Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

STRUCTURE AND SPECTRA OF NITRONES 193
Ph
+
N OH
Ph
O
Ph O
+
N OH
H
−
O
H N
H
E- isomer
OH
Z- isomer
Scheme 2.75
Also, an intramolecular version of E-Z isomerization seems possible
(Scheme 2.75) (304).
An NMR determination of the conÞguration exchange constant of methylenenitrone
(215) in 1,2-dichlorobenzene at 133 ◦ Cgavek= 88.6 c −1 . The estimated
energy of isomerization activation was ΔG = 20.3 kcal/mol (135).
AlowÞeld shift of proton signals of the OH-group in N -(salicylidene)phenylamine-N
-oxides (∼12,7–13,6 ppm) indicates the presence of an intramolecular
hydrogen bond. The value of this shift depends on the pKa value of the parent phenol
(400). While studying solvation effects of 1 H NMR spectra in α-(2-hydroxy-
1-phenyl)-N -(4-substituted-phenyl)nitrones, a Koppel-Palm three-parameter
correlation was detected (401).
Recently, 1 H NMR spectroscopy has been used to study the effect of substituents
in structural PBN (402) and 5,5-dimethylpyrroline N -oxide (DMPO)
(403) analogs on their complexing ability with natural β-cyclodextrin (402, 403).
From a comparative analysis of 1 H NMR spectra of structurally similar pairs of
nitroxyl radicals of 3-imidazoline and 3-imidazoline-3-oxide, it was concluded
that the nitrone group contributes to a more efÞcient long-range spin density
delocalization in the conjugated π-system of functional groups bonded with atom
C-4 (404).
2.3.4.2. 13 C NMR Spectra A systematic study of the effect of various structural
factors and solutions on the chemical shifts of carbon 13 C in the nitrone group and
their correlation with the changes in electronic density has been made (405–407).
Introduction of the N -oxide oxygen atom into the imino group leads to a shift
in the high Þeld of 30 to 33 ppm of the α-carbon signal as compared to the
corresponding imines; this agrees with an increase of electron density (405).
Similar results and conclusions were made from the study of the spectra of a
great number of (Z )-α-aryl N-tert-butylnitrones (395), pyrroline-1-oxides, and
2H - pyrrole -1-oxides (408, 409). The chemical shift of the carbon atom in
the nitrone group is located in the range of 117 to 152 ppm depending on the
electronic inßuence of a substituent in positions 1, 2, 4, and 5 of a heterocycle.
Increasing electrophilicity of the substituent in position 2 leads to a low Þeld α-C
shift due to the decrease of electronic density (405). The converse inßuence can
be observed for substituents in positions 1, 4, and 5 (406). In solutions, in which
hydrogen bonds are formed with the N-oxide group, one observes α-C signal shift
in the low Þeld at 1.5 to 2.5 ppm for solution in chloroform and at 5 to 9 ppm
for solution in methanol. The shift value depends on the substituent in position
−