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

XO
R 1
N
O
H
XO O
N
R
R 2 = H
R 3
XO O
N
R
Nu
E
R 3 F
−NuH
−OX
O
N
R 1
XO O
N
R
R 3
XO + O
N
R
R 3
Scheme 3.85
REACTIVITY OF NITRONATES 517
anions E. However, in the presence of a good leaving group OX − , the latter
can become kinetically unstable and rapidly transform into the corresponding
conjugated nitrosoalkenes. The problem of generation and efÞcient trapping of
these anions still exists.
It should be noted that nitronates can stabilize not only a negative but also
a positive charge on the β-carbon atom to give stabilized cations F. The latter,
however, can also be kinetically unstable in the presence of a good leaving group
X. It must be noted that this aspect of chemistry of nitronates also remains to be
studied.
From the above it follows that, in spite of numerous potential possibilities,
nitronates would not be “good reagents” in charge-controlled reactions. However,
the structures of these compounds are favorable for concerted reactions, which
are schematically shown in Scheme 3.86.
But, these processes can give rise to modiÞed nitro or nitroso compounds or
their isomeric oximes.
Since many of these processes occur through six-membered cyclic transition
states, they can be highly stereoselective. However, for these reactions to proceed,
not only high steric demands but also good leaving groups X are required. In
this respect, silyl and acyl nitronates are a priori most preferable.
In the reactions of cyclic nitronates, the leaving groups that are generated upon
cleavage of endocyclic N–O bonds are not eliminated and can be involved in
further transformations.
−OX −
XO
R
N
O
R 3
−X +