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

502 NITRONATES
Nitronates, in which the N -oxide fragment is absent, are characterized by
strong paramagnetic shifts of the signals of the α-C atom and the proton bound
to this atom (see, e.g., entry 5 in Table 3.10 and entries 2 and 9 in Table 3.11).
It is possible to choose between the structures of the true nitro compound and
nitronate based on direct spin–spin coupling constants 1 J 13 C, 15 N which increase
in nitronates by near to twice with reference to true AN (see entry 3, Table 3.12).
The question as to whether the boron atom is coordinated to the carbonyl
group in boryl nitronates can be solved by analyzing the chemical shifts of the
C=O fragment. For example, the signals of the coordinated and uncoordinated
carbonyl groups in the spectrum of boryl nitronate (entry 6 in Table 3.11) differ
by 12 ppm, although these signals in the spectrum of analogous alkyl nitronate
(entry 7 in Table 3.11) are very close to each other. At the same time, the signals
of two carbonyl groups in another boryl nitronate (entries 9,10 in Table 3.11) are
also very similar. This unambiguously demonstrates that the boron atom in the
latter nitronate is not coordinated to the carbonyl group.
The conÞgurations of two isomers of nitronates derived from primary AN
can be unambiguously determined by comparing the chemical shift of the proton
bound to the α-C atom. The signal of this proton in the cis isomer appears at
higher Þeld (cf . entries 3 and 4 or entries 7 and 8 in Table 3.10). (It should be
noted that the signals of the α-C-H protons in the spectra of SENAs are slightly
shifted to higher Þeld compared to the corresponding signals in the spectra of
analogous alkyl nitronates). If only one stereoisomer exists, its conÞguration can
be determined based on the presence (for cis isomers) or the absence (for trans
isomers) of the constant 2 JH, 15 N (see entries 2, 5, and 6–8, Table 3.10). An
analogous dependence is observed also for oximes (223).
The related conÞgurations of stereocenters in substituted cyclic nitronates can
be determined by analyzing the spin–spin coupling constants between the vicinal
protons in the stereoisomer discussed (Chart 3.7) (276). If needed, the results of
this analysis are supplemented by special NOE experiments.
Analysis of nitronates by 14 N and 15 N NMR spectroscopy has an auxiliary
character (see Table 3.12). The 14 N NMR signals are often broadened
and, hence, are difÞcult to observe and are poorly informative, although magnitudes
of their chemical shifts could in principle help in distinguishing between
covalent nitronates and salts. It is difÞcult to observe 15 N NMR signals in
natural-abundance NMR spectra of nitronates, while an introduction of a label is
an expensive procedure.
For most of SENAs, the 29 Si chemical shifts can easily be measured by the
Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) method (see Table
3.13).
These shifts are similar to the 29 Si chemical shifts in the spectra of silyl
derivatives of analogous oximes. This is evidence for the absence of essential
additional coordination of silicon in SENAs. The qualitative and quantitative
analyses of the 29 Si NMR signal can be considered as a simple method of NMR
monitoring of SENAs in solution.
The 11 B NMR chemical shifts (Table 3.13) demonstrate that the boron atom in
all known nitronates is tetracoordinated, that is, it is additionally coordinated by