Bridged, chiral (A) and open, achiral intermediates (B) can occur during the cationic norbornyl → norpinyl rearrangement. As stereochemical results show, CN and CF3 substituents favor A, double bonds (allyl resonance) in contrast favor B. Combination of these structural elements allows the proof of both types of carbocations.

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Stereochemistry of Norbornyl → Norpinyl Rearrangements.In order to clarify the nature of intermediates in the title reaction some stereochemical investigations are carried out. The formation of enantiomerically pure (II) in the deamination of (I) suggests that the intermediate has the bridged chiral structure (III). In contrast double bonds (allyl resonance) as in (IV) favor the achiral allyl cation (VI) as the product-determining intermediate as concluded from the formation of predominantly racemic (V). Combination of both these structural elements, double bond and CN or CF3 substituent, as in (VII) affords the isomers (VIII) and (IX) in a 1:2 ratio via an achiral allyl cation of type (VI). Only (VIII) can also be formed via the chiral type of intermediate.