Hydroaminomethylation of eugenol with di-n-butylamine catalyzed by rhodium complexes: Bringing light on the promoting effect of Br?nsted acids

Article abstract of DOI:10.1016/j.apcata.2012.08.033

The hydroaminomethylation of eugenol with di-n-butylamine was performed employing a bis[(1,5-ciclooctadiene)(μ-methoxy)rhodium(I)] as pre-catalyst. In the absence of phosphines, the catalyst was efficient in the process, but the regioselectivity for amines was poor. For phosphine-promoted catalyst, the chemoselectivity at the hydroformylation step improved, but the hydrogenation of enamine intermediates was hampered. The regioselectivity within the class of amines was surprisingly high (>96%) for the linear product. The addition of triflic acid (10-20 mol%) improved significantly the efficiency of HAM. Employing the 2,2′-bis((diphenylphosphino)methyl)-1,1′-binaphthyl as ancillary and triflic acid as a promoter, the linear product was obtained in up to 93% yield.

Full text of DOI:10.1016/j.apcata.2012.08.033

Applied Catalysis A: General 445–446 (2012) 204–208
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydroaminomethylation of eugenol with di-n-butylamine catalyzed by rhodium
complexes: Bringing light on the promoting effect of Brönsted acids
Kelley C.B. Oliveira, Alexandra G. Santos, E.N. dos Santos∗
Departamento de Química-ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2012
Received in revised form 15 August 2012
Accepted 17 August 2012
Available online 31 August 2012
The hydroaminomethylation of eugenol with di-n-butylamine was performed employing a bis[(1,5-
ciclooctadiene)(␮-methoxy)rhodium(I)] as pre-catalyst. In the absence of phosphines, the catalyst was
efficient in the process, but the regioselectivity for amines was poor. For phosphine-promoted catalyst,
the chemoselectivity at the hydroformylation step improved, but the hydrogenation of enamine inter-
mediates was hampered. The regioselectivity within the class of amines was surprisingly high (>96%)
for the linear product. The addition of triflic acid (10–20 mol%) improved significantly the efficiency of
HAM. Employing the 2,2ꢀ-bis((diphenylphosphino)methyl)-1,1ꢀ-binaphthyl as ancillary and triflic acid as
a promoter, the linear product was obtained in up to 93% yield.
Keywords:
Hydroaminomethylation
Acid promotion
Tandem catalysis
Renewable feedstock
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
reactions applied to natural products that can be obtained in kilo-
ton scale through sustainable methods [17–20]. We have recently
The hydroaminomethylation of olefins (HAM) is a three-step
reaction consisting of the hydroformylation of an olefin, the con-
densation of the formed aldehyde with an added primary or
secondary amine, and the catalytic hydrogenation of the result-
ing imine/enamine to yield the corresponding amine. The last two
steps are closely related as the amine condensation is reversible
and the hydrogenation step withdraws the imine/enamine from the
equilibrium, driving the reaction to completion. This tandem trans-
formation [1] is exemplified in Scheme 1 for the HAM of eugenol
(1) with di-n-butylamine.
reported the HAM of various monoterpenes to obtain amines,
which have potential bioactivity [21]. Herein we report the HAM of
eugenol (1) with di-n-butylamine (Scheme 1) to obtain three new
amines. Despite the potential bioactivity of the products, to the best
of our knowledge, it is the first time that HAM of arylpropenes of
natural sources is described.
2. Experimental
2.1. General procedure
Although known for a long time [2], only recently has HAM
gained importance in the synthesis of more complex molecules
[3], such as pharmaceuticals and other biologically active sub-
stances [4–9]. The use of special ligands such as diphosphines [10],
diphosphites [7], xanthene-based dipyrrolylphosphines [11], and
tetradentated phosphorus ligands [12] has allowed good selectivity
control associated with high activity. The development of biphasic
systems, in which the catalyst is dissolved in water [13], ionic liq-
uids [14], or multiphase thermophilic systems [15], has allowed
for the easier catalyst recycling, which favors industrial applica-
tions. The advances in this synthetically efficient process have been
recently reviewed [16].
Eugenol, di-n-butylamine, triphenylphosphine, sulfuric acid,
para-toluenesulfonic acid, and trifluoromethanesulfonic acid were
purchased from Aldrich. 2,2ꢀ-bis((Diphenylphosphino)methyl)-
1,1ꢀ-binaphthyl (NAPHOS) was kindly donated by Prof. B.
