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

630 NITRONATES
R 1
R 2
O
TfOH
R 1
N
N
OX R OX
2
OTf− 348 350
TfOSi
R 1
R 2
+
OH N
OSi
N
OTf
OX −
+
349
Scheme 3.206
R 1
R 2
348
O
N +
OX
Si – trialkylsilyl
N
H
OTf −
The activation parameters and a few other data characterizing the electrophilicity
of cationic intermediates (349) are given in Table 3.23.
As can be seen from Table 3.23, the electrophilicities E, which were determined
according to Mayr’s scheme (482), vary in a wide range and depend
mostly on the substituents at the C-3 atom (see the last entry in Table 3.23).
On the whole, the electrophilicities E for bis-oxyiminium cations are in the
same range as those for classical iminium cations (482) (e.g., see Ref. 483 for
[C=NMe2] + , E=−6.69).
Steric hindrance in the silyl groups of cation (349) and nucleophile (352) has
virtually no effect on the rate constant of the C,C-coupling reaction. Hence, it can
be concluded that, at least for silyl-containing nucleophiles (352), elimination of
the trialkylsilyl group from cationic intermediate A is not the rate-determining
step of the reaction sequence (Scheme 3.207).
As mentioned in the previous section, the position of the equilibrium between
the starting nitronates and the corresponding cationic intermediates are very labile
and depend on various factors. However, a preliminary analysis demonstrates that
C,C-coupling with nucleophiles can occur for cationic intermediates derived from
different types of nitronates.
3.5.2.3. Intermolecular C,C-Coupling of Bis-N,N-Oxyiminium Cations with
C-Centered Nucleophiles Readily available silyl ketene acetals CH2=C(OMe)-
OSi, where Si is Me3Si or Me2Bu t Si,wereusedasefÞcient test nucleophiles in
C,C-coupling reactions. These reactions with silyl ketene acetal, as well as with
other nucleophiles containing Si as the leaving group, can be performed under
very mild conditions in the presence of catalytic amounts of silyl Lewis acids.
(Catalysis of C,C-coupling reactions by silyl Lewis acids was discussed in detail
in Reference 297)
Both silyl- and alkyl esters derived from primary AN are readily involved in
C,C-coupling reactions with silyl ketene acetal (Scheme 3.208, Eq. 1) (484).