Homogeneous hydrogenations of nitriles catalyzed by rhenium complexes

Article abstract of DOI:10.1002/adsc.201000867

Rhenium(I) nitrosyl complexes bearing large bite angle diphosphines efficiently catalyze the hydrogenation of nitriles to the corresponding symmetrical secondary amines. Copyright

Full text of DOI:10.1002/adsc.201000867

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DOI: 10.1002/adsc.201000867
Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium
Complexes
Kunjanpillai Rajesh,a Balz Dudle,a Olivier Blacque,a and Heinz Berkea,*
a
Institute of Inorganic Chemistry, University of Zꢀrich, Winterthurerstrasse 190, CH-8057 Zꢀrich, Switzerland
Fax : (+41)-1-635-6802; e-mail: hberke@aci.uzh.ch
Received: November 18, 2010; Revised: April 4, 2011; Published online: June 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000867.
Abstract: Rhenium(I) nitrosyl complexes bearing
large bite angle diphosphines efficiently catalyze
the hydrogenation of nitriles to the corresponding
symmetrical secondary amines.
ondary amine. This can lead to a symmetrical tertiary
amine.[2] The composition of the reduction products
depends mainly on the nature of the reducing agent,
catalyst, reaction temperature and hydrogen pressure,
and on the structure of the nitrile.[3] Due to a facile
access to nitriles and their frequent avalability as
commodity chemicals, the reaction has great potential
in synthetic organic chemistry and in the production
of pharmaceuticals, agrochemicals, textile and rubber
chemicals.[2a,b] Apart from this aspect, syntheses of
secondary amines are particularly significant in view
of their role as biologically active molecules[3,4] and
Keywords: homogeneous hydrogenation; nitrile re-
duction; rhenium catalysts; secondary amines; tri-
ACHTUNGTRENNUNGethylsilane
Nitriles are difficult to hydrogenate[1] and if they can versatile ligands.[3,5]
be hydrogenated, a crucial selectivity problem arises
Catalytic reduction of nitriles using molecular hy-
concomitant with the reduction processes: the forma- drogen is of great interest as it is more efficient, eco-
tion of mixtures of primary, secondary and tertiary nomic and environmental friendly in processes as
amine, as well as intermediate imines.[2] Although they have been applied in both academia and indus-
many proposals on the mechanism of the formation try.[2b,6] The formation of amines is often achieved by
of secondary amines and tertiary amines have been heterogeneously[7] or homogeneously catalyzed pro-
put forward (Scheme 1, Scheme 2), it is still unclear cesses that involve the hydrogenation of nitriles, in
whether the reductions to amines occur through the which the latter is accomplished using Ni,[8] Ru,[2c,9]
hydrogenation of imines, enamines or the hydrogenol- Rh,[1b] Pd,[10] Ir[11] and Pt[12] complexes. Alkylamines
ysis of gem-diamines.[2a] But what is mechanistically can also be produced by amination of alcohols over
well established, is the reaction between the formed acidic catalysts, however this tends to form also unde-
primary amine and the intermediate imine accompa- sired alkenes.[13] Hydroaminomethylation of al-
nied by the expulsion of ammonia, which can give
rise to a symmetrical secondary amine and repetition
of this type of reaction sequence with the formed sec-
Scheme 1. Formation of different types of imines/amines in
the hydrogenation of aromatic nitriles.
Scheme 2. Formation of different types of imines/amines/en-
amines in the hydrogenation of aliphatic nitriles
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kenes,[14] reductive amination of carbonyl compounds,
treatment of primary amines with alkyl halides or di-
ACHTUNGTRENNUNGalkyl sulfates or sulfonates, addition of nucleophiles
or radicals to N-substituted imines etc., have been
widely reported for the synthesis of secondary
amines.[4] However, the traditional methods for secon-
dary amine formation are often problematic, because
of harsh reaction conditions, generally poor yields
and/or low chemoselectivities.[4]
Scheme 4. Structure of the diastereomers A and B of 7 and
8.
