Phosphoric acid bridged cobalt bis(dicarbollide) ion as a highly efficient catalyst for the organocatalytic hydrogenation of ketimines

Article abstract of DOI:10.1055/s-0033-1340847

The high-yield reduction of aromatic ketimines into amines by using a novel catalyst based on a metallacarborane structure, 8,8′-μ-phosphate[(1,2- dicarba-closo-undecaborane)-3,3′-cobalt(-1)(1′,2′-dicarba- closo-undecaborane)] acid, is described. Georg Th

Full text of DOI:10.1055/s-0033-1340847

LETTER
▌795
letter
Phosphoric Acid Bridged Cobalt Bis(dicarbollide) Ion as a Highly Efficient
Catalyst for the Organocatalytic Hydrogenation of Ketimines
Metallacarborane Phosphoric Acid as a Hydrogenation Catalyst
Takahiko Akiyama,*a Kodai Saito,a Slawomir Janczak,b Zbigniew J. Lesnikowski*b
a
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Fax +81(3)59921029; E-mail: takahiko.akiyama@gakushuin.ac.jp
b
Institute of Medical Biology, Polish Academy of Sciences, Laboratory of Molecular Virology and Biological Chemistry, 106 Lodowa St.,
Łódź 93-232, Poland
Fax +48(42)2723630; E-mail: zlesnikowski@cbm.pan.pl
Received: 31.12.2013; Accepted after revision: 28.01.2014
and is considered isolobal to the cyclopentadienyl (Cp–)
Abstract: The high-yield reduction of aromatic ketimines into
ligand that is present, for example, in ferrocene. Exo-
amines by using a novel catalyst based on a metallacarborane struc-
metallated carboranes, as well as metallacarboranes with
ture, 8,8′-μ-phosphate[(1,2-dicarba-closo-undecaborane)-3,3′-co-
metal centers covalently bound into a carborane cage,
have been extensively explored as catalysts in several
types of reactions.11–13 However, the potential of the metal
bis(dicarbollide) sandwich-type metallacarboranes re-
mains unexplored. Here, we describe a novel Brønsted
acid type catalyst constructed from a cobalt bis(dicarbol-
lide) complex, which provides a three-dimensional plat-
form to which a phosphoric acid diester system can be
attached that could serve as a catalytic unit and offer a
high catalytic efficacy.
balt(-1)(1′,2′-dicarba-closo-undecaborane)] acid, is described.
Key words: metallacarboranes, hydrogenation, catalysis, reduc-
tion, imines, amines
Amines are important structural elements in biologically
active natural products and pharmaceuticals.1 Imine re-
duction is a widespread approach to the preparation of
amines, and a range of methods for reducing imines have
been reported, including metal-catalyzed hydrogenation2
and hydrosilylation.3 Transfer hydrogenation of imines by
using a hydrogen donor in combination with an acid cata-
lyst has recently emerged as an efficient method for the
preparation of amines. In particular, the Hantzsch ester is
recognized as an effective and useful hydrogen donor.4
Several acid catalysts, including thiourea derivatives5 and
molecular iodine,6 have been used to reduce aldimines
and ketimines. Recently, ketimines, α-imino esters, and
quinolines were reduced with high enantioselectivity
through the combined use of a Hantzsch ester and chiral
phosphoric acids.7,8
– Et3NH+
= CH, = BH or B
O
O
OH
P
Co
O
Ar2
EtO2C
Me
CO2Et
Me
N
H
Ar2
1
+
N
Ar1
R
N
H
5 Å MS
toluene
Ar1
R
50 °C, 24 h
4a–k
2 (1.4 equiv)
3a–k
Scheme 1 Reduction of ketimines 3a–k to the corresponding amines
4a–k by using the phosphoric acid bridged cobalt bis(dicarbollide)
ion (1) as a catalyst and the Hantzsch ester 2 as a hydrogen donor: 3a:
Ar1 = 4-ClC6H4, Ar2 = 4-MeOC6H4, R = Me; 3b: Ar1 = Ph, Ar2 = 4-
MeOC6H4, R = Me; 3c: Ar1 = 4-MeC6H4, Ar2 = 4-MeOC6H4, R = Me;
3d: Ar1 = 4-MeOC6H4, Ar2 = 4-MeOC6H4, R = Me; 3e: Ar1 = 2-naph-
thyl, Ar2 = 4-MeOC6H4, R = Me; 3f: Ar1 = 4-ClC6H4, Ar2 = Ph, R =
Me; 3g: Ar1 = Ph, Ar2 = Ph, R = Me; 3h: Ar1 = Ph, Ar2 = 4-MeOC6H4,
Metallacarboranes are a vast family of metallocene-type
complexes that consist of at least one carborane cage and
one or more metal cations.9,10 Carborane clusters are ver-
satile and efficient ligands for cations of metals such as
Co, Fe, Ni, Cr, Re, Al, Au, Cu, Ir, Mn, and Pt. Metalla-
carboranes resemble metallocene-based catalytic systems
but with cyclopentadienyl ligands replaced by carboranyl R = H; 3i: Ar1 = Ph, Ar2 = 4-ClOC6H4, R = H; 3j: Ar1 = Ph, Ar2 =
3,4,5-(MeO)3C6H4, R = Me; 3k: Ar1 = Ph, Ar2 = 4-ClC6H4, R =
units. However, compared to metallocene-based catalytic
systems, metallacarboranes have greater versatility and
CO2Me.
