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

ELECTROCHEMICAL PROPERTIES AND EPR-SPECTRA 195
2.3.4.3. 14 Nand 17 O NMR Spectra NMR spectra of 14 Nand 17 O nucleus
are quite limited and mostly concern conjugated C,N -diaryl-nitrones (416–419).
Recently (420), for derivatives of 3-imidazoline-3-oxide, using 14 N and 17 O
NMR, the inßuence of substituents and hydrogen bonds on chemical shifts and
the range of chemical shift changes of nitrogen and oxygen of the nitrone group
has been determined. Both in 17 O NMR spectra, and 14 N NMR spectra in the
series of the derivatives examined, highest Þeld signals of N + -O − group are
those of amino derivatives, while the low Þeld ones are those of cyano derivatives.
Depending on the substituent (from amino- to cyano group), 17 O chemical
shifts tend to change in the range of 155 ppm, with the interval of such changes
in 14 N chemical shifts for the same substituents being 110 ppm. The range of
17 O chemical shifts in α,N -diaryl nitrones is 350 to 409 ppm (421).
The result of a study of 15 N NMR spectra of cyclic nitrones of 3-imidazoline-
3-oxide and of the corresponding nitroxyl radicals has been reported (422).
The hyperÞne interaction (HFI) 1 J ( 15 N 13 C) constants, both in E and Z -
isomers, have a negative sign and are close in absolute value (−21.5 and −21.2 Hz
respectively). Heminal constants 2 J ( 15 NH) in aldonitrones have close absolute
values, but differ in sign; in E-isomers the sign is positive ( + 2.1 Hz), in
Z -isomers it is negative (∼−2.3 Hz). Vicinal constants in keto nitrones 3 J( 15 NH)
in both isomers have a negative sign and a value close to ∼−3.3 Hz (423).
2.4. ELECTROCHEMICAL PROPERTIES AND ELECTRON
PARAMAGNETIC RESONANCE (EPR)-SPECTRA OF NITRONE
RADICAL IONS
EPR study of electrochemical properties of nitrones and registration of resulting
radical cation (RC) or radical anion (RA), such as, in the nitrone transformation
into nitroxyl radicals, allows us to get direct answers to the questions concerning
mechanisms of nitrone group reactions. The following schemes A–E can be
realized depending on conditions as below (Scheme 2.77):
A: nucleophilic addition of strong nucleophiles with further oxidation
(Forrester-Hapburn mechanism);
B: acid-catalyzed nucleophilic addition;
C: radical addition (spin trapping);
D: nitrone oxidation to RC with the subsequent addition of weak nucleophiles
(reversed spin trapping);
E: nitrone reduction to RA and addition of electrophiles.
To study mechanisms C–E, it seems reasonable to employ both, electrochemical
approaches and EPR-spectroscopy. It is important to be aware of the
electrochemical properties of nitrones if used as spin traps; for production of spin
adducts (SA) is possible not only via homolytic process (C) but also via ionic
processes shown in Scheme 2.77. In the case of (B), protonation can protect the