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Carbon Participation in the Solvolysis of 6-exo-substituted 2-exo- and 2-endo-Norbornyl p-Toluenesulfonates. Norbornanes part 5

✍ Scribed by Walter Fischer; Cyril A. Grob; Reinhard Hanreich; Georg von Sprecher; Adrian Waldner

Publisher
John Wiley and Sons
Year
1981
Tongue
German
Weight
680 KB
Volume
64
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

The solvolysis rates and products of the 6‐exo‐substituted 2‐exo‐ 1a‐1u, and 2‐endo‐norbornyl p‐toluenesulfonates 2a‐2u, have been determined. In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants σ of the substitutents at C(6); however, their sensitivity to σ is much larger in the 2‐exo‐series 1 than in the 2‐endo‐series 2. This differential transmission of polar effects is the cause of decreasing 2‐exo/2‐endo rate ratios from 2388 for R = t‐C~4~H~9~ to 0.37 for R = Br, i. e. with increasing electron attraction by the substituent. The high sensitivity of the rate constants for the 2‐exo‐p‐toluenesulfonates 1 to σ indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2). This interaction is ascribed to the participation of the pentacoordinate C(6)‐atom, i. e. to 1,3‐bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes. Donor substituents enhance 1,3‐bridging, lead to faster reactions and to the formation of 2‐exo substitution products. Conversely, acceptor substituents reduce 1,3‐bridging, decrease rates and facilitate the formation of 2‐endo substitution products. Graded 1,3‐bridging is discussed in the light of Winstein's nonclassical ion concept.

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