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

668 NITRONATES
R 1
NO2
R 2
E
E +
R 1
SiO OSi
N
R 1
N
BENA
R 2
− Nu
OH
Nu
Scheme 3.231
a general conclusion can be drawn that BENAs make it possible to functionalize
the β-C atom of AN in the reactions with both nucleophilic and electrophilic
agents.
3.5.4.2.1. Reactions of BENA with Electrophiles The reactivity of BENA toward
electrophiles was quantitatively estimated on Mayr’s scale by kinetic measurements
of their C,C-coupling reactions with reference electrophiles 512. The
parameter of nucleophilicity N , determined for several terminal BENAs varies
in a rather narrow range (N ≈ 4–5) and depends only slightly on the nature of
substituents at the α-C atom and steric hindrance of the trialkylsilyl group. The
nucleophilicity of BENA is approximately 9 orders of magnitude lower than
the nucleophilicity of standard enamines. It is somewhat lower than the nucleophilicity
of silyl enolates. However, in spite of low N , a rather large number of
electrophiles of different types can readily be coupled with BENA.
The most interesting reactions are shown in Schemes 3.232 and 3.233
(512–514).
This approach allows one to functionalize the β-C atom of AN. For this purpose,
AN are initially subjected to double silylation to prepare BENAs, which
are then coupled with various stabilized carbocations (512, 513), as well as
with sulfenyl and episulfonium cations (514). Molecules containing good leaving
groups (e.g., arenesulfenyl chlorides (514)) are used as sources (or precursors) of
R 2
R 1
NO 2
R 2
Nu