Herein we communicate the first example of the
homogeneous reduction of nitriles to amines using
rhenium catalysts bearing large bite angle bisphos-
phines. Bite angles were shown to be crucial ligand actions on these lines are available,[19] much more in-
À
parameters and their tuning allowed us to boost the teresting seemed the fact of the activation of the C H
performance of catalysts.[15] A large bite angle of the bond ortho to the phosphorus substituent giving rise
ligands induces distortion from octahedral geometry to the unusually stable four-membered rhenacycles
that can lead to activation of the metal center and the (Scheme 4). From X-ray diffraction studies (Figure 1)
bound atoms involved in the catalyses.[15] A series of it was derived that both 7A and 7B differ by the axial
novel rhenium(I) catalysts was prepared with the arrangement of Br and NO, as a consequence by
large bite angle sixantphos (1)[16] and the related, but which in 7A the sixanthene moiety gets close to Br,
previously unknown sixantphos-Ph2 (2) ligands of the whereas in 7B, it is close to NO. Although 8B is anal-
xanthphos family. Sixantphos-Ph2 (2) was prepared by ogous to 7B, surprisingly, 8A and 7A differ by the
an analogous procedure adopted for the synthesis of spatial arrangenent of the sixanthene moiety.[16]
sixantphos
(CH3CN)Br2(NO)] (3) and [Re(2)
(4) were prepared by reacting [ReBr5(NO)] [NEt4]2
(1).[17]
The
complexes
[Re(1)-
All the four complexes 3, 4, 7 and 8 turned out to
G
ACHTUNGTRENNUNG
[18]
responding symmetrical secondary amines. Although
with 1 and 2 in acetonitrile in 56% and 52% yields, the formation of dibenzylimine d was observed in the
respectively. Complex 3 was found to be in equilibri- hydrogenation of benzonitrile, no benzylimine b or
um with its bromo-bridged dimer [Re(1)Br2(NO)]2 (5) benzylamine c was detected during the course of the
expelling acetonitrile, a process which was much less reaction. However, tribenzylamine f was observed as
prominent for
(Scheme 3).
4
to give [Re(2)Br2(NO)]2 (6) a side product. The formation of a considerable
amount of dibenzylimine during the course of the re-
Complexes 3 and 4 were treated with excess of tri- action and its decrease at the end of the reaction indi-
ethylsilane and ethylene in dichloromethane to pro- cated that dibenzylamine was formed by the hydroge-
duce the ortho metallated complexes [Re
(oCPPh-1)(h2- nation of dibenzylimine rather than by hydrogenolysis
ethylene)Br(NO)] (7) and [Re
(oCPPh-2)(h2-ethy- of the gem-diamine; whereas the formation of addi-
lene)Br(NO)] (8) in 70% and 74% yield, respectively. tional tribenzylamine is attributed as a consequence
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Both these products 7 and 8 were obtained as a mix- of the hydrogenolysis of the gem-diamine. But when
ture of diastereomers A and B in a ratio of 1:0.4 and the aliphatic nitrile, phenylacetonitrile was subjected
1:0.3, respectively, which could be separated by to hydrogenation, diphenethyl-2-phenylethenamine g’
column chromatography. Although ortho metallations was detected which revealed that the tertiary amine f’
of aromatic compounds are common with transition was formed by hydrogenation of the enamine g’
metals and sufficient reports on rhenium-catalyzed re- rather than the hydrogenolysis of gem-diamine. Hy-
drogenation of benzonitrile using 0.1 mol% of 7 was
carried out under different conditions (Table 1). An
initial TOF of 90 hÀ1 was observed at 50 bar of hydro-
gen pressure and 1408C in THF as solvent (Table 1,
entry 1). When 25 equivalents of triethylsilane with
respect to the amount of the catalyst were added, the
TOF was raised to 205 hÀ1 (Table 1, entry 3) however,
addition of 5 equivalents of triethylsilane was found
to be inferior (Table 1, entry 5) and addition of
50 equivalents was found to have an effect compara-
ble to the reaction with 25 equivalents of triethylsi-
lane (Table 1, entry 6). A TOF of 156 hÀ1 was ob-
tained, when dichloromethane was used as the sol-
vent, but gave 31% of tribenzylamine (Table 1,
Scheme 3. Complexes 3 and 4 and their splitting equlibria
with 5 and 6 respectively, in acetonitrile.
