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

PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 511
to the nature of the solvent. All of these factors correspond to an intramolecular
process and the transition state A presented in Scheme 3.75 (Eq. 2).
This character of the process is indirectly conÞrmed by the X-ray diffraction
data for compounds (80) and (81) (Table 3.14), in which the distance from the
silicon atom to the valence bound O(1) atom is approximately 0.8 ûA smaller
than the sum of the van der Waals radii of these atoms. It should be noted that
the bond angles at the silicon atom are somewhat distorted. For example, the
O–Si–CMe3 angle is substantially smaller than the tetrahedral angle (100.6 ◦ and
97.5 ◦ , respectively). The Si–CMe3 and Si–O bonds are slightly elongated. In the
authors’ opinion (178), a distortion of the geometry of the silyl fragment in the
crystals of (80) and (81) cannot be attributed to a steric factor. This distortion is
evidence for the orientation of the silyl fragment in the Þeld of the oxygen atom,
which is not bonded to this fragment. The same intermediate structure should lie
on the path from SENA to the transition state A for a concerted process (Scheme
3.75, Eq. (2)). The relatively high negative entropy of activation of the process
also suggests that this process is electrocyclic.
Interestingly, the migration rate of the Si group in nitronates (95a,b) (Scheme
3.75) is so high that the methyl groups cannot be distinguished by the NMR
method by lowering the temperature at which the spectra are measured to -100 ◦ C.
(Now the migrations of organosilyl fragment in different organic compounds were
studied (286–288). A decrease in the 1,3-XY-migration rates of the silyl group
in the presence of electronegative substituents was documented (286).)
It should be emphasized that in no case were derivatives of AN containing
the C–Si bond detected, although the formation of these compounds in certain
silylation reactions as kinetic products cannot be ruled out. In particular, this can
be expected for nitronates (51 h,i) (Scheme 3.57) generated in the reactions of
silylated dimethylformamides with conjugated nitro oleÞns (202), because this
reaction with other Michael substrates affords products with the C–Si bond.
It is notable, that a reversible migration of the silyl fragment from oxygen
to the nitrogen atom bearing the nitro group occurs in silyl derivatives of
N-nitroamines (Scheme 3.76) (289). The mechanism of 1,3-N,O-migration
of the silyl fragment is analogous to that of 1,3-N,O-migration of the silyl
fragment in SENAs (through a cyclic transition state), and both of these processes
are characterized by similar kinetic parameters.
R O
RN(NO2)SiMe2R′ N N
OSiMe2R′
O
N N
R OSiMe2R′ R - Me, Et, NCCH2CH2; R′ - Me, Pr i , Bu t
Scheme 3.76