robustness. These advantages suggest that metallacarbo-
ranes have remarkable potential as catalysts that can be The monoanionic deltahedral metallacarborane cluster,
tailored to specific needs. A typical metallacarborane is a [(1,2-dicarba-closo-undecaborane)-3,3′-cobalt(_1)(1′,2′-
sandwich of two dicarbollide ([C2B9H11]2–) clusters with a dicarba-closo-undecaborane)]ate
{[3,3′-Co(1,2-
metal ion in the center. [C2B9H11]2– behaves as a Z5 ligand C2B9H11)2]–}, is the most extensively investigated mem-
ber of the metallabisdicarbollide family of type
[M(C2B9H11)2]n–. Interest in [3,3′-Co(1,2-C2B9H11)2]– ini-
tially emerged due to its capacity to extract 137Cs and 90Sr
SYNLETT 2014, 25, 0795–0798
Advanced online publication: 05.03.2014
from nuclear waste14,15 as well as its resistance to radioly-
sis and degradation in the presence of a concentrated
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340847; Art ID: ST-2013-D1176-L
© Georg Thieme Verlag Stuttgart · New York

796
T. Akiyama et al.
LETTER
strong acid, which is needed for the compound to remain Recently, these compounds were studied for their utility
intact in the presence of high-level nuclear waste.
as boron donors for Boron Neutron Capture Therapy
(BNCT),16,17 electrochemical labels,18,19 anticancer20,21
and antiviral agents,22,23 and others. The properties of a
metallacarborane can be adjusted by derivatizing the bo-
ron cluster ligands and changing the type of coordinate
metal.11 As an example of the exceptional potential of this
class of boron cage compounds, we have examined metal-
lacarborane derivatives as a scaffold for new catalysts; the
results are described herein.
Indeed, the chemistry and applications of metallacarbo-
ranes are relevant beyond the context of nuclear waste ex-
traction. The virtues of metallacarboranes as a catalyst
platform include a high chemical and thermal stability as
well as easy charge and redox potential tuning and chem-
ical modifications.