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Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
gen pressure at 1408C and the addition of 25 equiva-
lents of triethylsilane in THF gave 30% of tribenzyla-
mine and 4% of dibenzylimine in 18 h (Table 1,
entry 4), whereas without the addition of silane 13%
of tribenzylamine and 7% of dibenzylimine were ob-
tained within 16 h (Table 1, entry 2) with 90% conver-
sions in both the cases. Although the initial TOF was
comparatively higher, the longer reaction time would
be accompanied by the formation of higher amounts
of tertiary amine with much prominence in the
former case. Under the former conditions, reaction
with catalysts 3, 4 and 8 gave initial TOFs of 250 hÀ1,
229 hÀ1 and 161 hÀ1, respectively (Table 1, entries 10,
12 and 13).
The hydrogenation of the aliphatic phenylacetoni-
trile using catalyst 7 under these conditions gave a
TOF of 248 hÀ1, with a conversion of 50%, but
showed selectivity towards triphenethylamine f’
(Table 2, entry 1) and continuing this reaction gave a
conversion of 97% with 53% of triphenethylamine f’,
38% of diphenethylamine e’ and 9% of diphenethyl-
2-phenylethenamine g’ in 18 h (Table 2, entry 2). The
selectivity toward the tertiary amine in this case can
be due to higher reactivity towards nucleophilic
attack on the non-conjugated aliphatic imine func-
tionality. Hydrogenation of the aliphatic cyclohexane-
carbonitrile gave a TOF of 150 hÀ1 with a conversion
of 30%, but interestingly it showed selectivity towards
the secondary amine rather than the tertiary amine
(Table 2, entry 3), which can be attributed as a conse-
quence of the bulkiness, as well as electron richness,
which is expected to retard the nucleophilic attack on
this system. Further runs of this reaction gave a con-
version of 99% in 18 h with 94% of bis(cyclohexylme-
thyl)amine e’ along with 5% of tris(cyclohexylme-
thyl)amine f’ and 1% of cyclohexylidene(cyclohexyl-
methyl)methanamine D’ (Table 2, entry 4).
Keeping the other parameters unchanged, the se-
lectivity of the hydrogenation of benzonitrile was re-
versed (in comparision to Table 3, entry 2 and 4) with
respect to silane with a catalyst loading of 0.5 mol%
at a pressure of 75 bar (optimized conditions) gave a
TOF of 396 hÀ1 with a conversion of 99% forming
90% of dibenzylamine, 4% of tribenzylamine and 6%
of the dibenzylimine (Table 3, entry 2) in 0.5 h. How-
ever, only very little improvement of the conversion
was observed even after 3 h, but an increased portion
of tertiary amine (6%) and a reduced amount of di-
benzylimine (4%) was noticed.
Figure 1. Molecular structure of 8A and 8B. Anisotropic dis-
placement parameters are depicted at the 50% probability
level. Hydrogen atoms and solvent molecules are omitted
for clarity.
entry 9). When the reaction was carried out at a pres-
Under these conditions, reaction in the absence of
sure of 30 bar hydrogen and a temperature of 1408C, silane gave a TOF of 180 hÀ1 with 90% conversion
a TOF of 135 hÀ1 was accomplished with the forma- forming 66% of dibenzylamine, 24% of tribenzyl-
tion of 11% of tribenzylamine (Table 1, entry 7),
ACHTUNGTRENUNaNG mine and 10% of dibenzylimine in the first 1 h
whereas the reaction at 50 bar hydrogen and 1208C (Table 2, entry 1). The generality of the reaction was
gave a TOF of 79 hÀ1 with the formation of 5% of tri- tested by applying all the other catalysts in the benzo-
benzylamine (Table 1, entry 8). However, still longer nitrile reduction and reductions of a few other nitriles
reaction times using catalyst 7 with 50 bar of hydro- under the given optimized condition (Table 3).
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Table 1. Optimization table for the hydrogenation of benzonitrile.[a]
Entry
Et3SiH[b] [equiv]
Pressure [bar]/Catalyst Yield [%][c] after 2 h
Initial TOF [hÀ1] Initial Conv.[c] [%/2 h]
d
e
f
1
2
3
4
5
6
7
8
–
–
25
25
5
50
25
25
25
25
25
25
25
50/7
50/7
50/7
50/7
50/7
50/7
30/7
50/7
50/7
50/3
50/3
50/8
50/8
28
7
17
8
8
11
7
33
7
16
8
17
19
72
80
79
86
86
85
82
62
62
81
73
81
76
–
13
4
6
6
4
11
5
31
3
19
2
90
–
18
90/16 h
41
90/18 h
32
42
205
–
161
208
135
79
156
250
–
229
161
27
16[d]
31[e]
50
9
10
11
12
13
91/18 h
46
32
5
[a]
0.1 mol% of catalyst was used, reaction at 1408C in THF, TOFs were calculated as an average of the first 2 h.
