86318-23-2Relevant academic research and scientific papers
Stereochemistry and Product Distribution in the Singlet Oxygen Cycloaddition with 7,7-Disubstituted 1,3,5-Cycloheptatrienes
Adam, Waldemar,Adamsky, Friedhelm,Klaerner, Frank-Gerrit,Peters, Eva-Maria,Peters, Karl,et al.
, p. 1848 - 1859 (2007/10/02)
The photosensitized singlet oxygenation of 7,7-dimethyl-(1a), 7-(methoxycarbonyl)-7-methyl- (1b), 7-methyl-7-phenyl- (1c), and 7--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 were 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 CO2Me.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 substituent 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.
