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Stereochemistry and product distribution in the singlet oxygen cycloaddition with 7,7‐disubstituted 1,3,5‐cycloheptatrienes
Author(s) -
Adam Waldemar,
Adamsky Friedhelm,
Klärner FrankGerrit,
Peters EvaMaria,
Peters Karl,
Rebollo Hector,
Rüngeler Wolfgang,
von Schnering Hans Georg
Publication year - 1983
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19831160512
Subject(s) - chemistry , cycloaddition , isomerization , steric effects , singlet oxygen , singlet state , derivative (finance) , stereochemistry , medicinal chemistry , photochemistry , oxygen , organic chemistry , catalysis , excited state , physics , nuclear physics , financial economics , economics
The photosensitized singlet oxygenation of 7,7‐dimethyl‐ ( 1a ), 7‐(methoxycarbonyl)‐7‐methyl‐( 1b ), 7‐methyl‐7‐phenyl‐ ( 1c ) and 7‐[4‐(methoxycarbonyl)phenyl]‐7‐methyl‐ ( 1d ) 1,3,5‐cycloheptatrienes has been investigated. While 1b – 1d give only the respective norcaradiene endoperoxides 2b‐N – 2d‐N , the dimethyl derivative 1a affords a 60:40 mixture of tropilidene ( 2a‐T ) and norcaradiene ( 2a‐N ) endoperoxides. In addition to full spectral characterization, the norcaradiene endoperoxides 2b‐N – 2d‐N where thermally isomerized to their diepoxides 3b – 3d . X‐ray analysis confirmed that for 3b the stereochemistry of the substituents is 7‐ endo ‐methyl and 7‐ exo ‐methoxycarbonyl and for 3c it is 7‐ endo ‐phenyl and 7‐ exo ‐methyl, showing that the steric size of the substituents in the cycloheptatrienes is Ph < Me < CO 2 Me. While the tropilidene endoperoxide 2a‐T was a stable compound and could be fully characterized, the norcaradiene derivative 2a‐N was too labile and had to be reduced with diimide to the saturated endoperoxide 4a to enable complete characterization. It is shown that the cycloaddition of singlet oxygen is a useful method to assess qualitatively the substitutent effects on the tropilidene‐norcaradiene valence isomerization, especially for those cycloheptatrienes in which the N/T ratio is too low for measurement by dynamic NMR methods. Stabilization of the norcaradiene isomer by 7‐methyl substitution is rationalized in terms of steric effects.