Hanson (Virginiatech-USA). Toluene was refluxed over sodium/
benzophenone for six hours and distilled under argon. [Rh(cod)(␮-
OMe)]2 was synthesized according to literature procedure [22].
2.2. Catalytic runs
The pre-catalyst [Rh(cod)(␮-OMe)]2 (5.0 × 10−3 mmol), the
phosphorus ancillary (if any) and a PTFE-covered magnetic stir-
ring bar were placed in a stainless steel bomb, which was closed
and purged with three cycles of vacuum and argon. In a Schlenk
tube, a solution was prepared by adding toluene (30 mL), eugenol
(10 mmol), di-n-butylamine (10 mmol) and then the acid (if any).
The solution was transferred under inert atmosphere to the bomb,
Our group is involved in the preparation of useful or potentially
useful chemicals employing transition metal complex catalyzed
∗
Corresponding author. Tel.: +55 3134095743; fax: +55 3134095700.
E-mail address: nicolau@ufmg.br (E.N. dos Santos).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.033

K.C.B. Oliveira et al. / Applied Catalysis A: General 445–446 (2012) 204–208
205
Scheme 1.
which was pressurized with carbon monoxide (10–20 atm) and
then with hydrogen (to 40–80 atm). The bomb was placed in a pre-
heated silicone bath over magnetic stirring. Liquid samples were
taken periodically through a dip tube.
aldehydes (3–5) and enamines (6–8) were also observed in the reac-
tion solution. In Table 1 the results for HAM of 1 employing different
catalytic systems and reaction conditions are presented.
In entries
1 and 2 and Fig. 1 the unpromoted and the
triphenylphosphine-promoted rhodium catalysts are compared at
100 ◦C. For the unpromoted system (entry 1 and Fig. 1(a)), the reac-
tion was quite efficient as the substrate was completely converted
into products after 2 h. Nevertheless, the chemoselectivity was poor
as ca. 20% of the isomeric olefin 2 was formed. Under these condi-
tions, 2 was also converted into products, as 11 could be exclusively
formed from 2. After 24 h, 90% of the reactant was converted into
2.3. Product analysis
The products were quantitatively analyzed by gas chromatog-
raphy (GC) using a Shimadzu GC2010 instrument equipped with
a split/splitless injection port and flame ionization detector, fitted
with a Restek Rtx-5 capillary column (30 m × 0.25 mm × 0.25 ␮m).
Conversion and product distribution were determined by GC based
on the reacted eugenol. Qualitative analysis was made by GC
coupled with mass spectrometry in a Shimadzu GC2010/QP2010-
plus instrument fitted with a Restek Rtx-5 MS capillary column
(30 m × 0.25 mm × 0.25 ␮m), operating at 70 eV. The main products
were isolated by column chromatography and analyzed by and 1H,
13C, DEPT-NMR (Bruker Avance DRX 200, TMS, CDCl3).
(a)
80
70
60
50
40
30
20
10
2.4. Spectroscopic data
9: Mass spectrometry: (m/z/rel.int.): 307/5.0 (M+); 264/100;
168/15; 142/90; 100/50; 44/20 13C NMR: 14.02, 20.71, 26.05, 28.52,
29.66, 35.46, 53.49, 53.59, 55.78, 110.98, 114.23, 120.79, 134.33,
143.67, 146.43. 1H NMR: 0.90 (t, 6H, CH3, 3J = 7.0 Hz); 1.55–1.23
(m, 12H, CH2); 2.54–2.42 (m, 8H, CH2); 3.86 (s, 3H, CH3); 6.65 (d,
2H, Ar:CH, 3J = 7.6 Hz); 6.81 (d, 1H, Ar:CH, 3J = 7.6 Hz).
10: Mass spectrometry: (m/z/rel.int.): 307/5 (M+); 142/100;
100/58; 44/11. 13C NMR: 13.90; 18.05, 20.57, 29.62; 34.03, 35.32;
53.31, 54.29; 55.50, 110.95; 114.44; 120.55, 134.00, 143.74; 146.59.
1H NMR: 1.11 (t, 9H, CH3, 3J = 6.8 Hz); 1.68–1.47 (m, 8H, CH2); 1.99
(m, 1H, CH); 2.74–2.55 (m, 8H, CH2); 4.02 (s, 3H, CH3); 6.84 (d, 2H,
Ar:CH, 3J = 8.4 Hz); 6.99 (d, 1H, Ar:CH, 3J = 7.6 Hz).
