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

674 NITRONATES
the silyl derivative of substituted oxime (440), as well as to a new nitrosoalkene
molecule and the siloxy anion, which either reacts as an initiating nucleophile
(Nu ′ ) with BENA or reacts with the silyl derivative of azole at the N–Si bond,
thus generating the new Nu ′− molecule. Therefore, the reaction of nucleophiles
with BENA is considered as a chain process (502) with conjugated nitrosoalkenes
B as the key intermediates. This interpretation is supported by the following facts:
the presence of an evident induction period, instability of BENA toward nitrogen
bases, trapping of the nitrosoalkene intermediate by electron-rich dipolarophiles,
and Þnally, visual observation of a blue color of the mixture in some reactions
BENA + nucleophile.
This complex character of the above mentioned process and the involvement
in this interaction of unstable and active intermediates are responsible for the
formation of numerous by-products, and therefore, the reaction conditions must
be optimized in each particular case.
3.5.4.2.2.1. reactions of BENA with C-nucleophiles Russian researchers
performed comprehensive studies on C,C-coupling reactions of terminal BENAs
A with anions of nitro compounds (516, 517). This process enables one to assemble
β-substituted oximes from two different AN ((441) and442). It should be
noted that compound (442) must have the methyl group at the α-C atom necessary
for generation of terminal BENA. Both nitroalkanes should be “prepared”
for C,C-coupling, that is, AN (441) is transformed into the anion C by the reaction
with DBU, while AN (442) is successively transformed into BENA A and
nitrosoalkene B. The C,C-coupling reaction B+C is shown in Scheme 3.238.
This reaction was interpreted as a chain process. The major reaction is accompanied
by two side processes (Scheme 3.239), such as O-alkylation of the anion
C with electrophilic nitrosoalkene (reaction b) giving rise to oximes (445) and
(446) and the addition of the target oxime (443) to the second molecule of
nitrosoalkene B producing dioxime (447) (reaction d).
Both processes can be minimized by lowering the C,C-coupling temperature.
No evidence for the third possible side reaction, e.g., the addition of the
nitro carbanion at the nitroso group of conjugated nitrosoalkene (process c), has
been observed in C,C-coupling reactions due apparently to insufÞciently high
electrophilicity of the nitrogen atom.
After a number of attempts to optimize C,C-coupling, two rather general procedures
were developed, which can be performed with anions of both primary
and secondary AN (Tables 3.31 and 3.32).
These procedures differ in the nature of the initiating nucleophile (Nu ′− )and
the reaction temperature range. For anions derived from secondary AN a lower
concentration of active nitroso intermediate B is strongly recommended. Therefore,
the C,C-coupling of anions obtained from secondary AN is realized at higher
temperature (see Table 3.32).
The C,C-coupling reactions of BENAs can also proceed with SENAs (132,
500). However, in this case the part of side reactions is greater and the yields of
the target β-nitro oximes are generally lower.