Selective Pd-catalyzed hydrogenation of 3,3-diphenylallyl alcohol: Efficient synthesis of 3,3-diarylpropylamine drugs diisopromine and feniprane

Article abstract of DOI:10.1016/j.catcom.2017.08.025

The Pd-catalyzed selective hydrogenation of C[dbnd]C double bond in (3,3-diphenylallyl)diisopropylamine or 3,3-diphenylallyl alcohol was evaluated using different catalytic systems [Pd/C, Pd(OAc)₂/ionic liquid, isolated Pd(0) nanoparticles]. For the (3,3-diphenylallyl)diisopropylamine, hydrogenolysis is preferred over hydrogenation, and only moderate selectivities were obtained for the desired product. However, complete conversion and 100% selectivity were obtained for the hydrogenation of 3,3-diphenylallyl alcohol using isolated Pd(0) nanoparticles under mild condition. This successful strategy enabled the effective synthesis of diisopromine and feniprane drugs and opens new possibilities for the preparation of other biologically active compounds.

Full text of DOI:10.1016/j.catcom.2017.08.025

Accepted Manuscript
Selective Pd-catalyzed hydrogenation of 3,3-diphenylallyl
alcohol: Efficient synthesis of 3,3-diarylpropylamine drugs
diisopromine and feniprane
Thiago S. Claudino, Jackson D. Scholten, Adriano L. Monteiro
PII:
S1566-7367(17)30362-X
doi: 10.1016/j.catcom.2017.08.025
CATCOM 5174
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 June 2017
19 August 2017
21 August 2017
Please cite this article as: Thiago S. Claudino, Jackson D. Scholten, Adriano L. Monteiro
Selective Pd-catalyzed hydrogenation of 3,3-diphenylallyl alcohol: Efficient synthesis
,
of 3,3-diarylpropylamine drugs diisopromine and feniprane. The address for the
corresponding author was captured as affiliation for all authors. Please check if
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ACCEPTED MANUSCRIPT
Selective Pd-catalyzed hydrogenation of 3,3-diphenylallyl alcohol: Efficient synthesis
of 3,3-diarylpropylamine drugs diisopromine and feniprane
Thiago S. Claudino, Jackson D. Scholten*, Adriano L. Monteiro
Laboratory of Molecular Catalysis - Instituto de Química - UFRGS
Av. Bento Gonçalves, 9500 Porto Alegre 91501-970 - CP 15003 – Brazil
*
Corresponding author: Tel.: +55 51 33089628; fax: +55 51 33087304; e-mail:
adriano.monteiro@ufrgs.br, jackson.scholten@ufrgs.br.

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Abstract
The Pd-catalyzed selective hydrogenation of C=C double bond in (3,3-
diphenylallyl)diisopropylamine or 3,3-diphenylallyl alcohol was evaluated using different
catalytic systems [Pd/C, Pd(OAc)2/ionic liquid, isolated Pd(0) nanoparticles]. For the (3,3-
diphenylallyl)diisopropylamine, hydrogenolysis is preferred over hydrogenation, and only
moderate selectivities were obtained for the desired product. However, complete
conversion and 100% selectivity were obtained for the hydrogenation of 3,3-diphenylallyl
alcohol using isolated Pd(0) nanoparticles under mild condition. This successful strategy
enabled the effective synthesis of diisopromine and feniprane drugs and opens new
possibilities for the preparation of other biologically active compounds.
Keywords: hydrogenation, palladium, nanoparticles, allyl amine, allyl alcohol, 3,3-
diarylpropylamines