11: Mass spectrometry: (m/z/rel.int.): 264/5; 142/100; 100/55;
44/30. 13C NMR: 12.07; 14.00, 20.53, 27.58, 28.47, 45.71, 53.94,
55.87; 60.66; 110.42; 114.15, 120.34, 136.42, 143.86; 146.35. 1H
NMR: 0.75 (m, 3H, CH3, 3J = 7.4 Hz); 0.86 (m, 6H, CH3, 3J = 7.2 Hz);
1.42–1.21 (m, 10H, CH2); 2.49–2.39 (m, 4H, CH2); 2.62–2.56 (m,
3H,CH, CH2); 3.88 (s, 3H, CH3); 6.65 (d, 2H, Ar:CH, 3J = 7.6 Hz); 6.84
(d, 1H, Ar:CH, 3J = 7.8 Hz).
2
4
8
8
10 12 14 16 18 20 22
6
6
time /h
(b)
80
70
60
50
40
30
20
10
2
4
10 12 14 16 18 20 22 24
3. Results and discussion
time/h
The hydroaminomethylation (HAM) of eugenol (1) with di-n-
butylamine led to three isomeric amines (9–11), as depicted in
Scheme 1. Depending on the reaction conditions, the intermediate
Fig. 1. Hydroaminomethylation of eugenol: (a) unpromoted system and (b)
PPh3-promoted system (
conditions, see Table 1.
). For

206
K.C.B. Oliveira et al. / Applied Catalysis A: General 445–446 (2012) 204–208
Table 1
Hydroaminomethylation of eugenol (1) with di-n-butylamine: ligand effect.a
Entry
Ligand
P/Rhb
Conv. (%)
Product distribution (%)
Regioselectivity (%)c
2
Aldehydes
Enamines
Amines
9
10
11
1
2
3
None
PPh3
PPh3
NAPHOS
NAPHOS
0
2
10
2
100d
100
100
34
4
0
0
3
1
24
32
34
21
0
3
10
56
5
90
73
58
7
61
96
94
>99
97
33
4
5
0
3
6
0
1
0
0
4
5e
2
100
10
64
a
Conditions: 1 (10 mmol); di-n-butylamine (10 mmol); [Rh(cod)(␮-OMe)]2 (5.0 × 10−3 mmol), toluene (30 mL), 4.0 MPa (CO:H2 = 1:3), 100 ◦C, 24 h. For products, the value
“zero” means not observed or <0.5%.
b
Phosphorus/rhodium atomic ratio.
Related to the sum of amines (9 + 10 + 11).
c
d
5% of hydrogenation of substrate.
e
120 ◦C.
amines, but the regioselectivity was poor and ca. 5% of the hydro-
genation product (not shown in Scheme 1) was also formed.
In order to reach a better selectivity control, two equivalents
of PPh3 were added to the system (entry 2 and Fig. 1(b)). For
the PPh3-promoted system at 100 ◦C, the chemoselectivity at the
hydroformylation step increased, as the isomerization and hydro-
genation products were drastically reduced. After 2 h, 50% of the
linear amine 9 was formed, but the rate of the enamine hydrogena-
tion slowed down in comparison to the unpromoted system. Both
the aldehydes 3 and 4 and the enamine 6 were observed in signif-
icant amounts after two hours of reaction. The latter was slowly
converted into 9 and the equilibrium 3/6 was shifted to the prod-
uct. It is noteworthy that, although 20% of the branched aldehyde
4 was formed, no significant amount of the branched enamine 7
was observed (Fig. 1(b)), due to the fact that the equilibrium 4/7
was shifted to the reactant. As the equilibrium concentration of 7
was low and so was its rate of hydrogenation, the amine 10 was
formed at a very low rate. Although regioselectivity is defined at
the hydroformylation step (irreversible), it was possible to obtain a
high regioselectivity within the class of amines for the linear product
9 due to a kinetic resolution in the steps of amine condensa-
tion/enamine hydrogenation. Thus, with this protocol it is possible
to obtain the linear amine in a high regioselectivity (96%) employing
the cheap PPh3 ligand, which is produced worldwide in a multi-ton
scale.
