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

514 NITRONATES
O
98
N O
OH
HO
O
99
N
O
;
Scheme 3.81
( )n
SiO O N ( ) n
O O
N
OSi
O
100
n = 1, 2 101
Analogous tautomerism 98⇄99 has been observed earlier for similar functionalized
nitronic acids (294) (Scheme 3.81).
Evidently, the cleavage of the weak endocyclic N–O bond is the driving
force of the ring opening. The nucleophilicity of the N -oxide oxygen atom in
nitronates facilitates the backward cyclization reaction (in the case of minimization
of steric hindrance). With regard to the above mentioned one cannot exclude
the tautomerism between cyclic nitronates 100 and “open” (or chain) isomers
101.
3.3.4.3. Migrations of Dialkylboron Fragments in Acyclic Nitronates In dialkylboron
derivative of diethyl nitromalonate (102) (Scheme 3.82), in which
the boron atom is tetracoordinated due to bonding with one of the carbonyl
groups, the nonequivalence of the carboxy groups observed in the 1 Hand 13 C
NMR spectra disappears with increasing temperature. Since the rate of exchange
of the BEt2 group between the carboxy groups is virtually the same in both
acetonitrile and dichloromethane (285), the existence of the cationic intermediate
[BEt2(NCMe)2] + in this process can be excluded with reasonable conÞdence, and
MeO2C O
N
MeO O
O B
R′
102 R
MeO
*
H O
N
O
O B·
Et2
102a
Py
MeO2C
MeO2C
H
MeO 2C
*
O
N BRR′
O
A
O
N BEt2
O
A′
Scheme 3.82
MeO
MeO2C
O
R′
B
R
O
N
O
102
R = R' = Et (a)
ΔH ~ 63 kj/mol
ΔS = 8±0.5 e.e.
ΔG = 64.5 kj/mol
H
MeO 2C
298
*
.
OBEt2 N
N
O
103