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

H
H
X
+ O
N
H
RLi
O
N
E X = CH2, NCH3
N
229
H
Z
−
O
N
X
NITRONE REACTIONS 235
+ + +
RLi
N
N
O
−
RLi
RLi
− RH N
a
H
O
RLi
H
R
X
Li
− − −
O
N
+ + +
Scheme 2.128
O
N
Li
H
− −
Li +
Li
O
+
− − −
+ − +
Li
•
+
•
311
Scheme 2.129
b
N
N
N
O
X
N
311c
O
N
2.6.5.3.3. 13 C, 14 N, and 7 Li NMR Spectra of Dipole-Stabilized Organolithium
derivatives of cyclic nitrones Recently, NMR spectra of the compound resulting
from metalation of the cyclic aldonitrone 1,2,2,5,5-pentamethyl-3-imidazoline-
3-oxide (229) has been described (554). The chemical shift of the α-C atom
is located in the region typical of both carbanion (a) and carbenoid (b) carbon
atoms (213.14 ppm). The 14 N NMR spectrum of the metalated fragment and the
calculated charge distribution data in a model compound show a considerable
contribution of the carbenoid form (b) in structure (311c) in the ground state
(Scheme 2.129).
2.6.6. Nucleophilic Reactions of Nitrones
2.6.6.1. Reactions of Nucleophilic Addition The high tendency of nitrones
toward nucleophilic attack at the α-carbon atom is determined by both, the electrophilic
character of the nitrone group per se (Fig. 2.10, structure C) and by
Li
N
R
O −