In order to improve the regioselectivity for the linear product at
the hydroformylation step, we employed the chelating diphosphine
2,2ꢀ-bis((diphenylphosphino)methyl)-1,1ꢀ-binaphthyl (NAPHOS)
as ancillary. In a previous work [24], we successfully employed
this ligand to favor the regioselectivity for linear products in
the hydroformylation of arylpropenes, such as eugenol, eugenol
methyl ether and safrole. Nevertheless, under hydroaminomethy-
lation conditions at 100 ◦C and P/Rh molar ratio of two (Table 1,
entry 4), the system presented a poor performance: after 24 h
only 34% of the substrate was converted and only 7% of amines
were formed. With this chelating diphosphine, the amount of the
rhodium species containing one or no phosphorus atom bound
to the metal is reduced and the ones containing two phosphorus
atoms are too electron rich to carry out efficiently the enamine
hydrogenation under the conditions employed. At 120 ◦C (entry 5),
the HAM of eugenol employing the Rh/NAPHOS system showed to
be more efficient, but still only 64% of the substrate was converted
into amines after 24 h.
In previous works [6,11,13,25], it has been reported that the
addition of acids is beneficial for the hydroaminomethylation,
although the reasons remain unknown. In order to get some insight
into this matter, we performed a comparative study employing
strong acids of the same nature, but with different pKa and coor-
dination capacities. The results are presented in Table 2. For the
PPh3-promoted system at 120 ◦C in the absence of acids (entry 6),
the reaction was quite efficient, as 83% of the product was converted
into amines with a high regioselectivity within the amine class.
Surprisingly, under the same reaction conditions, but with the addi-
tion of sulfuric acid (10 mol% with respect to 1) made the system
less active both at the hydroformylation step and, especially, in the
enamine hydrogenation step. Only 2% of amines were formed after
24 h, in spite of the fact that 86% of the substrate was converted into
In Table 1, entry 3, the PPh3/Rh is 10. Under these conditions
the enamine hydrogenation was even less efficient, as the enamine
amount increased from 3% to 10% (c.f. entries 2 and 3). At a higher
concentration of PPh3, the rhodium species containing one or no
phosphorus ligand must be reduced and these species, which have
comparatively lower electron density at the metal center, seem to
play an important role in the enamine hydrogenation step [21,23].
Table 2
Hydroaminomethylation of eugenol (1) with di-n-butylamine: effect of added acid.a
Entry
Ligand
P/Rhb
Acid
Temp. (◦C)
Conv. (%)
Product distribution (%)
Regioselectivity (%)c
2
Aldehydes
Enamines
Amines
9
10
11
6
7
8
PPh3
PPh3
PPh3
PPh3
NAPHOS
NAPHOS
NAPHOS
2
2
2
2
2
None
120
120
120
120
100
100
100
100
86
0
13
0
1
5
17
76
23
11
9
0
9
0
0
0
0
0
83
2
96
94
89
84
99
99
98
4
6
11
14
1
0
0
0
2
0
0
0
d
H2SO4
HOTsd
HOTfd
HOTfe
HOTfe
HOTfe
100
100
100
100
100
75
88
86
93
95
9
10
11
12
20
40
2
2
5
3
1
2
Conditions: 1 (10 mmol); di-n-butylamine (10 mmol); [Rh(cod)(␮-OMe)]2 (5.0 × 10−3 mmol), toluene (30 mL), 4.0 MPa (CO:H2 = 1:3), 24 h. For products, the value “zero”
a
means not observed or <0.5%.
b
Phosphorus/rhodium atomic ratio.
Related to the sum of amines (9 + 10 + 11).
1.0 mmol.
2.0 mmol.
c
d
e

K.C.B. Oliveira et al. / Applied Catalysis A: General 445–446 (2012) 204–208
207
products (entry 7). The acid-catalyzed trimerization of the alde-
hydes, which is detrimental to the enamine formation, must also be
considered as an explanation for the lack in conversion of aldehydes
to enamines in entry 7. Comparing the system in the absence of
acid (entry 6) with the one in the presence of para-toluenesulfonic
acid (entry 8), it is possible to observed that, after 24 h, less of
amines were formed, but the proportion of 10 increased. The lat-
ter observation suggests that the system became more efficient for
the enamine hydrogenation, while the former observation suggests
that the acid hampers the amine condensation step. Employing the
considerably stronger trifluoromethanesulfonic acid (HOTf) under
the same reaction conditions (entry 9) the HAM became consid-
erably more efficient as 88% of amines were formed in 24 h. The
branched enamine was also converted at a higher rate, as 10 cor-
responded to 14% of the amine products after 24 h.
