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

RCH2CH2CHNO2
319
H a
B
− BH +
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 609
RCH2CH2CH(NO2)
RCH 2CH
A
321
CHN(OSi) 2
Scheme 3.187
− X− − SiX
BH
B
SiX
O
RCH2CH2CH N
OSi
320
OSi
+
RCH2CH2CH N
OSi
B
The above analysis shows that attempts to separate the two silylation steps of
AN can be successful due to the difference in the mechanism.
Main problems of monosilylation of AN were considered in detail in Section
3.2.1.3. Transformations of SENAs were described in Section 3.4. Hence, the
present section deals only with some aspects of the chemistry of these compounds
and derivatives which were not covered in previous sections and which
are associated with the process shown in Scheme 3.184.
3.5.1.1. Modeling of Classical Reactions of AN with the Use of Silyl Nitronates
Classical C,C-coupling reactions of AN anions (Henry, Michael, and Mannich)
involve complex systems of equilibria and, consequently, generally not performed
in protic solvents. The introduction of the silyl protecting group allows one
to perform these reactions in an aprotic medium to prepare or retain products
unstable in the presence of active protons. In addition, the use of nucleophiles
which are speciÞcally active toward silicon (e.g., the ßuoride anion) enables one
to design a process in which the effective concentration of α-nitro carbanions is
maintained low.
3.5.1.1.1. Silyl Nitronates in Henry Reactions Topologically, condensations of
SENAs with carbonyl and nitroso groups, as well as with an imino fragment,
belongs to Henry reactions.
In these processes, three transition states are possible (Scheme 3.188)
In the Þrst case, SENAs in the presence of various catalysts (primarily salts
containing the ßuoride anion) generate the corresponding α-nitro carbanions,
which are poorly solvated in aprotic solvents and, consequently, rapidly react
with substrates R 3 −Y = X to give functionalized nitro compounds through the
transition state A.
The second type of reactions is an electrocyclic process occurring as the ene
reaction (the transition state B in Scheme 3.188).
Finally, the possible transition state C contains a ligated metal atom and two
coordinated reagent molecules, which are involved in the C,C-coupling reaction.
X
−