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Cycloadditions of 2‐diazopropane to bicyclo[2.2.1]hept‐2‐enes. Direct experimental evidence for an asynchronous mechanism
Author(s) -
Majchrzak Michael W.,
Warkentin John
Publication year - 1990
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.610030511
Subject(s) - chemistry , norbornene , cycloaddition , bicyclic molecule , regioselectivity , concerted reaction , dicyclopentadiene , stereochemistry , photodissociation , double bond , computational chemistry , medicinal chemistry , photochemistry , organic chemistry , catalysis , polymerization , polymer
2‐Diazopropane, generated by photolysis of 2‐methoxy‐2,5,5‐trimethyl‐Δ 3 ‐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.