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

R 1
N(OSi)2
Nu:
(b)
R 2
EX
(a)
Si = SiAlk 3
N O SiO Si
R 1
R 1
SiO OSi
N
R 2
R 2
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 697
E – X
E – X
E – X
Nu:
N O SiO Si
R 1
R 1
R 2
R 2
Si – O OSi
N
Scheme 3.260
N O SiO Si
A
R 1
−NuSI; −OSi
R 1
R 2
NO
B
R 2
R 1
NOSi
Another approach is based on the reaction of nucleophiles with BENA at the
silicon atom (path (b)) generating nitrosoalkene B as the key intermediate. After
the addition of the OSi − anion and the reaction of the resulting anion with the
next BENA molecule in the chain process, this nitrosoalkene gives the target
derivative (504) (for more details, see the study (502)). Nitrosoalkenes B tend
to be involved in various side reactions, such as polymerization, rearrangements,
and so on (for more details, see Ref. 503). In addition, the reactivity of these
compounds toward the OSi − anion would be much lower than that of the cationic
intermediate A. Hence, the approach (a) seems to be a preferred strategy for the
rearrangement BENA→(504). Lewis acid E–X used for the rearrangement must
not passivate the OSi − anion, and the X − fragment involved in the LA must not
be nucleophilic toward the cation A, lest the acid be competitive with the OSi −
anion for this cation.
A priori, derivatives of trißic acid (silyl trißates, salts of trißic acid, etc.)
correspond to the above conditions. After a series of experiments, Zn(OTf)2 was
chosen as the most efÞcient LA for the rearrangement BENA→(504) (Scheme
3.261) (534). (It should be noted that the rearrangement BENA→(504) under the
action of silyl LA has been observed earlier (188, 465, 466), but the yields of
derivatives (504) have been disappointing.)
The rearrangement of BENA (505) containing various silyl fragments afforded
exclusively a derivative of oxime (506) containing various triorganosilyl fragments
in the oximino and hydroxy groups, that is, only the sterically least hindered
trimethylsilyl fragment (Scheme 3.262) migrated. These data rule out the formation
of nitrosoalkene A as the intermediate, which indicates that the rearrangement
occurs through the pathway (a).
504
−
OSi
R 2
OSi