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

PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 509
Conformation A Conformation B
Cyclohexene
Conformation q 1,° q 2,° Δ,°
A
B
q 1
Δ = | q1| – | q2| q 2
24.3 6.5 17.8
53.7 −46.6 7.1
Fig. 3.2 Calculations of conformations for six-membered cyclic nitronate
in qualitative agreement with X-ray diffraction data from the Cambridge Structural
Database for most of six-membered cyclic nitronates. Since the degree of
rotation about the N–O bond in the range from 0 ◦ to 50 ◦ has virtually no effect
on the energy of the conformation (see Fig. 3.1), the molecules of six-membered
cyclic nitronates can minimize steric and electronic interactions of substituents
by varying the deviation of the C-6 atom from the plane of the nitronate fragment
over a wide range.
The conformation B (Fig. 3.2) is a half-boat, in which the C-5 and C-6 atoms
deviate in the same direction from the plane of the nitronate fragment but at
different angles (θ 1 = 53.7 ◦ and θ 2 = −46.6 ◦ ,thatis,Δ = 7.1 ◦ ). The discussion of
the conformation A is valid for the conformation B. Interestingly, X-ray data for
one six-membered cyclic nitronate adopting the conformation B are available in
the Cambridge Structural Database.
3.3.4. Stereodynamics of Nitronates
3.3.4.1. Intramolecular Migrations of the Trialkylsilyl Fragment As was mentioned
in Section 3.3.2.2., acyclic alkyl nitronates are characterized by two sets
of signals in the NMR spectra corresponding to the E and Z isomers, that is, the
C=N bond in these products is conÞgurationally stable under usual conditions.
To the contrary, SENAs are, as a rule, characterized by one set of signals under
usual conditions. For example, spectroscopic data suggest that silyl derivatives