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

SYNTHESIS OF NITRONATES 463
In the reaction presented in Scheme 3.38, nitroalkenes (42) act as acceptors.
Consequently, the lowest unoccupied molecular orbital (LUMO) of these compounds
should be considered in terms of the frontier molecular orbital (FMO)
method as deÞned. The introduction of electron-withdrawing substituents at the
β-C atom of nitro oleÞne (42) should decrease the energy of LUMO of the latter
compound, thus decreasing the highest occupied molecular orbital (HOMO)
HOMO–LUMO energy gap and facilitating [4 + 2]-cycloaddition. However in
fact, the rate of the reaction of 4-methoxynitrostyrene with cyclopentene in the
presence of SnCl4 is 400 times faster than that of 4-trißuoromethylnitrostyrene
(103). Evidently, the reactivity of a particular heterodiene (42) in the cycloaddition
reaction depends most strongly on the concentration of a complex with
LA, which, in turn, should be decreased in the presence of electron-withdrawing
substituents in the diene.
The behavior of complexes of nitroalkenes (42) with LA toward conjugated
dienes is yet another factor underlying the role of these complexes. Conjugated
nitroalkenes (42) are considered as active dienophiles in classical Diels–Alder
reactions (104, 105). On the contrary, in the presence of SnCl4, nitroalkenes
(42) react with cyclopentadiene and 1,3-cyclohexadiene exclusively at one double
bond (103). Therefore, it is highly probable that the 42 + 43 cycloaddition
proceeds by a nonconcerted mechanism in the presence of LA (see Scheme
3.40).
Initially, a complex of nitroalkene (42) with LA (A) is reversibly formed.
The efÞcient concentration of the latter is determined by the reaction conditions
and the nature of heterodiene (42) and LA. This complex acts as a Michael
substrate and adds alkene (43) to give bipolar adduct B, which undergoes cyclization
to give cationic intermediate C. The latter eliminates LA to yield target
nitronate (35). In the case of nonconcerted cycloaddition, ionic intermediate B
can undergo different isomerization reactions, some of which are considered
below. The stereoselectivity of the process depends on the reactive conformation
R′
R
42
R′′
NO 2
LA
R′
R′′
R + N
O
O
LA
R – H, alkyl, EWG
R′ – alkyl, aryl
R′′ – H, alkyl, aryl
X – electron donating group
R′
+
R′′
+ −
R N O
R N O
O
O
LA
LA
A B
R′ R′′
R
O N
X
35
O
Scheme 3.40
−
X X +
43
−LA
R′
R
R′′
+
R′ R′′
LA
O
N
O
C
−
X
X