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

PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 493
Table 3.6 Physical characteristics of typical acyl nitronates
Entry Nitronate M.p., ◦ C Ref. Entry Nitronate M.p., ◦ C Ref.
1 Ph
2
HO
NC
Cl
O 115.8 249 3 OAc
N
O
N
O
Ph
OCO
N
Cl
O
Cl
Cl
188–190 225a 4
O
N
N
O
O
O
N
OAc
174–175 224
O 50/1 220
Actually, in these derivatives the silicon atom bonded to the oxygen atom of
the nitro group contains no protons. Hence, decomposition according to Eq. 1 is
impossible. The fragmentation according to Eq. 2 is also unlikely because the
Si=C double bond is thermodynamically unfavorable.
These effects undoubtedly increase thermal stability of SENAs (many of
these compounds are distilled in vacuo at temperatures higher than 100 ◦ C, see
Table 3.7). At the same time, SENAs are hydrolytically highly unstable (see
Section 3.4.2.2.). Besides, these compounds can undergo spontaneous decomposition
for unknown reasons. It is known that acidic impurities facilitate these
processes, whereas triethylamine, on the contrary, stabilizes SENAs (191). Hence,
SENAs are recommended to be either stored in a refrigerator with full protection
from atmospheric moisture or used in situ.
Typical physical characteristics of SENAs are given in Table 3.7. Gareev
and coworkers synthesized a large number of phosphoryl-containing SENAs and
characterized these compounds by densities and refractive indexes (255, 256).
Boryl nitronates (75–78), (217, 229–231) which have been poorly studied, are
as a rule unstable. It should be noted that dimers (75) are crystalline compounds,
which decompose in the solid state within a few days to give boron-containing
heterocycles (79) (Scheme 3.74).
Other boryl nitronates (76–78), in which boron is coordinated to the functional
group, are as a rule characterized by extraordinary high hydrolytic instability.
Complexes (77) are exceptions. They melt without decomposition at temperatures
higher than 100 ◦ C and are easily isolated from solutions.
3.3.1.3. Stability of Cyclic Nitronates For steric reasons, fragmentation of Þveand
six-membered cyclic nitronates cannot follow pathways presented in Scheme
3.72. Hence, stability of these compounds can be substantially higher than that
of alkyl nitronates. These compounds generally exist in the crystalline state and
can be puriÞed by recrystallization or liquid chromatography. Selected melting
points of nitronates are given in Table 3.8.