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

Abstract

(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

The Stability of Bridgehead Carbocations

“…The correlation between the standardized rate constants for the solvolysis of bridgehead derivatives, Δlog k (rate constants relative to 1-adamantyl- p -toluenesulfonate in 80% EtOH at 70° and Δ G °(X = Br) has been reported in a previous paper . These rate constants have been extrapolated from a variety of different sources with different leaving groups and solvents and at different temperatures by means of appropriate conversion factors or linear free energy correlations. , The datum for diadamantane was extrapolated from 1-chloro[1]diadamantane ( 3a ) solvolysis in EtOH at 25 °C, relative to 1-chloroadamantane ( k rel = 1.7 × 10 8 s -1 ) 15 by means of the Arrhenius equation.…”

Correlation of Gas-Phase Stability of Bridgehead Carbocations with Rates of Solvolysis and ab Initio Calculations

“…In Figure Δ G ° (4) values are plotted against the standardized rates of solvolysis (log k for solvolysis with OTs leaving groups, in 80% EtOH at 70°, relative to 1-adamantyl- p -toluenesulfonate). , The correlation spans about 23 log units for k . Taking into account that at 70° one order of magnitude in rate constants corresponds to 1.57 kcal mol -1 in Gibbs energy of activation, this amounts to 36.1 kcal mol -1 and almost 50 kcal/mol in Gibbs energies for bromide exchange.…”

“…This suggests that the high solvolytic reactivity of 1a and 2a , as well as that of the corresponding bromides, is due to a front-strain effect, and not to some particular stabilization of the carbenium ions. Although this F-strain effect is treated adequately by force-field calculations of 1a , b and 2a , b , respectively, it was not recognized as such in the past …”

Modern Molecular Mechanics and ab Initio Calculations on Benzylic and Cyclic Delocalized Cations

“…Hyperconjugation was introduced via a quantum chemical term . This new method allows a force field treatment of delocalized carbocations, such as allyl, benzyl, and 1,3-pentadien-5-yl as well as the calculation of resonance energies, − which are quite large in such species. , In contrast, early force field parametrizations were not devised to reproduce carbocation geometries and hyperconjugation effects, but rather to compute strain energies …”