Solvolysis of 2-Exo-Norbornyl Derivatives

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

The exo endo rate ratio can be correctly predicted without due regard for anchimeric assistance this points to its absence. Hence the initial product to ionize these systems is the classical tertiary 2-norbornyl cation. This conclusion, however, does not preclude the possibility of the subsequent rearrangement of the classical cation into a nonclassical one, nor does it give any evident of the relative stability of classical and nonclassical tertiary 2-horbomyl cations. 

Anchimeric assistance to ionization is only observed when the adjacent group participates by formation of an intermediate more stable than the one without such I rticipation. Consequently, the fact of anchimeric assistance in the solvolysis of 2-exo-norbornyl derivatives is a convincing evidence for the formation of the nonclassical ion 5 as an intermediate in this reaction. 

Jensen and Smart showed the benzoylation of exo-phenylnorbornane 67 to occur faster than that of endo isomer 68  

A similar effect has been discovered by Brown who has recorded a higher rate of solvolysis of p-exo-norbornyl-tert-cumyl chloride 69 in comparison with p-endo isomer 70  

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