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

454 NITRONATES
M + [XC(NO 2) 2] −
21
XC(NO 2)N 2
22
X − NO2 or EWG
−MNO 2
−N 2
O
XC(NO 2)
A
N
O −
C
X
B
+
Scheme 3.24
in their classical form A by the [1 + 2]-cycloaddition mechanism (path a) to
give nitrocyclopropanes (23). However, we are interested in another reaction
pathway, path (b), which involves the [3 + 2]-cycloaddition reaction of carbene
intermediates as 1,3-dipoles B leading to Þve-membered cyclic nitronates (24).
It has been expected that trinitromethane derivatives of the general formula
(NO2)3C − M (M is K, Cs, SiMe3, or SnBu n 3
(a)
(b)
23
O
24
N
X
NO2
) can generate dinitrocarbene through
solvolysis, and the latter can react as a 1,3-dipole with some oleÞns (74). However,
a next study (75) demonstrated that this interpretation of the processes is
erroneous at least for M=SiMe3. Attempts to detect nitrocarbenes in cycloaddition
reactions during thermolysis or photolysis of aliphatic nitrodiazo compounds
also failed (76).
Calculations of the reactivity of model nitrocarbenes (77a) demonstrated that
these species A much more rapidly undergo a rearrangement to give nitrosocarbonyl
intermediates B than are involved in [3 + 2]-cycloaddition with an external
trapping agent. Shortly thereafter, this fact was conÞrmed experimentally (77b)
because, at the same time, intermediates B can be detected in ene reactions with
certain oleÞns to give hydroxylamines (25) (Scheme 3.25).
Taken together, these facts demonstrate that the “nitrocarbene approach” to
the synthesis of nitronates (24) cannot be considered as a promising method for
their synthesis.
A more recent study (78) on thermolysis of silver salts of aryldinitromethanes
(26) did not change a negative attitude toward the approach shown in
Scheme 3.26, because target nitronates (24a) are generated in low yields and are
R
NO 2
fast
H
O C
R N O
O
O
R N H
R N
A B 25
Scheme 3.25
O
OH
X
O