and delocalized carbocations is described. Additional geometry parameters for carbocations were added to Allinger's MMP2 molecular mechanics program, which can treat delocalized π‐systems. The effect of hyperconjugation in carbocations is introduced via a quantum chemical term into force field calculations for the first time. The calculated heats of formation are in excellent agreement with a wide range of experimental data; the largest deviations are about 3.5 kcal/mol. The calculated structures agree very well with those computed at correlated ab initio levels (MP2(full)/6‐31G*). The relative energies and geometries of different conformations of representative carbocations also were in good agreement with MP4/6‐31G*//MP2(full)/6‐31G* results. © 1996 by John Wiley & Sons, Inc.

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Steric Effects on Reaction Rates. Part IX. Force‐Field Parameters for Bridgehead and Rigid Tertiary Carbenium Ions

Cited by 18 publications

References 32 publications

Steric Effects on Reaction Rates. Part XII. Force‐Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives

Steric Effects on Reaction Rates. Part XII. Force‐Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives

Steric effects on reaction rates. XI. Solvolysis of tertiary carbon substrates rationalized by molecular mechanics calculations

A new method for empirical force field calculations on localized and delocalized carbocations

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