Dehydrogenation of 2-Phenyl-1-pyrroline
Agilent J&W GC column HP-PONA (50 m 9 0.200 mm 9
was collected, but no reaction products were detected in the
1
H NMR spectra.
0
.50 lm), and equipped with a FID. The initial temperature
was set to 55 °C and the final to 280 °C, with a ramp of
0 °C/min. Injections of 0.3 lL of reaction aliquots, pure
1
2.3 Catalytic Tests in Closed Vessels
solvent and xylene solutions of reagent or products were
performed to identify all the components.
1
The H Nuclear Magnetic Resonance (NMR) spectra
In order to determine whether molecular hydrogen would
be present in the gas phase, the dehydrogenation reactions
were also performed in closed vessels. These tests were
carried out in 250 mL crown capped pressure bottles that
were previously dried in the oven at 140 °C. A solution of
1
were recorded on a Bruker Avance III 300 ( H,
3
00.130 MHz) spectrometer, the spectra being referenced
internally using the residual protio-resonance of the solvent
relative to tetramethylsilane (d = 0). Resonances were
characterized by their chemical shifts (d), quoted in ppm,
and coupling constants (J), given in Hz. Multiplicities were
abbreviated as broad (br), singlet (s), and multiplet (m).
The elemental analysis was performed in a Fisons Instru-
ment Mod EA-1108, at the Laborat o´ rio de An a´ lises (IST).
2
-phenyl-1-pyrroline (2 g, 13.8 mmol) in 40 ml of xylene
was prepared and 1 mol% of [10 %-Pd/C] or 0.4 mol% of
10 %-Pd/Al O ] catalyst added. The bottle was sealed
[
2
3
with neoprene septa, degassed by 3 short vacuum/nitrogen
cycles and finally filled with nitrogen gas. Then the mixture
was heated to 150 °C under nitrogen atmosphere. Aliquots
from the gas phase were taken through the septa with a
syringe and injected in a GC equipment to determine the
2
.2 Catalytic Tests—Determination of the Optimal
Reaction Conditions
amount of H present in the gas phase. After 3.5 h reaction
2
the reactor was allowed to cool and opened to air. The
typical work-up treatment was followed.
In a typical procedure, a solution of 2-phenyl-1-pyrroline
2 g, 13.8 mmol) in 40 ml of xylene was prepared and a
(
Immediately after opening the reactors to air, an aliquot
was taken from the liquid phase. After filtration and drying,
1
the sample was analyzed by H NMR spectroscopy. A
predefined amount of the selected supported palladium
catalyst added. The slurry was refluxed at 150 °C, under an
extremely slow stream of nitrogen, during the time required
to achieve a maximum level of conversion. The reaction
progress was monitored along time by taking aliquots that
were filtered and evaporated to dryness, the resulting
mass balance enabled the determination of the amount of
hydrogen missing at the end of the reaction (see Table S3
in Supplementary Material).
Aliquots of the liquid mixture were also taken for GC
analysis. These were filtered through syringe filters Waters
Acrodisc 25 mm GHP 0.2 lm to retain the catalyst and
analyzed by GC in order to determine the presence of any
other possible hydrogenation/dehydrogenation products in
the reaction mixture that were undetectable by NMR.
1
material being analyzed by H NMR spectroscopy (see a
typical example in Fig. S1 in Supplementary Material). The
molar percentages in 2-phenyl-1-pyrroline reagent (1) and
2
-phenylpyrrole (2) and 4-phenylbutylamine (3) products
were calculated by the relative intensities of the normalized
resonance integrals of substrate and products.
After cooling down to room temperature, the solution was
filtered and the catalyst washed with fresh xylene. The solvent
was removed under vacuum and the remaining solid washed
with cold n-hexane (-20 °C) to allow extraction of unreacted
reagent and reaction by-product 4-phenylbutylamine. Sub-
sequent drying under vacuum yielded 2-phenylpyrrole as a
white solid. Depending on the reaction conditions, namely
when long reflux times were used, the reaction product could
also present a red color. In the latter case, further purification
by static vacuum sublimation (typically ca. 40 °C and
2
.4 Synthesis of 2-Phenylpyrrole. Scale-up Reactions
In order to obtain a convenient procedure for the synthesis
of 2-phenylpyrrole, scale-up of the dehydrogenation tests
were performed with both types of Pd-supported catalysts,
using the optimal reaction conditions found with the cata-
lytic tests:
(a) For the [10 %-Pd/C] catalyst—A solution of
2-phenyl-1-pyrroline (14.52 g, 100 mmol) in
100 mL of xylene was prepared, the [10 %-Pd/C]
catalyst (1.06 g, 1 mmol Pd) added, and the suspen-
sion was refluxed for 4 h. The typical work-up
(evaporation of the solvent, washing the crude with
cold n-hexane and drying) was followed. Yield:
7.84 g (55 %).
0
.01 mbar) led to pure white 2-phenylpyrrole.
A blank test for the reactions carried out with catalyst
[
10 %-Pd/Al O ] was performed using only Type I acti-
2 3
vated alumina (Acr o¨ s). For this specific case, a solution of
-phenyl-1-pyrroline (1.5 g, 10.3 mmol) in 40 mL of
xylene was prepared, the activated alumina (106 mg,
0 mol % in relation to 2-phenyl-1-pyrroline) being sus-
pended and the resulting slurry refluxed at 150 °C, for
.5 h. At every 30 min, an aliquot of the reaction mixture
2
1
(b) For the [10 %-Pd/Al O ] catalyst—A solution of
2
3
2-phenyl-1-pyrroline (22.35 g, 154 mmol) in 80 mL
of xylene was prepared, the [10 %-Pd/Al O ]
3
2
3
123