With respect to catalyst.
By GC/MS.
Reaction at 1208C.
Solvent: dichloromethane.
[b]
[c]
[d]
[e]
Table 2. Hydrogenation of phenylacetonitrile and cyclohexanecarbonitrile[a]
Entry Et3SiH[b] [equiv.] Pressure [bar]/Catalyst Yield [%][c] after 2 h
Initial TOF [hÀ1] Initial Conv.[c] [%/2 h]
D’
e’
f’
1
2
3
4
25
25
25
25
50/7
50/7
50/7
50/7
–
–
17
1
31
38
83
94
37
53
–
248
–
150
–
50[d]
97[e,f]
30
5
99[e]
[a]
[b]
[c]
[d]
[e]
[f]
0.1 mol% of catalyst was used, reaction at 1408C in THF, TOFs were calculated as an average of the first 2 h.
With respect to catalyst.
By GC/MS.
32% of gꢂ was formed.
Conversion in 18 h.
9% of g’ was formed.
To exclude any heterogeneous reaction course in with the fact that rhenium metal has a very high atom
the hydrogenation catalyses, filtration and mercury binding energy (second largest in the periodic table of
poisoning experiments were carried out.[20] The hydro- elements) with low propensity for colloid or amalgam
genation of benzonitrile was carried out using catalyst formation.[20c]
7 under the conditions of Table 3. The reaction was
In summary, we have developed an efficient, air-
stopped at 20 min and the obtained clear pale yellow stable, homogeneous rhenium-catalyzed hydrogena-
solution, which showed a conversion of 71%, was fil- tion of nitriles with good selectivities for symmetrical
tered through a plug of celite into a new vessel with a secondary amines or tertiary amines. Addition of tri-
new stirring bar, and the reaction was continued for
ACHTUNGTRENeNUNG thylsilane could increase the TOFs and suppress
another 40 min under the same conditions showed over-alkylation of the amines at higher pressures with
identical results to Table 3, entry 2. A mercury poi- a relatively high loading of the catalyst. Secondary
soning experiment was carried out on the hydrogena- amines are anticipated to be formed by the hydroge-
tion of benzonitrile using catalyst 7 under the condi- nation of the imine (for aryl nitriles) or enamine (for
tions of Table 3. In the presence of 60 equivalents of alkyl nitriles) intermediates generated by the elimina-
Hg (per Re atom) the reaction again showed identical tion of ammonia from the gem-diamine species
results to Table 3, entry 2. The filtration and mercury whereas the tertiary amines were formed by the hy-
poisoning tests were thus all negative which rules out drogenolysis of the gem-diamines (for aryl nitriles) or
any colloid or amalgam formation. This is consistent hydrogenation of the enamines (for alkyl nitriles).
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Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
3.774 mmÀ1, R1 [I> 2s(I)]=0.048, wR2 (all data)=0.135 for
Table 3. Hydrogenation of nitriles under optimized condi-
tions.[a]
568 refined parameters, 11556 independent reflections
(qmax =28.28), GOF=1.080, maximum and minimum residu-
al electron density 2.760 and À1.526 eꢃÀ3.
Entry Nitrile
Cat. Yield [%]
TOF [hÀ1] Conv.[c]
d
e
f
Crystal data for 8B: C50H39BrNO2P2ReSi·CHCl3, Mr =
1161.33, triclinic space group P-1, a=11.2646(5), b=
14.3396(7), c=16.6916(6) ꢃ, a=67.050(4), b=79.232(4), g=
68.365(4)8, V=2304.95(19) ꢃ3, T=183(2) K, Z=2, 25807 re-
flections measured (Rint =0.036), 1=1.673 MgmÀ3, m=
3.814 mmÀ1, R1 [I> 2s(I)]=0.047, wR2 (all data)=0.114 for
563 refined parameters, 11394 independent reflections
(qmax =28.28), GOF=0.984, maximum and minimum residu-
al electron density 3.811 and À2.107 eꢃÀ3.
