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OH
N
N
C
S
NHNH
2
Ball-milling
+
NH
2
-NH
2
.
-H
2
O
S
O
2
N
O
2
N
OH
OH
N
S
O
Ball-milling
NHNH
2
NH
2
-NH
2
.
S
+
H
2
N
H
-H
2
O
H
O
OH
OH
O
O
OH
NH
Ball-milling
HOOC
+
NH
2
-NH
2
.
N
-2H
2
O
O
OH
COOH
O
OH
O
Ball-milling
NH
+
NH
2
-NH
2
.
COOH
Ph
-2H
2
O
N
OH
Ph
Scheme 1.3 Quantitative C-N bond forming reactions via condensation, ring open-
ing and ring closure under ball milling.
Naimi-Jamal, Kaupp and co-workers employed ''kneading ball-milling'' for
the stoichiometric quantitative preparation of synthetically versatile 2,4-
dinitrophenylhydrazones from low-melting aldehydes and ketones.
5
Owing
to the potential explosive nature of dry 2,4-dinitrophenylhydrazine, 50 wt%
deionized water was added to wet the crystals. Water was used as an auxiliary
to minimize the risk of explosive destruction. The stoichiometric reaction of
2,4-dinitrophenylhydrazine with aldehydes or ketones occurred rapidly in
the kneading ball mill at 25-70 1C for 10-20 min, giving the desired
hydrazones in 58-100% yields (Scheme 1.4). When these reactions are
performed in solution, strong acid catalysts are required.
6
However, they
occur much faster in the kneading ball mill in the absence of catalyst.
Similarly, aldehydes and ketones were stoichiometrically ball-milled with
hydroxylamine hydrochloride at 25-140 1C for 10-120 min to provide the
hydrated oxime salts in sticky form, which were then treated with a base to
obtain the oximes in 75-100% yields (Scheme 1.5).
5
NaCl in water and CO
2
from carbonate were found to be the only wastes produced. Aldehydes were
found to be more reactive than ketones towards hydroxylamine. The differ-
ence in reactivity of aldehydes and ketones can be utilized as a versatile
method for selective protection of an aldehyde by oximation in the presence
of a ketone.
Benzoylhydrazones were also obtained, by ball-milling benzhydrazide and
solid aldehydes in a molar ratio of 1 : 1 for 1 h at 25-30 1C. However,
the reaction of benzhydrazide with isatin required 3 h of ball milling for
completion (Scheme 1.6).
7
Liquid assisted grinding (LAG)
8
has been applied for the synthesis of
imine from 5-aminosalicylic acid and vanillin or 2-hydroxy-1- naphthal-
dehyde in a ball mill for 5-30 min in the presence of a small amount of EtOH
or EtOH-NEt
3
(Scheme 1.7).
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