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1
. Introduction
Aliphatic amine is one of the most important functionalities present in drugs and
drug candidates [1]. Secondary and tertiary 3-3-diarylpropylamines have an important
biological activity, and small variations in the amine moiety or aryl group allowed the
development of several pharmaceuticals used as antiallergic, spasmolytic, choleretic,
coronardilatic or antimuscarinic drugs [2]. Various methods for the synthesis of these
pharmaceuticals based on metal-catalyzed reactions have been developed. For instance, 3-
3
-diarylpropylamines were obtained via Rh-catalyzed hydroaminomethylation of 1,1-
diphenylethene [3-5], tandem Pd-catalyzed Heck reaction/hydrogenation of cinnamides
6,7] or cinnamic ester [8] substrates, and Cu-catalyzed asymmetric addition of aryl
Grignard reagents to oxazolidinones cinnamides [9].
[
Fig. 1. 3,3-Diaryl(hetearyl)propylamine drugs.
Pd-catalyzed cross-coupling reactions are powerful synthetic tools that are widely
employed in the synthesis of drugs and drug-candidates [10]. In our continuing efforts of
developing simple and efficient Pd-catalyzed methodologies [11], we developed selective

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protocols for the stereo-controlled synthesis of 3,3-diarylated allyl amines and alcohols
starting from inexpensive and commercially available trans-cinnamyl alcohol and trans-
cinnamyl chloride [12]. The bromination/dehydrobromination/Suzuki reaction sequence
furnished the (E)-3,3-diarylallyl amines and alcohols with good to excellent yields and with
high stereoselectivity (E >98%). The next step in our project was to obtain the 3-3-
diarylpropylamines drugs 3 via direct catalytic hydrogenation of (3,3-diphenylallyl)
diisopropyl amine (1) or via hydrogenation of 3,3-diphenylallyl alcohol (2), followed by
mesylation of 4 and nucleophilic substitution with the appropriated amine (Fig. 2). We
chose to investigate Pd catalysts because they are active for both Suzuki coupling and
hydrogenation including in sequential procedures that use the same catalyst [13]. Herein,
we report our results about the hydrogenation of allyl substrates 1 and 2 and their
application for the synthesis of diisopromine and feniprane drugs.
Fig. 2. Synthesis of diphenylpropyl amine drugs 3.
2
. Experimental
All reactions were carried out under argon atmosphere in resealable Schlenk tubes. The
chemicals were purchased from commercial sources and used without further purification.

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Methanol was degassed prior to use. (3,3-diphenylallyl)diisopropyl amine (1) and 3,3-
diphenylallyl alcohol (2) were prepared according to the procedure developed in the group
[12].
The
ionic
liquid
(IL)
1-butyronitrile-3-methylimidazolium
N-
bis(trifluoromethanesulfonyl)imide [(BCN)MI][NTf2] was obtained according to known
procedures from (BCN)MI.Cl [16]. Isolated Pd(0) nanoparticles (NPs) were obtained from
Pd(OAc)2 in [(BCN)MI][NTf2] as described in the literature [17]. The procedure for the
synthesis of diisopromine and feniprane was adapted from the literature [18]. The catalytic
hydrogenation reactions were carried out in a modified Fischer-Porter bottle with a glass
tube immersed in a silicone oil bath and pressurized with hydrogen gas (1-4 bar). The
temperature was maintained at 25, 40 or 80 °C using a hot-stirring plate connected to a
digital controller (ETS-D4 IKA). NMR spectra were recorded using a Varian 400
spectrometer and a Bruker Avance 400 MHz instrument. Mass spectra were obtained using
a GC/MS Shimadzu QP-2010 SE (EI, 70 eV) equipped with a 30 m DB-5 column. Gas
chromatography analyses were performed using a Shimadzu GC-2010 Plus equipped with a
3
0 m DB-5 column and FID detector. ICP-OES analyses were conducted using a Spectro
Arcos spectrometer (IQ-USP, Brazil).
2
.1. General procedure for the catalytic hydrogenation reactions
2
.1.1. Hydrogenation using Pd/C or isolated Pd(0) NPs as catalyst
The following procedure is representative of all performed reactions. The following
compounds were added to a Fischer-Porter bottle: 1 or 2 (0.5 mmol), Pd/C 10% (0.5-10
mol%) or Pd(0) NPs (5 mol%) and methanol (5 mL). The reaction bottle was evacuated and
filled with argon (5 cycles), then pressurized with hydrogen, and the mixture was stirred at

