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
Dedicated to Professor AndrP Dreiding on occasion of his 60th birthday (23.VII.79)
SummaryThe C, C-bond cleavage which occurs during chromic acid oxidation of t-butylphenylmethanol (1) and 1,2-diphenylethanol (2) to the extent of up to 67% is reduced to 3% for 1 and 13% for 2 when the reaction is run in acetone, and totally suppressed upon co-oxidation of the alcohols 1 and 2 with oxalic acid. Similarly, the yield of 7-norbornanone obtained from 7-norbornanol (7) is raised in going from acetic acid (24%) to acetone (41%) and approaches 100% in the co-oxidation. With the co-oxidation 5-endo-bicyclo [2.1 .l]hexanol (3) is converted to the corresponding ketone in 54% yield. Mechanistic implications of these results are discussed.
The rates of solvolysis of secondary p‐toluenesulfonates in acetic acid or 97% trifluoroethanol are interpreted in terms of strain changes between substrate and the corresponding ketone. Such strain changes are obtained from force‐field calculations (ΔEst) and from equilibration of alcohols and ketones (ΔGox). This simple model reproduces the behaviour of substrates reacting by kc‐pathways to afford unstrained carbenium ions. Anchimeric assistance and leaving group hindrance in the transition state are recognized in clear‐cut cases by deviations from the expected reactivity. However, the model breaks down when highly strained carbenium ions of the cyclobutyl or 7‐norbornyl type are involved.
SummaryThe rates of oxidation with chromic acid of 15 bi-and polycyclic secondary alcohols have been measured and correlated with strain changes calculated by the MM 1 -program between the alcohols and the corresponding ketones. A correlation of the same quality is obtained upon representation of OH-strain by CH,-strain. The significance of the correlations with respect to the oxidation mechanism as well as the limitations of the applicability of force-field calculations to reactivity problems are discussed.
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