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

A
G
N
R O
N
H O
O
A
Si
R 1
RR 1 C=N(O)OSi
+
ACONHG
SYNTHESIS OF NITRONATES 481
A
O
Si
G
N O
H
O
N
ACONHG
Si = SiMe3
B
+
R
R 1
RR 1 C=N(O)OSi
Chart 3.1 A possible mexanistic models for electrocyclic silylation of AN with
N-silyl-amides.
Deprotonation
K (mol -1 sec -1 )
The silylation:
Ph NO 2 >> MeNO2 >> EtNO 2 >Pr n NO 2 >> Me 2CHNO 2
144 30 5.6 4.7 0.26
Chart 3.2 The competition of the rates of silylation and deprotonation of various AN.
The rates of silylation of AN with BSA (Chart 3.2) were estimated by the
competitive reaction method (204). These rates change in parallel with the rates
of deprotonation of these AN, determined by treatment with NaOH in aqueous
methanol (213).
It is most likely that silylation of AN with silyl derivatives of amides, like
the processes considered in Section 3.2.3.2, involve the formation of α-nitrocarbanions
as the key step. It is also possible that only aci forms of AN can react
with DPSU. This is evidenced by a comparison of the results of entries 8 and 9
in Table 3.3.
However, this process is multistep, many steps are reversible, and the reactions
are heterophase in the case of DPSU. These circumstances do not allow one to
give a general interpretation. It cannot be ruled out that the rate-determining step
in the reaction with DPSU differs from that with bis-trimethylsilylacetamides.
3.2.3.5. Silylation of Functionalized AN Silylation of α-functionalized AN can
afford not only “classical” SENAs but also their structural isomers, in which the
silyl group is bound to the electronegative atom of the functional group. This
could occur due to the possible stabilization of the negative charge by its transfer
to functional groups (Scheme 3.60). However, only the corresponding SENAs
occur (isolated or detected) as products of silylation of functionalized AN (e.g.,
see Eqs 1 and 2 in Scheme 3.60). Apparently, this is associated not with kinetic