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

R 1
R 2
OSi
N
O
E—Nu
R 1
R 2
REACTIVITY OF NITRONATES 521
O Si
N Nu −Si–Nu
R 1 – alkyl, EWG – group;
R 2 = H, alkyl, EWG – group; Si – trialkylsilyl
E-Nu = H-OH Ref. 174,203-205 ; H-OMe Ref. 175,203-205 ;
O
E
R 1
O
N
OE
>90%
R 2
E=H
R 1 R 2 CHNO 2
H-ClRef. 175,204,205 ; H-NEtRef. 175,204 ; Na-OHRef. 204 ; XMg-AlkRef. 306
Scheme 3.92
O-Si bond can also be cleaved in the presence of Si-active nucleophiles. Hence,
eliminations play the major role in reactions of SENAs.
SENAs are readily desilylated by reagents containing an active proton (175,
203–205) and by sodium ethoxide (204, 306) (Scheme 3.92).
In these reactions, the starting AN are generally recovered in good yields or
their salts are generated.
Unlike alkyl nitronates (302), their silyl analogs react with Grignard reagents
at the silicon atom rather than at the α-C atom (306). All these processes may
be represented as electrocyclic.
Organolithium reagents cause deprotonation of the α-C atom of SENA (306).
If these protons are absent, deprotonation occurs at the β-carbon (Scheme 3.93).
These transformations produce oximes (109) or(110), respectively.
The formation of the latter compounds can be attributed to the result of the
direct attack of the nucleophile R on the α- orβ-carbon atoms of SENAs after
elimination of the corresponding protons. However, it is most likely that the
reaction proceeds through nitrile oxides or conjugated nitrosoalkenes (see Scheme
3.93). This interpretation is evidenced by generation of silyl esters of hydroxamic
acids R’CONHOSi as by-products. The reactions with more saturated solutions
give the latter compounds as the major products.
By contrast, softer nucleophiles, such as thiols (111), evidently do react with
SENAs at the α-C atom (307) (see Scheme 3.94). This interpretation is conÞrmed
by a substantial difference in the conÞguration of thiohydroxamate 112a isolated
in the reaction with silyl nitronate (a) and analogous product 112b (b) prepared
from authentic nitrile oxide.
The reactions of salts of nitro compounds (113) (Scheme 3.95) with silylated
thiols (308), hexamethyldisilathiane (308, 309), and hexamethyldisilane (310)
afford oximes (114), thiohydroxamates (115), or thiohydroxamic acids (116)
as Þnal products depending on the structures of the starting nitronates and the
reagents used.
All reactions initially produce silyl nitronates, which react with nucleophiles
to give nitroso intermediates A. The latter give products 114 to 116 either during
the reaction or upon aqueous treatment.