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

184 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
O
C N
C
O
−
C N
O
C N
A B C D
E
−
+ + +
O
N
−
C
F
O
+
N
Fig. 2.10
+
C
O
C N
of a pure double bond nature, and that between N and O atoms is of a partial
double bond nature. This is established not only by zwitterionic A and B structures
but also by the hypervalent E structure. However, neither experimental
evidence, nor computational data have been reported for C and D structures,
which are characterized by the single nature of all bonds.
For the Þrst time, the primary nitrone (formaldonitrone) generation and the
comparative quantum chemical analysis of its relative stability by comparison
with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described
(357). Both, experimental and theoretical data clearly show that the formaldonitrones,
formed in the course of collision by electronic transfer, can hardly be
molecularly isomerized into other [C,H3,N,O] molecules. Methods of quantum
chemistry and molecular dynamics have made it possible to study the reactions of
nitrone rearrangement into amides through the formation of oxaziridines (358).
On the basis of photoelectron spectra and quantum-chemical computations, the
effect of variation of substituents at N − 1 atoms in derivatives of 1-R-3-imidazoline-3-oxide
has been studied. It has been found that the increase in ionization
energy π−C=NO-MO occurs in the series CH3 ≤ H ≤ OH ≤ O . ≤ NO
(359, 360).
Recently (361), on studying the full charge distribution in (Z )-N -methyl-Cphenylnitrone
by high resolution X-ray diffraction and by comparing theoretical
high level computational data, it was concluded, surprisingly, that in the N + -O −
group both atoms carry a negative charge.
The study of the stabilizing effect of the nitrone group in cumyl radical (205)
(Scheme 2.72) (362) and cumyl cation (206) (Scheme 2.73) (363) has shown that
the nitrone group appears to be a “super radical stabilizer” and, at the same time,
a “weak cation stabilizer.”
Analysis of the conformational and structural stability of N -vinylnitrone
CH2=CH–N(O)=CH2 and N -(2,2-dichlorovinyl)nitrone CCl2=CH-N(O) =
CH2 with DFT-B3LYP and MP2 methods revealed that they have a planar structure
resulting from the apparent conjugation between C=C and N=C bonds.
−
G
O
N
−