The promoting effect of the acid on HAM has been explained
in at least two manners [6,13]. Employing the strong tetrafluro-
boric acid (HBF4) as a promoter, Routaboul et al. [6] suggested
that the role of the acid would be to protonate the intermediate
imine or enamine, as the corresponding imminium or enaminium
salt could be hydrogenated more easily. Behr et al. [13] employed
the ammonium salts of acids with different strengths instead of
the amine counterparts. The authors suggested that the effect of
the acid could be related to the formation of cationic rhodium
species that are more efficient for the imine/enamine hydrogena-
tion. Indeed, cationic rhodium complexes are efficient catalyst
to hydrogenate C C double-bonds in molecules containing other
coordinating groups [26], such as enamines. In a stoichiometric
study, Crozet et al. [27] demonstrated the interplay of cationic and
neutral rhodium species under conditions relevant for the HAM.
When para-toluenesulfonic acid was added to a neutral rhodium
species, it was readily converted into a cationic one. Nevertheless,
when piperidinium para-toluenesulfonate was employed in place
of HOTs, the formation of the cationic species did not occur even
after 24 h.
The strength of the acids employed in this work varies signifi-
cantly, as the pKa1 for sulfuric acid is −3, for HOTs is −2.8, and for
HOTf is −14. On one hand, as we used 5 to 10-fold excess of di-n-
butylamine, a leveling effect is expected and, at the beginning of
the reaction, the strongest acid must be the di-n-butylammonium
formed due to the protonation of the di-n-butylamine by the acids.
On the other hand, the counter-anions have different coordina-
tion abilities (SO42− > OTs− > OTf−) and the enamine hydrogenation
ability of the systems follows the reversal of this order. Even that
the ammonium ion is acidic enough to promote the formation
of cationic complexes, in the presence of coordinating counter-
ion, a neutral complex would be immediately formed. Only strong
acids, which generate a non-coordinating ion by hydrogen loss, can
produce a stable cationic complex. This observation can be inter-
preted as favoring the hypothesis that the ammonium ion is acidic
enough to promote the formation of cationic complexes, which
would efficiently hydrogenate the enamines, provided the counter-
ion is a non-coordinating one. Nevertheless, it does not rule out the
hypothesis that the effect of the acid is to protonate the interme-
diate enamine (or imine) favoring its hydrogenation through the
enaminium (or iminium) salt.
in entries 10–13. Under these conditions the HAM of eugenol with
di-n-butylamine was highly efficient and the linear product was
formed in up to 99% selectivity. The increase in regioselectivity
with the increase in NAPHOS concentration was already observed
previously [25] and the effect was attributed to the entrapment of
rhodium species without phosphorus ligands that are less regios-
elective. An alternative explanation is that in such conditions the
phosphorus atoms of the ancillaries are partially protonated, which
may explain the need for a larger P/Rh ratio to keep good selectiv-
ity. Indeed surprising is the fact that the enamine hydrogenation
seems to be more efficient at higher NAPHOS concentrations, as
in the absence of the acid the opposite behavior was observed.
This fact suggests that, in the presence of strong acid bearing non-
coordinating anion such as HOTf, the more efficient catalyst for
enamine hydrogenation contains electron-donating ligands. This
behavior is exactly the opposite of the one observed for systems
unpromoted by acids [23]. Finally, in entry 12, we demonstrate
that by employing the Rh/NAPHOS/HOTf system, eugenol could be
converted into the linear amine 9 in nearly quantitative yield.
4. Concluding remarks
The hydroaminomethylation of eugenol, a bio-renewable sub-
strate available from essential oils of various plants, was performed
for the first time. Di-n-butylamine was used as amine counterpart.
Three novel amines (9–11) were obtained in high yields, with their
relative amounts depending on the catalyst and the reaction condi-
tions. The reaction was highly selective to the linear amine 9, which
was produced in up to 93% yield, by the appropriate choice of the
ancillary ligand, its amount and the acid additive. Three strong acids
of the same nature, but with different pKa and coordination abil-
ity of the counter-ion have been tested and the latter feature has
showed to be critical: the less coordinating is the counter-ion, the
more efficient is the process. Triflic acid, which is more stable than
the previously reported HBF4, proved to be an excellent promoter
for the hydroaminomethylation of eugenol catalyzed by rhodium
complexes in the presence of phosphines as ancillaries.
Acknowledgments
CNPq and FAPEMIG are gratefully thanked for financial support
to this work. CAPES is thanked for KCBO scholarship. Prof. Elena V.
Gusevskaya is thanked for valuable discussions.
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