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

X
NO 2
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 657
BSA
X: Cl > CH3COO >> PhNH
MeO 2C
X
Si
X = OAc
A
N O
O
CO 2Me
CH 2OCOMe
O N OSiMe 3
416
Scheme 3.225
−SiX
X = Cl
3.5.3.2. Elimination Reactions with N,N-Bis(siloxy)Enamines
3.5.3.2.1. 1,2-N,O Elimination BENAs are chemical equivalents of α-nitrosoalkenes
and, consequently, can give very active intermediates under various conditions
(Scheme 3.226).
First, α-nitrosoalkenes can be generated from BENAs in the presence of bases,
for example, of triethylamine (499), which can initiate chain decomposition of
these nitroso acetals (Eq. 1). Then, the more active trialkylsiloxy anion is involved
in this process. (Apparently, this is responsible for the well-known spontaneous
decomposition of BENA in the presence of nitrogen bases.)
Analogous processes with BENAs also occur in the presence of ßuoride anion
(500) (Eq. 2).
Evidently, some acids can also initiate similar chain reactions (501, 502)
(Eq. 3).
Finally, it is known that BENA are thermally unstable (464). α-Nitrosoalkenes
generated from BENAs are very labile intermediates having multiple reactivities
(503). Hence, special efforts should be made to choose the reaction conditions
under which the transformations of BENA follow the desired pathway.
3.5.3.2.2. 1,4–N,C Elimination The reactions of standard BENAs with bases
were considered in the previous section. As a rule, these reactions proceed at
the silicon atom of the nitroso acetal fragment. However, if a EWG-group is
adjacent to the γ-C atom of BENA, the allylic proton (Hγ) at this carbon atom
becomes so labile that it can be eliminated already in the presence of bases at
room temperature (504), thus initiating the transformation of such BENA into
conjugated en oximes (Scheme 3.227).
To summarize Section 3.5.3, the general conclusion is that depending on the
reaction conditions and the nature of the nitro substrate, silylation of AN and
cyclic nitronates afford silyl nitronates (SENA), bis(siloxy)enamines (BENA, or
NO 2
NO2