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

PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 515
consequently, the exchange occurs through the cyclic transition state A shown in
Scheme 3.82.
The kinetic parameters of the exchange process in product (102a) are similar
to those of the corresponding exchange process in the related trimethylsilyl
derivative (285).
In the dialkylboron derivative of nitroacetic ester (102a) adopting the Z- con-
Þguration, the tetracoordinated boron atom is bonded with the carbonyl group
(NMR spectroscopic data) (230). Also, a complex of this product with pyridine
(103) has the E- conÞguration (272). Apparently, the reaction of (102a) with
pyridine also proceeds through the cyclic transition state A ′ (Scheme 3.82).
3.3.4.4. Stereodynamics of Cyclic Nitronates From general considerations and
according to X-ray diffraction data (263a), molecules of Þve-membered cyclic
nitronates should adopt an envelope conformation with the C-5 atom deviating
from the plane. This atom ßuctuates together with its substituents (R 4 and R 5 )
(Scheme 3.83, process a).
An analogous situation was observed for the C 5 and C 6 atoms in six-membered
cyclic nitronates (Scheme 3.83, process b).
Both of these processes have no effect on the conÞguration of nitronates.
Owing to low energy barriers, they are unobservable on the NMR time scale.
(The ring-inversion barrier in cyclohexane (295) is 22 kJ·mol −1 ; the calculated
inversion barrier for unsubstituted six-membered cyclic nitronate (293) should
also be not higher than 20 to 25 kJ/mol.). Hence, the NMR data reßect averaged
most favorable thermodynamical conformations of cyclic nitronates.
O
O
O
O
R 4
R 4
N
N
R 5
R 5
R 3
R2
1 R
R 2
R3
R1 O
O N
R 4 R 5
R 5 R 4
O N
O
a b
Scheme 3.83
R 2
R 3
R 3
R 2