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

468 NITRONATES
NO 2
O N
NO2 O
O N
42 47 42a 47a
O
O
•
N
R
H
CHClNO 2
O
N2O4
CHRNO2
NO 2
48 42b–d 47b–d
slowly
O
COCl
47c 47e 47f
N
O
Scheme 3.47
NH 3
O
N
CH(NO2)R
O
O
N
CONH 2
O
R = H (b)
Cl (c)
Br(d)
four-membered cyclic nitronates (47), viz., oxazete N-oxides. Clearly, the equilibrium
is virtually completely shifted toward nitroalkenes due to large strains in
molecule (47). Isomers (47) were discussed only as reactive unstable intermediates
in thermal (165) and photochemical (166) destruction of α-nitroalkenes (42).
However, the introduction of sterically hindered substituents at the β-carbon
atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated
nitroalkenes. Apparently, steric hindrance caused by two bulky Bu t groups
in product (42a) (Scheme 3.47) prevents effective conjugation of the π systems of
the C,C double bond and the nitro group, thus causing its deviation from the plane
of the C=C bond as a result of which isomer (47a) becomes thermodynamically
more favorable.
As a result, nitroalkene (42a) is almost completely isomerized into N-oxide
(47a), which is stable up to 100 ◦ C (167). Isomerization proceeds smoothly both
in solutions and the solid state. The isomerization rate is approximated by the
Þrst-order equation and depends on the polarity of the solvent (isomerization in
hexane occurs 70 times more slowly than that in ethanol).
Nitronate(47a) is not the only oxazete derivative. For example, sterically hindered
nitroalkenes (42b–d) can be prepared by nitration and halogenation of
readily available allenes (48). Compounds (42b–d) are rather smoothly isomerized
into the corresponding four-membered cyclic nitronates (47b–d) bythe
Þrst-order reaction equation (168). Storage of nitronate (47c) is accompanied by
its slow transformation into acid chloride (47e) from which amide (47f) can be
easily synthesized.