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

662 NITRONATES
Table 3.27 Some calculations for model BENA and related compounds
Entry Compound R X Conform. rel.E a kJ/mol
1 (422) H H A ′b 0
2 (422) H H B 30.3
3 (422) H H TSR 2 37.0
4 (422) H H TSN 4.5 b
5 (423) OSiMe3 H A + 0.3
6 (423) OSiMe3 H B 0
7 (423) OSiMe3 H TSR1 8.8
8 (423) OSiMe3 H TSR2 23.9
9 (423) OSiMe3 H TSN 65.3
10 (424) Me 4-NO2-C6H4- B 45.0 c
11 (425) Me EtO2C B 51.0 d
a The values were calculated taking into consideration the zero-point vibration energy.
b Experimental 4.2 kJ/mol (506a).
c By NMR data (see Table 3.25).
d Ref. (505)b.
For standard vinylamine (422), the change between stable conformations
A⇄A ′ through the transition state B, caused by rotation about the C,N bond
requires more than 30 kJ/mol, whereas both the calculated and experimental
nitrogen inversion barriers are not higher than 5 kJ/mol. (The data for compounds
(424) and (425) (entries 10 and 11) demonstrated that the introduction of
electron-withdrawing groups at the β-C atom of enamines leads to a substantial
increase in the barrier to rotation about the C,N bond in enamines.) The nitrogen
atom in vinylamine (422) is substantially ßattened (the calculated sum of bond
angles is 341.4 ◦ ).
However, a quite different situation is observed for ene nitroso acetal (423).
First, a stable conformation and, correspondingly, new transition states TSR1 and
TSR1* with a low barrier (8.8 kJ/mol) appear on the rotation coordinate about
the C,N bond at the place of the transition state of enamine (422). Therefore, the
barrier to rotation about the C,N bond decreases so that the process is fast on the
NMR time scale and cannot be detected by this method.
On the other hand, for compound (423) both the nitrogen inversion barrier
(65.3 kJ/mol) and pyramidalization at the nitrogen atom are substantially higher
(the sum of the bond angles at the nitrogen atom is 309.7 ◦ and 321.0 ◦ for the
conformations B and A, respectively). A noticeable decrease in the degree of
pyramidalization at nitrogen in the conformation A is indicative of a certain
degree of nπ conjugation.) The nitrogen inversion barrier is suitable for observation
on the NMR time scale (see below).
Special calculations for hypothetical nitroso acetal (426) demonstrated that the
energy minimum for the conformation B is due to the partial compensation of
the absence of stabilization through nπ conjugation by favorable steric factors
(Fig. 3.5).