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

500 NITRONATES
NO 2
CH 3
CH 3
OMe
O
O
OMe O
76b
N O
BEt 2
ΔV C=0 ~ 150 cm −1 ΔV C=0 < 40 cm −1
R
H
cis
OMe
N
O
R - CO2Me (1.8D)
CN (2.25 D)
Chart 3.4
Chart 3.5
R
N
O
H
trans
OMe
R - CO 2Me (2.63D)
CN (4.7 D)
NO 2
CH 3
CH 3
NH 2
O
O
N O
O NH2 77b
BEt 2
By contrast, the analogous low-frequency shift in the spectrum of nitronate
(77b), in which the boron atom is coordinated at the nitrogen atom of the amide
group, is no larger than 40 cm −1 . (In addition, coordination at nitrogen is con-
Þrmed by the low-Þeld shift and splitting of the signals of the protons of the
NH2 fragment in the 1 H NMR spectrum of product 77b (7.8 and 8.2 ppm) compared
to their signal in the 1 H NMR spectrum of the starting amide (7.2 ppm,
one broadened signal).
French scientists (270) suggested that the conÞgurations of stereoisomeric
acyclic alkyl nitronates can be determined from the relative dipole moments
which for trans- isomers of nitronates containing EWG at the α-C atom are
substantially larger than those of the cis isomers (Chart 3.5).
3.3.2.2. NMR Spectroscopy of Nitronates Due to the informativeness of NMR
spectra and the fact that it is relatively easy to measure these spectra on modern
NMR instruments, NMR spectroscopy is the most important and useful method
for structural and stereoisomeric assignments of nitronates.
Tables 3.10 to 3.13 contain principal characteristic NMR parameters for different
types of nitronates. In spite of evident differences in the structures of the
model compounds used, their 1 Hand 13 C NMR spectra have many common
features.