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Norpinyl‐norbornyl rearrangements: Destabilized 2‐norpinyl cations
Author(s) -
Fendel Wolfgang,
Kautz Cornelia Brigitte,
Kirmse Wolfgang,
Klar Matthias,
Siegfried Rainer,
Wonner Aribert
Publication year - 1995
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.1995199510244
Subject(s) - 2 norbornyl cation , chemistry , carbocation , bicyclic molecule , stereochemistry , counterion , medicinal chemistry , ring (chemistry) , ion , electron acceptor , photochemistry , organic chemistry
In order to explore the effect of electron‐withdrawing substituents on 2‐norpinyl (bicyclo[3.1.1]hept‐2‐yl)→2‐norbornyl (bicyclo[2.2.1]hept‐2‐yl) rearrangements, cyano and penta‐fluoroethyl groups were introduced into the α and β (= bridgehead) positions. The substrates were synthesized from norpinan‐2‐one ( 8 ) by addition of HCN or C 2 F 5 Li, from 2‐aminonorbornane‐2‐carbonitrile ( 25 ) by rearrangement, and from 1‐(pentafluoroethyl)bicyclo[2.1.1]hexan‐2‐one ( 51 ) by ring expansion. Arenesulfonates and diazonium ions were used to generate the 2‐norpinyl cations. – Electron acceptors in the α position of 2‐norpinyl substrates promote the norpinyl→norbornyl rearrangement. Internal return of the counterion leads to endo ‐2‐norbornyl sulfonates, often without formation of the analogous alcohols, i.e., the destabilized norpinyl cations do not escape from tight ion pairs. Electron acceptors in the β (bridgehead) position favor displacement and elimination processes with retention of the norpinyl structure while the norpinylnorbornyl rearrangement is inhibited, more strongly by β‐C 2 F 5 than by β‐CN. The tendency to separate the positive charge from the electron acceptor explains the divergent effects of α and β substitution. – The k H / k α‐CN and k H / k β‐CN rate ratios in the 2‐norpinyl and 2‐norbornyl series are virtually the same, and the stereoselectivity of the intervening carbocations is not significantly affected by electron‐withdrawing substituents.