OMe
OMe
8,8′-μ-Phosphate[(1,2-dicarba-closo-undecaborane)-3,3′-
cobalt(_1)(1′,2′-dicarba-closo-undecaborane)]ate, the tri-
HN
+
HN
ethylammonium salt 1(HNEt3 )2 and the free phosphoric
+
acid form 1(HNEt3 )(H+) were prepared from the corre-
sponding chlorophosphate according to a reported proce-
dure.14 Upon treatment of ketimine 3a, which was derived
from acetophenone and p-anisidine, with the Hantzsch es-
Cl
4a
(94%)
4b
(94%)
ter 2 in the presence of phosphoric acid 1(HNEt3 )(H+)
+
OMe
OMe
and 5 Å MS in benzene at 50 °C for 24 h, the transfer hy-
drogenation of ketimine proceeded smoothly to furnish
the corresponding amine 4a in 94% yield (Figure 1,
Scheme 1). A range of ketimines were subjected to the
same reaction conditions to give the corresponding
amines in high chemical yields24 (4b–f and 4j). Aldimines
also proved to be suitable substrates (4h and 4i). In addi-
tion, the α-imino ester underwent transfer hydrogenation
to provide the corresponding α-amino ester in good yield
(4k). The structures of these compounds were determined
on the basis of their 1H and 13C NMR spectra.25
HN
HN
MeO
4d
(75%)
4c
(96%)
OMe
HN
HN
Cl
– Et3NH+
4f
(97%)
4e
(88%)
H
Ar2
N
OMe
O
O
R
Ar1
E
P
Co
O
O
Me
N
HN
HN
H
H
Me
H
1
E
4g
4h
Figure 2 A possible transition state of the hydrogen transfer reaction
catalyzed by the phosphoric acid bridged cobalt bis(dicarbollide) ion
1 in the presence of the Hantzsch ester 2; E = CO2Et
(75%)
(75%)
OMe
Cl
OMe
OMe
A screen of the reaction conditions revealed that the cata-
lyst loading could be reduced to 5 mol% without compro-
mising the yield, using only a 40% molar excess of the
Hantzsch ester 2. The turnover number (TON) of 1 was
10, which was lower than that for metal-catalyzed hydro-
genation and similar to that obtained for organocatalysts.
The reactivity of 1 was compared to the reactivity of a rep-
resentative Brønsted acid catalyst comprising a phosphoric
acid derived from BINOL and bearing a 2,4,6-(i-Pr)3C6H2
group at the 3,3′-position. The reactions were conducted
at 50 °C for 95 min to afford amine 4a in 70 and 85%
yield, respectively. These results showed that the catalytic
activity toward the reduction of the aromatic imine was
somewhat lower for 1 than for the BINOL-derived cata-
lyst. Further modifications of 1 can be envisioned that
may improve its catalytic activity.
HN
HN
4i
(75%)
4j
(99%)
Cl
HN
CO2Me
4k
(78%)
Figure 1 Substrate scope and yields of reduction of ketimines 3a–k
to amines 4a–k by using phosphoric acid bridged cobalt bis(dicarbol-
lide) ion (1) as catalyst
Synlett 2014, 25, 795–798
© Georg Thieme Verlag Stuttgart · New York

LETTER
Metallacarborane Phosphoric Acid as a Hydrogenation Catalyst
797
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In conclusion, a new catalyst for the hydrogenation of ar-
omatic ketimines based on a metallabisdicarbollide-type
structural scaffold bearing a phosphoric acid bridge was
proposed. In the presence of this catalyst, the hydrogena-
tion of a range of substrates was achieved in high yield.
The versatility of the metallacarborane structure and
chemistry opens the way for the further development of
this new type of Brønsted acid catalyst.
Acknowledgment
This work was partially supported by a Grant-in-Aid for Scientific
Research on Innovative Areas, ‘Advanced Transformation by Orga-
nocatalysis’, from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan; by a Grant-in Aid for Scientific
Research from the Japan Society for the Promotion of Science, and
by a grant from the European Regional Development Fund under
the Operational Programme Innovative Economy, POIG.01.01.02-
10-107/09 (SJ, ZJL). Partial funding from the Statutory Fund of
IMB PAS is also gratefully acknowledged. We thank Prof. Jaromir
Plešek (1927–2010) for the kind gift of compound 1 (chlorophos-
phate form) and Dr. Bohumir Grűner for his expertise and discus-
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(24) Preparation of 4a–k; General Procedure: All the reaction
flasks were dried by flame, and all reactions were carried out
under N2. All the solvents were distilled under nitrogen and
stored over 4 Å MS prior to use. Thin-layer chromatography
was performed on Merck 60 F254 silica gel plates and
visualization was accomplished by irradiation with UV light
or by treatment with a solution of phosphomolybdic acid
solution followed by heating. Ketimine 3a–k (13.0 mg,
+
0.05 mmol), 1(HNEt3 )(H+) (1.1 mg, 0.0025 mmol), and
Hantzsch ester (2; 0.07 mmol, 1.4 equiv) were mixed in
benzene (1 mL) in a flame-dried test tube containing dried
molecular sieves (5 Å, 50 mg) under a nitrogen atmosphere
at 50 °C for 20 h (not optimized). The resulting mixture was
filtered through Celite (washed with CH2Cl2) and then
evaporated under reduced pressure. The residue was purified
by preparative silica gel thin-layer chromatography
(hexane–EtOAc, 5:1 v/v).
