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

R 2
R 3
R 1
NO 2
SiX/Et3N
(1)
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 619
R 2
R 2
R 3
R 3
R 1
AN SENA
(2)
333
R 1
O
N
SiX/Et3N
OSi
N(OSi)2
Scheme 3.197
SiX/Et3N
(3)
SiX/Et3N
(4)
OSi
R1 R3 R2 +
NEt3 R1 R3 R2 can be rapidly rearranged into oximes (334) or give quaternary ammonium salts
(335). (These reactions will be considered in more detail in Section 3.5.4.3.)
Stability of BENA (333) depends not only on the nature of the silylating reagent
used but also on the reaction conditions and the nature of the starting AN.
Interestingly, bases and water have different effect on stability of BENA and
intermediate SENA. Additives of tertiary amines stabilize SENAs (191), whereas
bases cause spontaneous decomposition of BENAs. SENAs are hydrolytically
unstable, whereas BENAs are inert toward water under neutral conditions.
An increase in the strength of silyl Lewis acid leads to an increase in the rate
of the AN→BENA process and allows one to use milder conditions. In addition,
the rate ratio K4/K2 (Chart 3.20) sharply increases. For example, the generation
rate of respective BENA is approximately equal to the rate of consumption of
the corresponding intermediate SENA in double silylation of 2-nitropropane with
Me3SiCl (187). This SENA was not detected among the products of silylation of
2-nitropropane in the presence of even a smaller amount of Me3SiOTf.
In special studies, several procedures were developed (465–468) for the onepot
synthesis of the corresponding BENA in good yields from the overwhelming
majority of various AN, the amounts of impurities being minimum (Table
3.18).
To increase stability of the target substrates, washing of the resulting reaction
mixture with an aqueous NaHSO4 solution was included as a necessary step in
standard procedures for the synthesis of BENA.
However, in some cases, it is worthwhile to synthesize BENA in two steps by
isolating intermediate SENA (see, in particular, entries 10 and 22, Table 3.18).
It is particularly difÞcult to synthesize BENA from AN disubstituted at the
β-carbon atom.
N
334
X −
N
335
OSi
OSi