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

488 NITRONATES
R OBEt2O R'
R N R N
C O C O
or
or
−NaCl
C BEt2 C BEt2
70a-j
R' OBEt2O R MeO O O N
71a-e 71f-h
H2
_a:
R = Me, R' = CHMeEt, Br instead Cl (42%); _b:
R = R' = Me (42%); 71i,j
c: _ R = H; R' = Me (54%); d: _ R = H, R' = C3H7 (21%); e: _ R = Me, R' = Ph (42%);
f: _ R = H, R' = CO2Me (84%, crude); g: _ R = Me, R' = CO2Me (97%, crude);
h: _ R = R' = CO2Me (86%, crude); _i:
R = H, R' = CONH2 (94%);
_j:
R = Me, R' = CONH2 (60%).
[RR'C(NO2)] − Na Et2BCl
R'CH CR''NO2
72a-c
Et3B/Et2O
35–40°C
Scheme 3.69
EtR'CH O B(Et) 2
N
R'' O B(Et)2
O
Scheme 3.70
R''
O
O CHR'Et
71k-m
O
R' = R'' = Me (72%);
R' = Et, R'' = Me (43%);
R' = Me, R'' = Pr i (15%)
Another synthetic route to boryl nitronates (71k–m), which is, however,
poorly developed (231), is based on the reaction of trialkylborons (exempli-
Þed by triethylboron) with α-nitrooleÞns (72a–c) proceeding by the 1,4-addition
mechanism (Scheme 3.70, cf. Scheme 3.56).
Unfortunately, an increase in the size of substituents at the α- andβ-C atoms
of nitro oleÞn (72) leads to a sharp decrease in the yield of target nitronates.
It casts doubt on the importance of this method or at least on the procedure
suggested.
Finally, boryl nitronates can be synthesized by a method based on the replacement
of the trialkylsilyl fragment in SENAs (217) (Scheme 3.71).
To successfully apply this approach, it is necessary to use boron halides as
reagents. This equilibrium process (cf. Scheme 3.61), is being shifted to the
right toward dimeric nitronates (71n–s) because the energy of the B–O bond
(544 kJ/mol) (218) is higher than that of the B–Cl bond (489 kJ/mol) (218).
The fact that there is an additional gain in energy in dimeric nitronates (71n–s)
because boron is tetracoordinate, is of Þrst importance. It should be emphasized
that boron polyhalides (e.g., BF3) can also be involved in exchange reactions,
which extends the range of available boron derivatives of nitronic acids. Besides,
this exchange process occurs under very mild conditions.
In this section, we considered virtually complete data on the preparation of
covalent nitronates. Data on nitronates including other elements are scarce (see,
e.g., Ref. 232) but they refer to either salts or nitro compounds in which the
elementorganic fragment is bound to the carbon atom bearing the nitro group or
to intermediates of unknown structures.