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

498 NITRONATES
The physicochemical characteristics of different types of nitronates were considered
in detail in a review (266). Many data on this problem were included in
a recent monograph (267).
3.3.2.1. UV and IR Spectroscopy of Nitronates Table 3.9 contains selected
data from UV and IR spectroscopy of different types of nitronates. It is quite
evident that characteristic spectral parameters are virtually independent of the
nature of the fragment bound to the oxygen atom of the nitro group.
The UV spectra of nitronates, which are not functionalized at the α-C atom,
have an intense absorption at 230 to 240 nm, which is very similar in characteristics
to UV absorption of salts of nitro compounds and solutions of aci-nitro
compounds in protic solvents. Since standard alkyl- or silyl nitronates cannot
have ionic structures, the presence of the above mentioned absorption in the UV
spectra of nitronates, unambiguously conÞrms, that these compounds have the
structures of O-esters.
The introduction of substituents, which have a mesomeric effect and extend
the chain of conjugation, at the α-C atom of nitronates leads to a shift of the
absorption 260 to a maximum of 320 nm. It should be noted that the UV spectra
of dimers of dialkylboron nitronates (entry 8) are virtually identical to those of
other types of nitronates.
The IR spectra of unfunctionalized nitronates show an intense C=N stretching
band between νas of the nitro group (1550–1560 cm −1 )andtheC=N stretching
band in oximes (1650–1685 cm −1 ) (269).
This band in the spectra of acyclic nitronates is moderate in intensity, whereas
the intensity of this band in the spectra of cyclic nitronates is substantially higher.
In Reference 267, the vibrational frequencies of nitrones, nitronates, and
oximes were compared and it was concluded that the C=N vibrational frequency
in the IR spectra of these derivatives decreases in the following series:
oximes > nitronates > nitrones.
In the analysis of nitronates containing substituents, which extend the conjugation
chain, the IR spectroscopic data are uninformative for the structural
assignment because C=N absorption is shifted to lower frequencies (see entries
3, 6, 7, 9, 13, and 15), whereas the νas band of the nitro group is, on the contrary,
slightly shifted to higher frequencies with the result that the characteristic ranges
of these vibrations overlap. However, study of nitronate MeO2CCH=N(O)OMe
(see entry 6) demonstrated that the C=N stretching band can be revealed by
Raman spectroscopy (203). Actually, this band is clearly observed in the Raman
spectrum of the nitronate, whereas the intensity of the νas band of the nitro group
in the spectra of analogous nitro compounds is, on the contrary, very low.
IR spectroscopy can be used with advantage for the determination of the
coordination site of the boron atom in boryl nitronates (230) (see Chart 3.4).
In the IR spectrum of nitronate (76b), the νstretch.C=O band is shifted to lower
frequencies by approximately 150 cm −1 accompanied by a decrease in the intensity
of this signal by more than twice due to coordination of the boron atom at
the carbonyl group.