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

C N OH
OH
C N OH
+
+
OH
B
Si - triorganosilyl group
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 627
C N OSi
OSi
C N OSi
+
+
OSi
B′
or
C N OAlk
OSi
C N OAlk
+
+
OSi
B′′
Chart 3.22 New approach to the generation of iminium cations from AN.
temperature) and, consequently, it is not promising as a methodological approach
for most of synthetically important carbon-centered nucleophiles Nuc. In this
connection, the use of AN silylation instead of AN protonation and, consequently,
the use of aprotic solvents holds considerable promise. Recently, this approach
has been developed by Russian researchers (274) (Chart 3.22).
It can easily be seen that intermediates B ′ and B ′′ are generated, respectively,
in silylation reactions of SENAs (Chart 3.20) or alkyl nitronates (Scheme
3.202). To estimate the efÞciency of this approach, it is necessary to reveal
whether cationic intermediates B ′ and B ′′ can exist as kinetically independent
species.
3.5.2.1. Direct Low-temperature Observation of Bis-N,N-Oxyiminium Cations
For this purpose, the reactions of various types of nitronates (348) with trialkylsilyl
trißates in CD2Cl2 were studied by low-temperature NMR spectroscopy. More
than 10 iminium cationic intermediates (349) were detected (Scheme 3.204)
(478).
It should be noted that the signals of the –CH=N fragment in all cations
(349) are characterized by strong paramagnetic shifts compared to the analogous
resonances of the starting nitronates (348) (the signal for the proton is shifted
O
O
R 1
R 2
N
O
N SiOTf
R
N
OR
OR
1
R2 +
OSi
+ _
OTf
348 349
OEt
+
An
Me2Bu t SiOTf
OEt
O N
SiO
Ionic pair
An
H
SiO +
O N
348a 349a 349a′
+
OEt
H
An
Si – Me 2Bu t Si, An – 4-MeOC6H4 349a : 349a′ ~ 1 : 1,5
Scheme 3.204
+