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

510 NITRONATES
O
CF 3
R
O
N
H OSi
trans-
R
O
N
R′ OSi
N OSiMe 2Bu t
H
92Ref. 132,
O
R O
N Si
R′ O
A
MeO2C CO2Me
93Ref. 277, 285
N OSiMe 3
H
OSi
N
R O
cis-
O
N
R′
OSiMe2Bu t
Ref. 178, 94
N
R O
O
OSi
Me Me
(1)
(2)
N OSiMe 2R
Ref. 178, 285
95a,b
R = Me(a),But (b)
for 92: ΔH≠ = 36.4 kj/mol; ΔS≠ =−75.4k j/mol; ΔG≠ 298k = 58.9 kj/mol; ΔG ° 293k = 2.07
kj/mol;
for 93: ΔH≠ = 47.4 kj/mol; ΔS≠ =−30.2 kj/mol; ΔG≠ 298k = 56.4 kj/mol;
for 94: EA = 42.0 kj/mol.
Scheme 3.75
of primary AN are trans isomers, as evidenced by a number of characteristic
features (see Tables 3.10 and 3.11). However, in some cases, cis- isomers of
the starting nitronates were identiÞed based on the conÞgurations of the resulting
products generated in 1,3-dipolar cycloaddition reactions (for more details,
see Section 3.4.3.4.4). Consequently, a fast exchange process occurs in these
derivatives (Scheme 3.75, Eq. 1), and trans isomers are thermodynamically more
favorable due to steric factors.
Product (92) containing the CF3 group in the α position is the only compound,
in which the contribution of the cis isomer of SENA is rather high, and both
isomers can be observed by 19 F NMR spectroscopy. However, the trans isomer
of this compound is also thermodynamically more favorable.
For symmetric nitronates 93 to 95 (Scheme 3.75), the substituents at the α-C
atom in the NMR spectra are equivalent under usual conditions, that is, they give
one set of signals. However, at lower temperature, the signals of two MeO2C and
α-CH2 fragments in the spectra of products (93) and (94), respectively, become
nonequivalent (178, 285, 277). The kinetic parameters of the process described by
Equation 2 and the kinetic and thermodynamic parameters of equilibrium (1) for
product (92) (see Scheme 3.75) were determined by dynamic NMR. The presence
of pentacoordinated silicon in products presented on Scheme 3.75 can be ruled
out based on the 29 Si chemical shifts in the spectra of these SENAs. Hence,
the stereodynamic process described by Equation (1) can be explained only as
a 1,3–O,O-migration of the silyl fragment. The real rate of this migration is
independent of the concentration of the respective SENA and is weakly sensitive