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Generation and rearrangement of spirocyclopropane‐substituted 2‐norbornyl cations
Author(s) -
Kirmse Wolfgang,
Landscheidt Heinz,
Schleich Annette
Publication year - 1992
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610050104
Subject(s) - 2 norbornyl cation , chemistry , bicyclic molecule , carbocation , cyclopropane , heptane , nucleophile , alkene , nucleophilic substitution , stereochemistry , ring (chemistry) , reactivity (psychology) , computational chemistry , medicinal chemistry , organic chemistry , medicine , alternative medicine , pathology , catalysis
Access to spiro(bicyclo [2.2.1] heptane‐2,1' ‐cycloprop‐6‐yl) derivatives was gained from the alkene spiro(bicyclo [2.2.1] hept‐5‐ene‐2,1'‐cyclopropane) via separation of positional isomers. Spiro(bicyclo [2.2.1]‐2,1'‐cycloprop‐ exo ‐6‐yl) p ‐toluenesulphonate (10) and spiro(bicyclo [2.2.1] heptane‐2,1'‐cycloprop‐ exo ‐6‐yl) trifluoroacetate were found to solvolyse faster than the analogous exo ‐2‐norbornyl esters, as predicted by theory. Ion‐pair recombination, with the formation of tricyclo [4.2.1.0 3,7 ] non‐3‐yl p ‐toluenesulphonate, accounts for previous failures to assess the true reactivity of 10. An intervening bridged carbocation (3), labelled with deuterium, was shown to achieve equivalence of C‐1 and C‐6 prior to ring expansion. The rate of the formal Wagner–Meerwein rearrangement is estimated to be of the order of molecular vibrations, thus supporting the symmetrical bridged structure of 3. Methyl substitution at C‐6 was found to direct nucleophilic attack exclusively to the tertiary carbon, and ring expansion preferentially to the secondary carbon. An equilibrating pair of 6(1)‐methylspiro(bicyclo [2.2.1] heptane‐2,1′‐cycloprop‐6‐yl) carbocations is thought to explain these observations most reasonably.