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

O
Me 2RSi NMe 2
+ −
Me2N=C—OSiMe2R 57a,b
R=Me(a); But (b)
Me 2NCOCH 2C(Me)
51g, only in solution
N
C 6H 6, 75°C
O
OSiMe3
H +
NO 2
0,5 h
2 h
NO 2
SYNTHESIS OF NITRONATES 477
Me2NCOCH2C(Me) 51h
O
N
OSiMe 2R
CONMe2
51i
Me2NCOCH2CH(NO2)Me
Scheme 3.57
O
N
OSiMe2R R = Bu t ; 46%
R = Bu t ; 79%
This result was considered by the authors in terms of C,C-coupling of silyl
derivatives (57) with Michael substrates, such as electron-deÞcient alkenes, the
NO2 group being the most activating of all the substituents examined (EtCO,
EtO2C, CN, etc.). The mechanism of this process remains unknown. Among other
approaches, the authors suggested the single electron transfer (SET) procedure
involving the formation of an activated complex of the reagents as the Þrst step.
It was noted that more hindered silyl derivative (57b) reacts more rapidly than
trimethylsilyl analog 57a (202).
SENAs (51 h) and (51i) are quite stable, whereas derivative (51 g) can be
detected only by physicochemical methods in solution ( 1 H NMR) or (after protodesilylation)
as β-nitrobutyric acid N,N -dimethylamide.
The use of strongly stabilized nucleophiles, for example, of [(EtO)2P(O)CHX] −
Li + 58a–d, where X is a powerful EWG group, such as MeO2C, CN, SO2Me,
or P(O) (OEt)2, in the conjugated addition with α-nitrooleÞns gives rise to more
complex processes (201b) (Scheme 3.58).
Probably, at high temperature, the initially formed anion A undergoes the
1,3-proton shift to give apparently more stable anion A ′ , which is silylated at
the oxygen atom of the nitro group to form (after elimination of LiCl) two new
intermediates—trimethylsilyl methylmethanenitronate and substituted styrene,
which are involved in [3 + 2]-cycloaddition followed by elimination of trimethylsilanol
to give isoxazolines 59a–d. One of these products (59a) was obtained
in very high yield. (The C,C bond cleavage in anionic intermediate A ′ could
alternatively occur through silylation of the group X followed by the transfer of
the SiMe3 fragment to oxygen of the nitro group.)
3.2.3.4. Silylation of AN with Silylated Amides One of the most important
approaches to the synthesis of SENAs is based on silylation of AN with neutral
silylating agents, that is, in the absence of bases. N -Silylated amides presented