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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 607
Let us consider the reaction of the standard silylating agent B/Si with AN
(314) containing at least three carbon atoms. For simplicity, all processes will
be described as electrocyclic schemes, although many of them evidently involve
nonconcerted reactions.
In the Þrst step (1), the Hα proton having the highest acidity is eliminated as
BHα + giving rise to SENA (315). However, now the Hβ proton in this substrate
becomes ßexible because it becomes heteroallylic. Hence, the BHβ + fragment
is eliminated in the second step (2) to give bis(siloxy)enamine (316). It should
be noted that the Hγ proton in nitroso acetal (316) also has high acidity because
it becomes allylic. Elimination of the latter proton affords the corresponding
conjugated enoxime (317).
Therefore, AN containing protons at the Þrst three carbon atoms can be considered
as sources of SENA, nitroso acetals, and conjugated enoximes. The process
shown in Scheme 3.184 is actually a redox process, in which the nitro group
is partially reduced, while the carbon skeleton of AN is oxidized, the silylating
agent serving as a mediator, which makes it possible to interrupt the sequence
in the required step. Actually, the strength of both components in the silylating
agent B/SiX can be varied over a wide range. Substituents at the C1 –C3 atoms
also act as mediators and either facilitate the elimination of the corresponding
protons or, on the contrary, hinder the proton abstraction.
It should be noted that enoxime (317) in Scheme 3.184 is not always the last
one in the reaction sequence under consideration. For primary AN, this sequence
can be extended to give Þnally α-silylated conjugated enenitriles (Scheme 3.185).
Evidently, this approach is not limited to the formation of nitronates, nitroso
acetals or enoximes. The rearrangements of these compounds by elimination
reactions, the trapping of intermediates and Þnally their reactions with various
reagents are of equal importance. It should be emphasized that silylation of AN
as a process in organic chemistry is characterized by an unrivalled completeness
and diversity of transformations. Hence, the silylation can be considered as a
separate Þeld of application of AN in organic synthesis.
317
N OSi
Si
C
N
B− / Si + X−
H
A′
N O
B Si
N
Si
Scheme 3.185
− BH
− SiOSi
BH
318
H
N
B− /Si + X−
B
Si
N