Structural Effects in Solvolytic Reactions. 27. Solvolysis of the exo-and endo-1, 2-Diphenyl-2-norbornyl and -1, 2-Dimethyl-2-norbornyl p-Nitrobenzoates and Chlorides. Definitive Evidence for the Classical Nature of the 1, 2-Disubstituted Tertiary 2-Norbornyl Cations and Implications for the Structure of the Parent 2-Norbornyl Cation

Herbert C. Brown*, M. Ravindranathan, C. Gundu Rao, Frank J. Chloupek, Min Hon Rei

*Corresponding author for this work

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Abstract

exo and endo-1, 2-diphenyl- and -1, 2-dimethyl-2-norbornyl p-nitrobenzoates were synthesized and their rates of solvolysis determined in 80% aqueous acetone. The tertiary chlorides were also synthesized and their rates of solvolysis measured in 100% ethanol. The exo/endo rate ratios for the solvolysis of 1, 2-diphenyl-2-norbornyl p-nitrobenzoates (350) and of 1, 2-dimethyl-2-norbornyl p-nitrobenzoates (564) are similar to the ratios observed for the corresponding tertiary 2-phenyl and 2-methyl derivatives, as well as to those for the secondary 2-norbornyl tosylates. Similarly, the exo/endo ratio observed for the 1, 2-dimethyl-2-norbornyl chlorides (178) is similar to the value previously determined for the epimeric 2-norbornyl chlorides (170). Consequently, the presence of substituents at the 2 position or at the 1, 2 positions has little effect upon the observed exo/endo rate ratios. The introduction of a 1-phenyl substituent into the 2-phenyl-2-norbornyl p-nitrobenzoate does not increase, but decreases sig ificantly the rate of solvolysis (by factors of 21 in the exo and 58 in the endo). A 1-methyl substituent, introduced into 2-methyl-2-norbornyl p-nitrobenzoate, increases the rate. The effect is the same in both the exo (8.5) and the endo (8.6). Similar effects were realized for the ethanolysis of the corresponding tertiary chlorides. The effects of the 1-phenyl and 1-methyl substituents reveal the absence of significant charge delocalization from the 2 to the 1 position in the solvolytic process. It is concluded that these tertiary derivatives must undergo solvolysis without σ bridging and accompanying charge delocalization to the 1 position associated with such bridging. Yet the free-energy diagram for the solvolysis of 1, 2-dimethyl-2-norbornyl p-nitrobenzoate is remarkably similar to 2-methyl-2-norbornyl p-nitrobenzoate and to 2-norbornyl tosylate. It does not appear reasonable to attribute such similar behavior to the operation of totally different physical phenomena. Yet such has been claimed. Tree such proposals which have been advanced are considered and refuted on the basis of available experimental data. Comparison of the rate of solvolysis of 2-methyl-endo-norbornyl chloride with that for endo-norbornyl chloride reveals a relative rate of 53 000. Ignoring minor differences in the ground state energies, this yields a difference in the energies of the tertiary and secondary transition states of 6.5 kcal mol-1. This corresponds to an estimated difference in energy of the 2-methyl-2-norbornyl cation and 2-norbornyl cation under stable ion conditions of 7.5 kcal mol-1 and a difference in the calorimetric heats of ionization of 2-methyl-exo-norbornyl chloride and of exo-norbornyl chloride in SO2CIF of 7.4 kcal mol-1. These results establish that the magnitude of the positive charge at the developing cationic center in these transition states must approach that in the intermediate ions or ion pairs, providing strong support for the validity of the Hammond postulate as applied to solvolytic processes. The similarity in the tertiary/secondary rate ratio for the exo isomers to the value for the endo isomers supports the absence of any significant nonclassical resonance contribution to the rate of solvolysis of exo-norbornyl derivatives. These data yield essentially identical values of 2-Me/2-H, 5.3X104 for endo and 5.5X104 for exo, incompatible with the presence of major nonclassical resonance contributions in the exo secondary and its reduction or absence in the exo tertiary. Other approaches for extrapolating from the tertiary derivatives to the secondary fail to support the presence of a major nonclassical resonance contribution in the exo secondary, absent in the endo secondary and in the exo and endo tertiary derivatives, as postulated in some current proposals.

Original languageEnglish
Pages (from-to)3667-3678
Number of pages12
JournalJournal of Organic Chemistry
Volume43
Issue number19
DOIs
StatePublished - 1978
Externally publishedYes

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