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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 719
In any case, the data considered in Section 3.5.6 lead to the conclusion that AN
can be considered in the context of silylation as preparatively valuable precursors
of conjugated enoximes.
3.5.7. Exhaustive Silylation of Aliphatic Nitro Compounds
A standard procedure for the silylation of AN affords silyl derivatives of conjugated
enoximes as the Þnal products. However, the reactions with primary AN can
proceed further (see Scheme 3.185). The reaction of 1-nitropentan-4-one (579)
with a large excess of the powerful silylating agent Me3SiOTf/Et3N was monitored
by NMR (548) (Scheme 3.281, all products and intermediates, which are
not enclosed in square brackets, were detected by NMR). At −78 ◦ C, exclusive
silylation of the more nucleophilic carbonyl group giving rise to two isomeric
silyl enolates (580a) and (580b) inaca 1:1 ratio was observed, only the E
isomer being detected for (580b). At higher temperature (−30 ◦ C), both silyl enolates
undergo double silylation of the nitro fragment to give, respectively, BENA
(582a) and the silyl derivative of enoxime (584) in a ratio of 1:1. (After quenching
of the reaction mixture, the E isomer of enoxime MeCOCH=CHCH=NOH
can be isolated in 56% yield.) Most likely, the silyl derivative of oxime (584) is
generated as a result of the elimination of silanol from BENA (582b) which is
unstable under these conditions.
The cleavage of the weak N–O bond promoted by the πσ* interaction in
BENA (582b) is the driving force of the reaction (582b→584). The rise of
the monitoring temperature to 0 ◦ C and an increase in the exposure time enable
the detection of silylated enoxime (583) as the rearrangement product of BENA
(582a). Since the (583:584) ratio is substantially lower than the (580a:580b) ratio,
the partial transformation (582a→584), involving apparently the corresponding
α-nitroso alkene cannot be ruled out.
At room temperature, silylation of nitro ketone (579) affords nitriles (585) and
(589) or(586) and (589) astheÞnal products depending on the exposure time
and the excess of the silylating agent (Scheme 3.281) (548).
The probable pathway giving rise to silylated cyclic nitrile (589), which is the
most unusual reaction product, is shown on the left of Scheme 3.282. Apparently,
this compound is generated through the cationic intermediate A. It undergoes
cyclization at the terminal electron-rich C,C-double bond to form silylated oxime
(587), which is transformed into nitrile (588). After silylation of the latter, nitrile
(589) can be isolated. Desilylation of (589) according to standard procedures
affords nitrile (588).
The probable pathway resulting in the stereoselective formation of silylated
ene nitrile (586) from enoxime (584) is presented on the right of Scheme 3.282.
At higher temperature, the latter eliminates trimethylsilanol to give ene-nitrile
(586) under the action of silyl Lewis acid (TfOSiMe3). Evidently, the reaction of
compound (585) with TfOSiMe3 at room temperature involves initial silylation
of the nitrogen atom to form the cationic intermediate B, which is deprotonated
with triethylamine, followed by the thermodynamically favorable 1,3-N,C-shift