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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 663
Steric Conjugative
Fig. 3.5 The plots of steric and conjugative constituents of C,N-rotation potential for
CH2 = CHN(OSiH3)2 (426)
In addition, conformation B can be stabilized due to πσ* interactions between
the π system of nitroso acetal and antibonding orbitals of both N–O bonds.
Table 3.28 presents the results of investigation of the stereodynamics in BENA
and related compounds by dynamic NMR spectroscopy (467).
In similar nitroso acetals (427) and (428) (292, 493), which do not contain a
C,C double bond, the nitrogen inversion barrier is so high that it cannot be directly
determined by dynamic NMR spectroscopy due to instability of the nitroso acetals
at high temperatures. The nπ conjugation between the nitrogen lone pair and the
C,C double bond substantially decreases the barrier and slightly ßattens the nitrogen
atom, thus increasing its s character. (It should be noted that the exchange
process in bis-N,N -(siloxy)aniline (429) (entry 3, Table 3.28) can be interpreted
only as nitrogen inversion.) The introduction of electron-withdrawing substituents
at the β-C atom of BENA would strengthen the nπ conjugation, thus resulting
in ßattening of the nitrogen atom and in a decrease in the inversion barrier. This
situation is observed for BENA (cf., for example, entries 4–6 with entries 9 and
14 in Table 3.28).
In contrast, the opposite situation is observed for standard enamines, in which
the introduction of electron-withdrawing substituents at the β-C atom leads to
an increase in the barrier to rotation about the C,N bond (505). It should be
noted that the experimental inversion barrier in BENA (423b) (52 kJ/mol, entry
4, Table 3.28) is rather similar to the calculated barrier for its very close analog
(423a) (65.3 kJ/mol, entry 9, Table 3.27). As can be seen in Table 3.28, steric