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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 659
are resistant to water treatment but undergo rapid and uncontrolled decomposition
in the presence of acids or bases. Investigations of their structures and
stereodynamics are of considerable interest.
3.5.4.1. Structure and Stereodynamic of Ene- Nitroso Acetals On the one
hand, BENA can be considered as close analogs of enamines, in which the
nitrogen lone pair is efÞciently conjugated with the C,C double bond, resulting in
strengthening of the C,N bond and ßattening of the nitrogen atom. As a result, the
nitrogen inversion is accelerated, whereas rotation about the C=N bond becomes
much slower. In enamines containing an EWG-group at the β-carbon atom, the
activation free energy for rotation about the C,N bond is 40 to 80 kJ/mol (505),
whereas the inversion barrier is very low (4–6 kJ/mol (506)).
On the other hand, BENA are nitroso acetals, that is, compounds in which
the nitrogen atom is bound to two electronegative substituents. The nitrogen
inversion in nitroso acetals is strongly hindered. The barrier for this process is,
as a rule, higher than 80 kJ/mol (395), and the nitrogen atom is characterized by
a high degree of pyramidalization (508).
Therefore, the nitrogen conÞguration in BENA and the energy barriers for
both stereodynamic processes are inßuenced by two opposite factors and it is a
priori difÞcult to predict the Þnal result.
Preliminary experiments demonstrated that only one slow dynamic process
can be generally detected in BENA by dynamic NMR spectroscopy.
The equivalence or nonequivalence of siloxy substituents in BENA in possible
dynamic processes and conformations are considered in Fig. 3.3 (467).
Pyramidal nitrogen is favorable for slow inversion. In this case, two methyl
groups in the SiMe2Bu t fragment are nonequivalent, whereas two SiMe3 groups
are, on the contrary, equivalent. However, any two identical substituents at the
nitrogen atom become nonequivalent in the presence of the asymmetric center
G* attached to the C,C double bond.
A consideration of hindered rotation about the C,N bond is a more complex
problem. Here it is necessary to analyze conformations with “planar” or ßattened
nitrogen. Two frontier situations are possible. In the Þrst, most evident case,
the conformations A and C, in which the nitrogen conÞguration is favorable for
efÞcient conjugation between the nitrogen lone pair and the C,C double bond,
are predominant. In these conformations, the groups R at the nitrogen atoms are
always nonequivalent in the case of slow rotation of nitrogen. However, it cannot
be ruled out that the conformation B, in which the substituents at the nitrogen
atom (R = Me or OSiMe3) are equivalent in the case of slow rotation of nitrogen,
is predominant.
To study this problem in detail, the structures and stereodynamics of BENA
were investigated by several physicochemical methods (X-ray diffraction, quantum
chemical calculations and dynamic NMR spectroscopy) (467).
Principal X-ray diffraction data for typical BENAs (416a–d) and related compounds
(417) to (421) are shown in Table 3.26 (264, 467, 468, 507–509).