15013-89-5Relevant academic research and scientific papers
Geometrical Isomerization and Restricted Rotation in Iminoxyl Radicals from Benzaldoximes
Lucarini, Marco,Pedulli, Gian Franco,Alberti, Angelo
, p. 1980 - 1983 (2007/10/02)
The E geometrical isomers of benzaldoxime, p-chlorobenzaldoxime, and p-methylbenzaldoxime have been oxidized to the corresponding iminoxyl radicals within the cavity of an EPR spectrometer by using photolytically produced tert-butoxy radicals.The observed iminoxyls retain the same geometry (E) of the starting oximes at low temperature (ca. 190 K), while at room temperature they are almost completely converted to the more stable Z isomers.At an intermediate temperature (233 K), where both isomeric species were simultaneously detected, the rate of geometrical isomerization could be determined as 18 +/- 6 s-1 for the radical from p-chlorobenzaldoxime.The estimated energy barrier (ca. 12 kcal mol-1) is much lower than that calculated in the parent oximino compounds (ca. 60 kcal mol-1).A rationalization of this effect in terms of Molecular Orbital theory is given.In the Z isomers of the examined iminoxyls, selective broadening of the EPR lines due to restricted rotation of the aryl ring was observed in the temperature range from 250 to 330 K.An analysis of the line shape variations with temperature performed on iminoxyl 5 provided a value of the rotational barrier of 6.18 kcal mol-1.
STEREOCHEMISTRY OF IMINOXY RADICALS
Alberti, Angelo,Barbaro, Gaetano,Battaglia, Arturo,Guerra, Maurizio,Bernardi, Fernando,et al.
, p. 742 - 750 (2007/10/02)
Iminoxy radicals of general structure Ar-C(X)=N-O, where X=H, CH2OH, n-Bu, t-Bu, SiEt3, SiPh3, GePh3, SnMe3, SnBu3, SnPh3, SMe, SBu, SPh, Cl and Br, have been photolytically generated from the parent oximino compounds or from aromatic nitrile N-oxides in an aprotic solvent.Two configurational isomers, interconvertible in solution, have been detected by electron spin resonance spectroscopy for the majority of these radicals.The preferred geometry of iminoxyls derived from ortho-unsubstituted benzaldoximes is that which places the aryl ring and the oxygen atom on the same side of the C=N double bond (anti).Substitution of the azomethine proton leads to a stabilization of the syn configuration, the effect being larger the greater the atomic number of the leading atom of the substituent group.The relative stability of the syn isomer is also increased by substitution of the aromatic ortho protons.INDO calculations have been carried out on several model systems in order to rationalize the experimental results.The effects responsible for the configurational preference of the different terms of this series of radicals are discussed in terms of a perturbation molecular orbital (PMO) approach.