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the desired temperature and time. After filtration to remove the catalyst, the resulting
product mixtures were analyzed using GC and GC-MS.
2
.1.2. Hydrogenation using Pd(0) NPs prepared in situ from Pd(OAc)2 in [(BCN)MI][NTf2]
The following procedure is representative of all the reactions performed. The following
compounds were added to a Fischer-Porter bottle: 1 or 2 (0.2 mmol), Pd(OAc)2 (1 or 5
mol%), [(BCN)MI][NTf2] (0.5 mL) and dichloromethane (1 mL). The reaction bottle was
stirred at room temperature for 5 minutes and, then, CH2Cl2 was removed via vacuum.
Next, the system was evacuated and filled with argon (5 cycles), the bottle was pressurized
with hydrogen, and the mixture was stirred at the desired temperature and time. The
resulting product mixtures were analyzed using GC (undecane as internal standard) and
GC-MS after extracting the final mixture with diethyl ether. In the study on recyclability,
the Pd NPs catalyst were separated from the reactional mixture by centrifugation, then
washed with dichloromethane, dried under vacuum at room temperature and then reused in
the hydrogenation reaction.
2
.2. Synthesis of diisopromine and feniprane
In a Schlenk tube, a solution of 4 (150 mg, 0.71 mmol) and triethylamine (118 L, 0.85
mmol) in 2 mL of anhydrous dichloromethane was cooled at 0 °C under argon. Then, mesyl
chloride (60 L, 0.77 mmol) was added dropwise, and the mixture was stirred for 1 h at 0
°C, warmed to room temperature and stirred for additional 3 h. The reaction mixture was
extracted with dichloromethane (3 × 15 mL), and the organic phases were combined,
washed with 10% aqueous HCl solution and dried over magnesium sulfate. The solvent

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was evaporated under reduced pressure, and the product was transferred to a Schlenk tube
containing diisopropylamine (0.4 mL, 2.8 mmol), Li2CO3 (104 mg, 1.4 mmol) and
acetonitrile (8 mL). The reaction mixture was maintained at reflux under argon for 48 h.
The mixture was concentrated in a rotatory evaporator, and the product was purified using
chromatography on silica gel (hexane:ethyl acetate = 50:50) giving a yellowish oil (74%
1
yield) analyzed using GC, GC-MS and H-NMR.
The same procedure was used for the synthesis of feniprane with an 88% yield, using
piperidine as amine and reflux for 24 h.
3
. Results and Discussion
Because 3-3-dihenyllpropylamines can be obtained directly via catalytic hydrogenation of
1
, we started to investigate the catalytic hydrogenation of 1 using Pd/C as a catalyst (Table
1
). We initially investigated a set of experiments changing catalyst loading, hydrogen
pressure, temperature and reaction time. In addition to the expected hydrogenation product
3,3-diphenylpropyl)diisopropyl amine (3), 1,1-diphenylpropene (5) and 1,1-
(
diphenylpropane (6) were also observed. Even after 24 h at room temperature, only 11%
conversion was obtained using 1 mol% Pd/C, 4 bar of hydrogen (Table 1, entry 1). In
addition, the major product was the olefin 5 (48%), which was obtained via hydrogenolysis
of 1. After hydrogenolysis, the olefin 5 was hydrogenated to 1-diphenylpropane (6). To
exclude the formation of 6 from 3a, we submitted hydrogenated amine 3a to the
hydrogenation conditions (Table 1, entry 4) and after 18 h only 2% conversion was
observed. In addition, any reaction was observed for the hydrogenated alcohol 4 (see Table
2
) under the same hydrogenation conditions (Table 1, entry 4, 16 h). The hydrogenolysis of
allylic compounds in the presence of different hydride sources is catalyzed by palladium

