Steric Effects on Reaction Rates. Part VIII. The Significance of Front Strain Release in Solvolysis of Bridgehead Derivatives

Abstract

The steric requirements of leaving groups for 14 bridgehead derivatives have been examined using MM2 calculations. The strain varies almost monotonously throughout the series upon variation of the leaving group from H to C1, OH, CH3, CH3CH2O, (CH3)3CO, (CH3)3C and no significant trends for differential F-strain effects are detected expect for the perhydrophenalene derivative 13. The experimental rates of solvolysis of bridgehead derivatives correlate well with the calculated steric energy differences between substrate R-X and the corresponding carbenium ion R. However, the strain calculations using the more recent force-fields (MM2) disagree, in part, with those reported in the literature: chloride and p-toluenesulfonate leaving groups correlate with identical slopes, and the perhydrotriquinacene derivative 10 shows no anomalous behavior. The calculations suggest that F-strain and C,C-hyperconjugation should not play any dominant role in bridgehead solvolysis

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

“…On the one hand, the forcefield applied to carbenium ions (UNICAT 1) was a trial set of parameters and had not been extensively tested [5]. On the other hand, Bingham and Schleyer [3] had reported important variations of the kOTJkBr rate ratios which are irreconcilable with our results.…”

“…Comparison of this correlation with the previously reported one which is based on rates relative to I-adamantyl [5] reveals a fit of about the same quality. Similarly, with OH as leaving-group model, the correlation is almost unchanged, except for the intercept (Eqn.…”

“…A factor of 5.0 x lo9 was used in order to convert these rates to those of OTs derivatives solvolyzing in 80% EtOH/70". Values of dE,,(R+ -RH) for 1 6 1 9 were calculated as for the bridgehead substrates using UNICAT 1 [5] (Table I ) and are included in Fig. I (stars).…”

“…UNICAT 3 was obtained by modification of UNICAT 2 in the following way: The 'ideal' bond angles adjacent to the cationic center were varied according to their substitution pattern [5]. The constant for C-C+-C deformation was set to 2.0 and that for out-of-plane deformation to 1.1.…”

“…We have attempted a re-parametrization using the solvolysis rates of 6 bridgehead (1, 2,7,8,9 and 10) and 4 tertiary methyl-substituted compounds (16-19) as reference. The parameters in the previously reported force-fields UNICAT 1 [5] and UNICAT 2 [15] were systematically modified, first by changing the balance between those for deformation of the valence-bond angle and the out-of-plane deformation of the cationic center 1161. Later on, small modifications were applied to other terms.…”