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
7[d]
7
3
4
8
3
4
7
8
7
8
3
3
10 66 24 180
90
99
99
99
99
99
99
99
99
99
99
99[e]
99
6
5
8
7
2
4
5
5
2
90
85 10 198
86 198
82 11 196
4
198
6
Ph
3-tolyl
3-tolyl
2-thienyl
2-thienyl
3-tolyl
3-tolyl
PhCH2
cyclo-hexyl
89
87
9
9
198
198
83 12 162
85 10 160
81 17 170
CCDC 787796 and 787797 contain the supplementary
crystallographic data for compounds 8A and 8B. These data
can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif and are contained in the Supporting Information.
9
10
11
12
13
21 80 18 169
–
2
10 85 176
95 198
3
[a]
[b]
0.5 mol% of catalyst with 25 equiv. of triethylsilane with
respect to catalyst at 75 bar H2, 1408C in THF, run for
1 h except for entries 1 and 8–12 for which reaction was
run for 1 h and 4 h respectively.
Ar=R for entries 12 and 13 which corresponds to
Scheme 2, wherein d=D’, e=e’, f=fꢀ and for entry 13,
RCH2 =cyclohexyl.
By GC/MS.
No silane was added.
5% of g’ was formed.
General Procedure for the Catalytic Hydrogenation
of Benzonitrile
Catalyst 7 (0.002 g, 2.18ꢄ10À3 mmol) was taken in a stainless
steel autoclave and benzonitrile (0.045 g, 0.44 mmol) was
added followed by THF (0.2 mL) and triethylsilane (9 mL,
5.63ꢄ10À2). Naphthalene (0.028 g, 0.22 mmol) was added as
internal standard. It was pressurized with 75 bar of hydrogen
and kept in an oil bath maintained at 1408C. After an ap-
propriate reaction time, the vessel was immediately cooled
to room temperature and the hydrogen was released slowly
in a fume hood. The reaction mixture was filtered through a
[c]
[d]
[e]
Rhenium as a neighbouring element to precious short plug of silica gel and the yield was measured by GC/
MS (CP-3800 Saturn 2000MS/MS spectrometer, benzoni-
trile: 2.90 min (m/z=103.1), dibenzylamine: 7.87 min (m/z=
197.1), dibenzylimine: 8.15 min (m/z=195.0), tribenzyl-
metals can represent a ligand sphere tuned to adopt
similar catalytic properties providing an appropriate
alternative to precious metal catalyses.
AHCTUNGTREGUNaNN mine: 10.65 min (m/z=287.1). For the GC/MS data of
other compounds, see Supporting Information.
Experimental Section[22]
Crystallographic Details for 8A and 8B
Acknowledgements
Intensity data were collected at 183(2) K on an Oxford Xca-
libur diffractometer (4-circle kappa platform, ruby CCD de-
tector, and a single wavelength enhance X-ray source with
MoKa radiation, l=0.71073 ꢃ).[21a] The selected suitable
single crystals were mounted using polybutene oil on the
top of a glass fiber fixed on a goniometer head and immedi-
Funding from the SwissNational Science Foundation and the
University of Zurich is gratefully acknowledged.
ately transferred to the diffractometer. Pre-experiment, data References
collection, data reduction and analytical absorption correc-
tions[21b] were performed with the Oxford program suite
CrysAlisPro.[21c] The crystal structures were solved with
SHELXS-97[21d] using direct methods. The structure refine-
ments were performed by full-matrix least-squares on F2
with SHELXL-97.[21d] All programs used during the crystal
structure determination process are included in the WINGX
software.[21e] The program PLATON[21f] was used to check
the result of the X-ray analyses.
Crystal data for 8A: C50H39BrNO2P2ReSi·CHCl3, Mr =
1161.33, triclinic space group P-1, a=11.478(2), b=
14.542(3), c=16.4780(14) ꢃ, a=67.281(15), b=78.654(11),
g=66.87(3)8, V=2329.8(9) ꢃ3, T=183(2) K, Z=2, 37199 re-
flections measured (Rint =0.061), 1=1.655 MgmÀ3, m=
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Adv. Synth. Catal. 2011, 353, 1479 – 1484