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

R'
R
PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 495
R O
R O
O B(Et)2 O R'
N
N
N
N
MeO O O O
O B(Et) 2 O R
O
NH
B(Et)2
2 B(Et)2
75a-f 76a-c 77a-b
R = H, R'=Me (a), Pr n (b), Ph (c) (oils);
R = R'=Me (d) (mp 64–66°C),
R = Me, R'=Pr i (e) (mp 62–65°C),
R = Me, R'=CHMeEt (f) (mp 63–65°C).
R'
R
O
N
B(Et) 2 O
N
O B(Et)2 O
R'
R
R = H(a), Me (b), CO2Me (c)
(only in solutions)
R'
75 79
R
Scheme 3.74
N
O B(Et) 2
O
O B(Et) 2
R = H(a) (mp 110°C),
Me (b) (mp 109–115°C)
MeO
MeO
O
O
O
N
O
78a
(only in solution)
B(Et) 2
These data conÞrm the general conclusion about high thermal stability of
cyclic nitronates.
3.3.2. Spectral Characteristics of Nitronates
During the period of more than four decades of extensive development of chemistry
of covalent nitronates, at least a thousand of these compounds have been
synthesized. Their overwhelming majority has been spectroscopically characterized.
Hence, a consideration of all spectral characteristics of these products within
the framework of this short section is of no signiÞcance, the more so as some
data have been summarized in a recent monograph 3.
A discussion of problems, which could be solved by physicochemical studies
of nitronates, seems to be much more useful.
In this respect, ways of detecting the nitronate fragment in substrates under
study are considered Þrst.
However, in our opinion, the rigorous assignment of products to covalent
nitronic esters rather than to their structural isomers, which are true nitro compounds
or ionic salts, is a more important and complex problem. This problem
involves difÞculties, because ambident anions of nitro compounds (which are evident
precursors of nitronates) have comparable O- and C-nucleophilicities and,
therefore, the resulting substrates can belong to any of the above mentioned series.
Incorrect structure assignments of derivatives of polynitro compounds prepared
from tetranitromethane were made in former studies. In addition, the structures
of nitronates assigned to some products in early studies, should not have been
accepted without the use of modern spectral methods.