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

610 NITRONATES
SiO
N
O
R 1 R 2
R 3
X
Y
X O NR
=
Y
3 HC
R 3 R 4 O
; ;
HC N
3 3 R R
+
F − M + (K + , Bu4N + )
− SiF
en – reaction
MXn; M = Sc,Cu
− SiX
R 1
R 2
O
O
O O
N
R 1
N
R 1
O
Scheme 3.188
N +
−
O
A
R 2
B
R 2
X
Y R 3
Si
X
Y
R3 M X
C
Y R 3
R 1
R 2
R 1
R 2
R 1
R 2
NO 2
NO2
NO2
Y
R 3
or (and)
3
R
3
R
X H
Y X Si
or (and)
Y X M
The C,C-coupling reactions of SENAs with carbonyl compounds were studied
in most detail (132, 179, 182, 183, 185, 253, 254, 449, 450). Only aldehydes are
involved in the Henry reaction with SENAs and activating additives of ammonium
ßuorides (or triethylamine (185)) are required for this process (Scheme
3.189).
The reaction starts with desilylation of nitronate. The α-nitro carbanion which
is eliminated gives (through the transition state A) the coupling product, the anion
B. The latter desilylates the next nitronate molecule to form the target product
and continues the chain.
SENAs derived from primary AN are most efÞcient in this process, erythro
isomers of β-siloxy-nitro compounds being formed as the major products. (In
contrast, the threo isomer is primarily generated from CF3CH=N(O)OSi under
these conditions (132).) It should be emphasized that the stereoselectivity of this
process strongly depends on the quality of tetrabutylammonium ßuoride. The
latter should be thoroughly dried, because the presence of moisture in the reaction
mixture leads to epimerization of the starting products. On the other hand, in some
cases the addition of water increases the total yield of the coupling product (182).
SENAs derived from secondary AN react with aldehydes with difÞculty to give
mixtures of silylated and nonsilylated condensation products in low yields.
Recently, C,N condensation of salts of nitro compounds with the nitroso group
has been discovered (251, 451) (Scheme 3.190). This reaction can be considered
as a convenient procedure for the synthesis of nitrones from AN. The yields of
the target products are 40 to 97%.
SENAs can be involved in these reactions but only in the presence of ßuoride
anion as the catalyst (see the lower part of Scheme 3.190). As can be seen from