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[
19]. However, because amine is a poorer leaving group, we expected that the
hydrogenolysis could be suppressed. By slightly increasing the temperature to 40 °C, the
conversion after 5 h increased to 60% but without the significant change in selectivity
(Table 1, entry 2). Increasing the palladium loading, it was possible to achieve almost total
conversion with a better selectivity of the desired product 3a (35%) under the same reaction
conditions (Table 1, entry 4). Similar results were achieved when the reaction was carried
out at 80 °C for 2 h (Table 1, entry 5), although the formation of the hydrogenolysis
product 6 was preferred after a long reaction time (Table 1, entry 6). It is important to note
that low yields for the hydrogenation of 3,3-diarylallyl amines using Pd(OH) /C have also
2
been described [20, 21].
Because metal NPs have been used as excellent catalysts in several catalytic reactions [22-
2
4] due to their high surface-to-volume ratio and quantum effects [25,26], they appear as
alternative promoters for the hydrogenation of the substrate 1. For this purpose, Pd(0) NPs
were prepared in situ via the reduction of Pd(OAc)2 in a nitrile-functionalized imidazolium
IL, [(BCN)MI][NTf2] (see details in the experimental section). Surprisingly, the selectivity
in the product 3a remained low (Table 1, entries 8 and 9) and was not improved compared
with the classical Pd/C catalyst. Moreover, the use of isolated Pd(0) NPs produces a modest
conversion together with a high generation of the products from the hydrogenolysis
reaction (Table 1, entry 10), which is typical of surface metal catalysts [27].
a
Table 1. Pd-catalyzed hydrogenation of amine 1
Entry
[Pd]
Mol % H2 (bar) Temp.
Time
Conv.
Selectivity (%)

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o
Pd
1
( C)
(h)
24
(%)
11
3
a
5
6
Pd/C
Pd/C
4
4
25
18
19
48
34
1
2
1
5
40
25
5
2
60
76
43
37
38
53
Pd/C
Pd/C
Pd/C
Pd/C
1
4
4
4
10
35
33
22
3
4
5
6
5
5
5
40
80
80
3
2
98
3
0
0
62
67
78
100
100
24
Pd/C
10
5
4
4
40
25
1
100
53
10
20
0
90
57
7
8
9
Pd(OAc)2/IL
18
23
Pd(OAc)2/IL
Pd(0) NPs
5
5
4
4
40
40
22
24
56
68
15
8
41
56
44
36
10
a) Reaction conditions: the reactions were performed in methanol (5 mL), except for entries 8 and 9,
where IL (BCN)MI.NTf2 (0.5 mL) was used.
To overcome the difficulty of obtaining the hydrogenated product 3a, another strategy
starting from the hydrogenation of alcohol 2 was employed. The classical heterogeneous
catalyst Pd/C showed excellent activities providing high conversions and selectivities for
the desired product 4. In addition, the increase of the amount of catalyst causes a decrease
of the reaction time, and no considerable changes of the selectivities were observed (Table
2
, entries 1-4). Comparing the entries 4 and 5 (Table 2), it is possible to note that even after
a prolonged reaction time there is no occurrence of hydrogenolysis towards product 6. In
the cases of Pd(0) NPs in IL (Table 2, entries 6 and 7), up to 96% of conversion was

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verified reaching similar or superior selectivity compared with Pd/C. The long reaction
time observed for the hydrogenation in IL is typical of such systems, where the low
solubility of H2 and the diffusion of the compounds into the IL phase are important aspects
to be considered [21]. To our delight, the hydrogenation of 2 in the presence of isolated
Pd(0) NPs afforded a total conversion and unique selectivity for the desired compound 4
after 1 h at 25 °C (Table 2, entry 8). Using lower time (30 min, Table 2, entry 9) or lower
Pd loading (1 mol%, Table 2, entry 9) incomplete conversions were obtained. After a new
reaction under the optimized conditions (Table 3, entry 8), the Pd catalyst was separated
and reused three more times affording 97% conversion for the first and second recycle and
9
5% conversion for the third recycle. Finally, we tested a different 3,3-diarylallyl alcohol
under optimized conditions (Table 2, entry 8). Then, (E)-3-(4-methoxyphenyl)-3-
phenylprop-2-en-1-ol was hydrogenated to the 3-(4-methoxyphenyl)-3-phenyl-1-propanol
in 100% conversion and selectivity. There is no doubt that this is one of the best results
reported for the hydrogenation of C=C double bond in diarylallyl alcohols.
a
Table 2. Pd-catalyzed hydrogenation of alcohol 2
Selectivity (%)
Entry
[Pd]
Mol % Pd Time (h)
Conv (%)
4
6
Pd/C
Pd/C
Pd/C
0.5
0.5
1
2
19
1
19
96
4
1
2
3
100
100
90
91
10
9

