Chemistry Reference
In-Depth Information
N
2
BF
4
Te
KOH (0.75 equiv.)
Neutral Al
2
O
3
600 rpm, 15-30 min.
+ (PhTe)
2
(0.5 equiv.)
R
1
R
1
Some representative examples:
Te
Te
O
F
3
C
68%
76%
Te
Cl
Te
NC
76%
62%
Scheme 1.25
Synthesis of aryl-phenyl tellurides under ball milling.
1.3
Synthesis of Heterocycles
1.3.1 Nitrogen-containing Heterocycles
1,2,3-Triazoles, five-membered nitrogen-containing heterocycles, have
received considerable interest because of their useful applications as
agrochemicals, dyes, corrosion inhibitors, photostabilizer, materials and in
pharmaceutical industries.
27
Stolle and co-workers have reported the first
ligand- and solvent-free mechanochemical synthesis of triazole from azides
and alkynes using a planetary ball mill (Scheme 1.26).
28
The applicability of this protocol to a broad variety of substrates enables
easy access to a wide range of complex triazoles. Click polymerization in a
ball mill without destroying the polymer backbone is also successful
(Scheme 1.27).
Ranu et al. introduced a different protocol where 1,4-disubstituted-1,2,3-
triazoles were synthesized by a simple one-pot three-component reaction of
alkyl halide/aryl boronic acid, sodium azide and terminal alkynes on the
surface of Cu/Al
2
O
3
catalyst by ball-milling under solvent-free conditions
(Scheme 1.28).
29
Usually no chromatographic separation/purification was
required and no toxic organic solvent is used in the entire process. This
protocol avoids handling of hazardous organo-azide and provides easy ac-
cess to aryl-alkyl and aryl-aryl substituted 1,2,3-triazoles. From the X-band
EPR spectrum and XPS study it was found that the copper was present in the
þ
2 oxidation state throughout the reaction cycle. SEM and AFM analysis
exhibited the uniform spherical morphology of the catalyst.
Another nitrogen-containing five-membered heterocycle, pyrazoline, can be
effectively synthesized from chalcones and phenyl-hydrazines in the presence
of NaHSO
4
H
2
O using high speed ball milling (HSBM) (Scheme 1.29).
30
It was found that better yield of pyrazolines was obtained when the sub-
strates contained electron-donating groups. The easy catalyst recovery and
its reusability make this procedure more attractive. When other a,b-
unsaturated ketones were treated with thiosemicarbazide/phenyl hydrazine
in the presence of this catalyst the corresponding 2-pyrazoline derivatives
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