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O
R
2
R
2
R
1
OH
O
O
O
O
NH
2
R
2
R
2
R
1
R
1
R
O
ball-milling (30 Hz)
rt, 30 min
R
1
O
O
ball-milling (30 Hz)
rt-70
o
C, 1 h
H
O
H
R
R
100%
99-100%
Scheme 1.13 Enamino ketone synthesis under ball milling.
R
2
O
O
MM (30 Hz)
rt, 0.5-2 h, 61-97%
NH
O
R
2
+
NH
2
R
1
R
1
Scheme 1.14
Synthesis of enamino ketone and ester using aliphatic amine under
ball milling.
Kaupp's group has reported quantitative synthesis of enamino ketones by
the reaction of cyclic 1,3-dicarbonyl compounds such as 1,3-cyclohexanedione,
dimedone and dehydroacetic acid with aniline derivatives without any
catalyst under ball milling within 1 h, followed by drying at 0.01 bar at 80 1C
(Scheme 1.13).
7
Li and co-workers have reported the mechanochemical reaction of
aliphatic primary amines with acyclic 1,3-dicarbonyl compounds such as 1,3-
pentadione and ethyl acetoacetate in the absence of catalyst and solvent
(Scheme 1.14).
15
A series of enamino ketones and esters were obtained in
61-97% yields by ball milling the mixtures of amines and 1,3-dicarbonyl
compounds in a ratio of 1 : 1 in a mixer mill at 30 Hz for 0.5-2 h.
Stolle et al. have developed a solvent-free methodology for the synthesis of
enamines by the addition of amines to dialkyl acetylene dicarboxylates or
alkyl propiolates using a planetary ball mill at 800 rpm (13.3 Hz)
(Scheme 1.15).
16
Fused quartz sand (SiO
2
) was used as inert grinding
auxiliary to facilitate the energy entry in the presence of liquid substrates
by adsorbing them on the surface.
17
Significantly, reactions with several
anilines and secondary alkyl amines were complete within five minutes with
excellent yield of products. Besides the (E-/Z)-isomers, no other product was
formed. Interestingly, addition of aniline or p-toluidine to dialkyl acetylene
dicarboxylate produced the (E)-enamine as the major product whereas
addition of the same amine to alkyl propiolate produces the (Z)-enamine as
the major product.
Lamaty and co-workers reported the condensation of aldehydes with
equimolar amounts of N-substituted hydroxylamines in a ball mill at a
frequency of 30 Hz for 0.5-2 h to obtain various C-aryl and C-alkyl nitrones in
71-100% yields (Scheme 1.16).
18
Significantly, reactions were performed in
the presence of air and moisture and the products were obtained pure.
Though urea is very unreactive toward alkylation, the reaction of urea with
4-bromobenzyl bromide under mechanical milling in the presence of NaOH
produced di(4-bromobenzyl)urea with 41% conversion for a total milling
time of 34 h (Scheme 1.17).
19
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