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(25) NMR characteristics of 4a–k: NMR spectra were recorded
with a Unity Inova-400 instrument (Varian Ltd., 400 MHz
for 1H, 100 MHz for 13C) using CDCl3 as solvent. Chemical
shifts (δ) for 1H were referenced to tetramethylsilane (δ =
0.00 ppm) as an internal standard. Chemical shifts (δ) for 13
were referenced to the solvent signal (CDCl3; δ =
77.00 ppm).
C
N-(4-Methoxyphenyl)-1-(4-chlorophenyl)ethylamine
(4a):27 Yield: 94%; pale-yellow oil.1H NMR (400 MHz,
CDCl3): δ = 1.47 (d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 4.37 (q,
J = 6.8 Hz, 1 H), 6.42–6.46 (m, 2 H), 6.66–6.71 (m, 2 H),
(9) Grimes, R. N. Coord. Chem. Rev. 2000, 200, 773.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 795–798

798
T. Akiyama et al.
LETTER
7.24–7.32 (m, 4 H). 13C NMR (100 MHz, CDCl3): δ = 25.1,
53.8, 55.7, 114.6, 114.7, 127.3, 128.7, 132.3, 141.1, 144.0,
152.1.
MHz, CDCl3): δ = 24.9, 53.5, 113.3, 117.2, 125.8, 126.8,
128.6, 129.1, 145.1, 147.2.
N-Benzyl-4-methoxybenzenamine (4h):29a,b Yield: 98%;
colorless oil. 1H NMR (400 MHz, CDCl3): δ = 3.74 (s, 3 H),
4.28 (s, 2 H), 6.60 (d, J = 7.6 Hz, 2 H), 6.77 (d, J = 7.6 Hz,
2 H), 7.24–7.38 (m, 5 H). 13C NMR (100 MHz, CDCl3): δ =
49.5, 56.0, 114.3, 115.1, 127.4, 127.8, 128.8, 139.9, 142.6,
152.4.
N-(4-Methoxyphenyl)-1-phenylethylamine (4b):27 Yield:
94%; pale-yellow oil. 1H NMR (400 MHz, CDCl3): δ = 1.50
(d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 4.41 (q, J = 6.8 Hz, 1 H),
6.45–6.50 (m, 2 H), 6.66–6.71 (m, 2 H), 7.19–7.24 (m, 1 H),
7.29–7.38 (m, 4 H). 13C NMR (100 MHz, CDCl3): δ = 25.0,
54.4, 55.7, 114.7, 114.7, 125.9, 126.8, 128.6, 141.3, 145.3,
152.0.
N-Benzyl-4-chlorobenzenamine (4i):30 Yield: 95%; white
solid. 1H NMR (400 MHz, CDCl3): δ = 4.14 (br s, 1 H), 4.31
(s, 2 H), 6.54–6.59 (m, 2 H), 7.10–7.15 (m, 2 H), 7.27–7.34
(m, 1 H), 7.34–7.39 (m, 4 H). 13C NMR (100 MHz, CDCl3):
δ = 48.3, 113.9, 122.1, 127.3, 127.4, 128.7, 129.0, 138.8,
146.5.
N-(4-Methoxyphenyl)-1-(4-methylphenyl)ethylamine
(4c):27 Yield: 96%; colorless oil. 1H NMR (400 MHz,
CDCl3): δ = 1.47 (d, J = 6.8 Hz, 3 H), 2.32 (s, 3 H), 3.69 (s,
3 H), 4.37 (q, J = 6.8 Hz, 1 H), 6.45–6.50 (m, 2 H), 6.66–
6.71 (m, 2 H), 7.11 (d, J = 8.0 Hz, 2 H), 7.24 (d, J = 8.0 Hz,
2 H). 13C NMR (100 MHz, CDCl3): δ = 21.0, 25.1, 53.9,
55.7, 114.5, 114.7, 125.8, 129.2, 136.3, 141.6, 142.4, 151.8.
