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

506 NITRONATES
N–O bonds in nitronates (80 to 85) strongly differ in length. One of these bonds
(semipolar) is shortened and its length in all nitronates is even smaller than that in
reference nitrone (86). To the contrary, another N–O bond length is similar to that
in oxime derivative (87). As expected, the latter bond length is most sensitive
to the electronegativity of the substituent, the higher the electronegativity, the
larger being the bond length. Accordingly, this bond in acyl nitronate (82) isthe
weakest one, which facilitates generation of the corresponding nitrile oxide after
elimination of the OAc-group and the proton from the α-C atom.
The sum of the bond angles at the nitrogen atom in nitronates (80 to 85) is
close to 360 ◦ , which correlates with sp 2 hybridization of the nitrogen atom in
these derivatives. However, all three angles substantially deviated from “classical”
120 ◦ (see Table 3.14). The O–N–O and O(2) –N–C angles are smallest.
Evidently, a decrease in the O–N–O angle reßects a decrease in the grade of s
character of the nitrogen atom on these orbitals. It cannot be ruled out that the
O(1)–N–C angle increases due to a stereoelectronic repulsion between the substituent
at the α-C atom and the O(1)- atom. (Analogous destabilizing interactions
have been discussed earlier in connection with isomerization of sterically hindered
α-nitroalkenes (167, 168).) The destabilizing repulsion between the phenyl
substituent at C-3 and the O(1) atom in nitronate (81) causes rotation of this
substituent so that it becomes orthogonal to the plane of the nitronate fragment,
whereas another phenyl ring is in the plane of the nitronate fragment and is conjugated
with this fragment. These are very Þne effects because after removal of
the “planar” phenyl ring, the remaining phenyl substituent (nitronate 80) returns
to the plane of the π system of nitronate to provide efÞcient conjugation with its
π system.
In six-membered cyclic nitronates (e.g., in (85)) adopting a half-chair conformation,
the C-6 and C-5 atoms deviate from the plane of four atoms in the
opposite directions, the deviation of the C-6 atom being substantially larger (the
deviation of the C-5 atom. The latter is generally at most 0.05 ûA). It should
be noted that one six-membered cyclic nitronate adopts a half-boat conformation
(data from the Cambridge Structural Database, see also Fig. 3.2 and its
discussion).
The geometry of nitronates has not been adequately studied by quantumchemical
calculations. For example, the bond lengths in nitronate
Me2C=N(O)OMe were calculated by the INDO method for ideal geometry (248).
Dil’man has optimized the geometry of several model α-halogen-substituted
SENAs by the B3LYP/6–31G(d) method (87, 282) (Table 3.15).
The calculated bond lengths and bond angles are in satisfactory agreement with
the X-ray diffraction data for the nitronate fragment in model SENAs (Table 3.14)
although the introduction of the halogen atom clearly leads to a slight change
in the bond lengths and bond angles. Interestingly, the structural parameters of
stereoisomers of α-halogen-substituted silyl nitronates are somewhat different.
This is particularly true for the O←N=C angle. The latter is substantially larger
in the E isomer, which apparently minimizes the destabilizing interaction between
the oxygen and halogen atoms.