Abstract 2‐Diazopropane, generated by photolysis of 2‐methoxy‐2,5,5‐trimethyl‐Δ3‐1,3,4‐oxadiazoline, was trapped in situ by cycloaddition to norbornene, endo‐ and exo‐5‐methylnorbornene, endo‐ and exo‐5‐phenylnorbornene, endo‐ and exo‐5‐methoxycarbonyl norbornene and endo‐ and exo‐dicyclopentadiene. In all cases, only exo addition was observed. In spite of the exo approach of diazopropane to the norbornene double bond, endo substituents at C‐5 and/or C‐6 influenced the regiochemistry of addition whereas exo substituents did not. The results are interpreted in terms of a concerted, asynchronous mechanism in which CC bond formation runs well ahead of NC bond formation, as predicted from theory. The regiochemical control exercised by apparently remote endo substituents provides a new experimental criterion for asynchrony or synchrony of cycloadditions that are known to be concerted.
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Majchrzak et al. (1990) studied this question.
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