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Pd/C
Pd/C
5
5
1
5
5
5
1
0.08
1
100
100
64
87
88
13
12
0
4
5
6
7
8
9
Pd(OAc)2/IL
Pd(OAc)2/IL
Pd NPs
22
24
1
100
97
96
3
100
55
100
100
100
0
Pd NPs
0.5
20
0
Pd NPs
7
0
10
a) Reaction conditions: the reactions were performed at 4 bar of H2 at 25 °C in methanol (5 mL), except
for entries 6 and 7, where IL (BCN)MI.NTf2 (0.5 mL) was used as a solvent.
Finally, diisopromine (3a) and feniprane (3b) were synthesized via the reaction of alcohol 4
with mesyl chloride followed by the addition of the respective amine in a non-protic solvent
(Fig. 3). Alcohol 4 was mesylated with a quantitative yield and without a need for
purification after the work-up. A mixture of the mesylated product with diisopropylamine,
Li2CO3 and acetonitrile was refluxed for 48 hours. After the work-up, the product was
purified via chromatography on silica giving diisopromine (3a) with a 74% yield.
Piperidine is a less hindered amine compared with diisopropylamine, and feniprane was
obtained with a higher yield (88%) in a short time (24 h). A significant problem for the
pharmaceutical industry is the potential for palladium-containing impurities to remain in
the desired compound after isolation [28]. Substrate 2 was obtained by Pd-catalyzed
Suzuki cross-coupling reaction [12] and the residual palladium content was found to be
relatively high (156 ppm). However, for the final drugs diisopromine (3a) and feniprane
(3b), the residual palladium contents were found to be <1 ppm.

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Fig. 3. Synthesis of the diisopromine (3a) and feniprane (3b) drugs from the allylic alcohol
(2).
4
. Conclusions
In this study, we developed a new successful strategy for the synthesis of diisopromine and
feniprane drugs, starting from the hydrogenation of 3,3-diphenylallyl alcohol catalyzed by
Pd(0) NPs under mild conditions. In fact, this protocol allowed the total conversion and
selectivity of the substrate to the desired product 4 without any hydrogenolysis side
reaction. Other catalysts tested, such as Pd/C and Pd(0) NPs in IL, also demonstrated
satisfactory results, but the best result was observed when isolated Pd(0) NPs was
employed. In contrast, attempts to perform the selective hydrogenation of (3,3-
diphenylallyl)diisopropyl amine as substrate provided only moderate selectivities for the
desired product, although high conversions were reached using different catalytic systems.
However, it is important to consider that even these moderate selectivities can be consider

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as positive results taking into account the difficulty of reducing the C=C double bond in
diarylallylamines. Therefore, we demonstrated that the selective hydrogenation of 3,3-
diarylallyl alcohols to the respective 3,3-diarylpropyl alcohols catalyzed by Pd(0) NPs may
be a potential methodology for preparing intermediate compounds for the synthesis of
several drug candidates.
Acknowledgments
We acknowledge the following Brazilian agencies for financial support and scholarship:
CNPq (307514/2013-7), PRONEX-FAPERGS 16/2551-0000, CAPES, and INCT-Catálise
(465454/2014-3).
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Highlights:





New successful strategy for the synthesis of diisopromine and feniprane drugs.
Pd- Catalyzed hydrogenation of 3,3-diphenylallyl alcohol under mild conditions.
Pd nanoparticles gave complete conversion and 100% selectivity.
Hydrogenation generates a common intermediate for different drugs.
Diisopromine and feniprane drugs were obtained in three steps.