N-(4-Methoxyphenyl)-1-(4-methoxyphenyl)ethylamine
(4d):27 Yield: 75%; colorless oil. 1H NMR (400 MHz,
CDCl3): δ = 1.47 (d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 3.78 (s,
3 H), 4.36 (q, J = 6.8 Hz, 1 H), 6.46–6.51 (m, 2 H), 6.67–
N-(3,4,5-Trimethoxyphenyl)-1-phenylethylamine (4j):31
Yield: 99%; pale-yellow oil. 1H NMR (400 MHz, CDCl3):
δ = 1.51 (d, J = 6.8 Hz, 3 H), 3.67 (s, 6 H), 3.71 (s, 3 H), 4.03
(br s, 1 H), 4.42 (q, J = 6.8 Hz, 1 H), 5.75 (s, 2 H), 7.19–7.25
(m, 1 H), 7.29–7.35 (m, 2 H), 7.35–7.39 (m, 2 H). 13C NMR
(100 MHz, CDCl3): δ = 24.9, 54.1, 55.6, 60.9, 90.8, 125.7,
126.9, 128.6, 129.7, 144.0, 145.3, 153.6.
6.71 (m, 2 H), 6.83–6.87 (m, 2 H), 7.25–7.29 (m, 2 H). 13
C
Methyl (4-Chlorophenylamino)phenylacetate (4k):32
Yield: 78%; colorless oil. 1H NMR (400 MHz, CDCl3): δ =
3.73 (s, 3 H), 5.03 (s, 1 H), 6.47 (d, J = 8.8 Hz, 2 H), 7.06 (d,
J = 8.8 Hz, 2 H), 7.31–7.37 (m, 3 H), 7.45–7.47 (m, 2 H).13C
NMR (100 MHz, CDCl3): δ = 52.9, 60.7, 114.5, 122.8,
127.2, 128.5, 128.9, 129.1, 137.7, 144.3, 172.0.
(26) (a) Simón, L.; Goodman, J. M. J. Am. Chem. Soc. 2008, 130,
8741. (b) Shibata, Y.; Yamanaka, M. J. Org. Chem. 2013,
78, 3731.
NMR (100 MHz, CDCl3): δ = 25.0, 53.6, 55.2, 55.7, 113.9,
114.6, 114.7, 126.9, 137.4, 141.6, 151.8, 158.4.
N-(4-Methoxyphenyl)-1-naphthaylethylamine (4e):27
Yield: 88%; off-white sticky oil. 1H NMR (400 MHz,
CDCl3): δ = 1.57 (d, J = 6.8 Hz, 3 H), 3.67 (s, 3 H), 4.56 (q,
J = 6.8 Hz, 1 H), 6.50–6.56 (m, 2 H), 6.64–6.70 (m, 2 H),
7.40–7.47 (m, 2 H), 7.48–7.52 (m, 1 H), 7.78–7.82 (m, 4 H).
13C NMR (100 MHz, CDCl3): δ = 25.1, 54.5, 54.5, 55.7,
114.8, 124.3, 124.4, 125.5, 125.9, 127.6, 127.8, 128.4,
132.7, 133.6, 141.4, 142.9, 152.0.
(27) Storer, R. I.; Carrera, D. E.; Ni, Y.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2006, 128, 84.
N-Phenyl-1-(4-chlorophenyl)ethylamine (4f):28 Yield:
97%; colorless oil. 1H NMR (400 MHz, CDCl3): δ = 1.46 (d,
J = 6.7 Hz, 3 H), 4.38 (q, J = 2.2 Hz, 1 H), 6.45 (d, J =
7.6 Hz, 2 H), 6.63 (t, J = 7.4 Hz, 1 H), 7.06–7.12 (m, 2 H),
7.21–7.30 (m, 4 H), 7.48–7.52 (m, 1 H), 7.78–7.82 (m, 4 H).
13C NMR (100 MHz, CDCl3): δ = 25.4, 53.2, 113.4, 117.3,
129.0, 129.3, 132.6, 144.1, 147.2.
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N-(Phenyl)-1-phenylethylamine (4g):27 Yield: quant.;
colorless oil. 1H NMR (400 MHz, CDCl3): δ = 1.51 (d, J =
6.8 Hz, 3 H), 4.10 (br s, 1 H), 4.47 (q, J = 6.8 Hz, 1 H), 6.50
(d, J = 8.4 Hz, 2 H), 6.62–6.68 (m, 1 H), 7.06–7.12 (m, 2 H),
7.19–7.26 (m, 1 H), 7.28–7.40 (m, 4 H). 13C NMR (100
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Synlett 2014, 25, 795–798
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