Enantioselective hydrogenation of cyclic imines catalysed by Noyori-Ikariya half-sandwich complexes and their analogues

Article abstract of DOI:10.1039/c5cc06712j

A method for enantioselective hydrogenation of cyclic imines with gaseous hydrogen has been developed. Easily accessible Noyori-Ikariya Ru(ii) and Rh(iii) complexes can be used directly without an inert atmosphere. Substrate activation has been achieved by trifluoroacetic acid. A new hydroxyl-functionalized complex is reported, showing high activity in transfer hydrogenation.

Full text of DOI:10.1039/c5cc06712j

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Václavík, P. Šot, J. Pecháek, J. Zápal, R. Pažout, J. Maixner, M. Kuzma and P. Kaer, Chem. Commun., 2015,
DOI: 10.1039/C5CC06712J.
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COMMUNICATION
Enantioselective hydrogenation of cyclic imines catalysed by
Noyori-Ikariya half-sandwich complexes and their analogues†
B. Vilhanová,*a J. Václavík,bc P. Šot,a J. Pecháček,a J. Zápal,c R. Pažout,d J. Maixner,d M. Kuzmac and
Received 00th January 20xx,
Accepted 00th January 20xx
P. Kačer*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A method for enantioselective hydrogenation of cyclic imines with with AgSbF6. For acyclic imines, a Cp*-Ir(III) catalyst with a chiral
gaseous hydrogen has been developed. Easily accessible Noyori- phosphate anion was employed.6b Ikariya and co-workers
Ikariya Ru(II) and Rh(III) complexes can be used directly without developed an alternative strategy for the AH of acyclic imines
inert atmosphere. Substrate activation has been achieved by with an Ir(III) catalyst and AgSbF6, proposing that Ag+ can
trifluoroacetic acid. A new hydroxyl-functionalized complex is activate the substrate.6c Imine AH was further screened by Chen
et al. by testing various counteranions.6d,e They also synthesized
reported, showing high activity in transfer hydrogenation.
enantioenriched 1,2,3,4-tetrahydroquinolines via AH of
Efficient methods toward optically enriched amines are in the
forefront of modern synthetic chemistry.1 One such method is
the asymmetric transfer hydrogenation (ATH) of imines
catalysed by chiral half-sandwich Ru(II),2 Rh(III)3,4 and Ir(III)3
complexes. Asymmetric hydrogenation reactions directly using
gaseous hydrogen (AH) are often preferred, and the existing
ATH catalytic systems have thus been modified to be applicable
under AH conditions.5 The first use of complexes [Ru(II)Cl(η6-
quinolines using the Ru-triflate complex under a variety of
reaction conditions.6g–j
In this work, we present a method for the AH of cyclic imines
with the aim of simplifying the reaction conditions: standard
metal-chloride complexes are employed and the necessity of an
inert atmosphere is avoided.
For the initial experiments, catalyst [RuCl(η6-p-cymene)(S,S)-
TsDPEN]
(
A
)
and substrate 6,7-dimethoxy-1-methyl-3,4-
) were
arene)(N-R-sulfonyl-DPEN)]
(where
DPEN
=
1,2-
dihydroisoquinoline (6,7-dimethoxy-1-methyl-DHIQ,
1
diphenylethylene-1,2-diamine and R = aryl or alkyl) in AH was
reported by Ohkuma et al. in 2006 in the reduction of ketones.5a
The key difference from ATH was a switch from basic to mildly
acidic conditions (i.e., methanol as solvent). The authors found
autodissociation of the Ru–Cl bond to be essential.5c To
facilitate this a Ru-triflate complex was used.5a–e Wills’s5f and
Ikariya’s5g tethered complexes proved useful in AH of ketones
without modification, i.e. as Ru-chloride complexes.
The first AH of cyclic imines with this catalytic system was
shown by Li et al. on a Cp*-Rh(III) (Cp* = 1,2,3,4,5-
pentamethylcyclopentadienyl) catalyst.6a They generated a
[Rh]+SbF6– complex in situ by reacting the Rh-chloride complex
selected because both substances are very often used to
benchmark ATH.7 Screening of solvents (Table S1‡) revealed
that the reaction did not proceed in acetonitrile whilst only
minimal reactivity was observed in DMSO (<5% conversion) and
methanol (10% conversion). In this catalytic system, it is
presumed that imines require activation by polarization of the
C=N bond in order to undergo reduction.8 This activation has
been achieved by Brønsted8a or Lewis acids,8a,9 electron-
withdrawing effect of a CF3 group,10 or conversion of the imine
to a quarternary iminium salt.11 Therefore, we envisaged we
could activate the C=N bond by adding a suitable acid (Table
S2‡). Using methanol as solvent, tetrafluoroboric acid (48% wt.
solution in water, 1 equiv) enhanced the reaction only
moderately (19% conversion) and trifluoromethanesulfonic
acid (1 equiv) had no effect (9% conversion). Gratifyingly,
trifluoroacetic acid (1 equiv) increased the conversion to 57%.
Lesser amounts were found to be insufficient, probably because
the substrate could not be fully protonated.§,12 Excess
trifluoroacetic acid gave no improvement, but could be
detrimental by causing partial or full protonation of the TsDPEN
ligand. Conversion was further improved to 96% by increasing
temperature to 40 °C. At this point, it was found that the
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reaction was equally feasible in dichloromethane under the We still could not achieve full conversion. Neither addition of
View Article Online
DOI: 10.1039/C5CC06712J
conditions developed for methanol. However, this avenue was acid or catalyst, increasing reaction temperature, nor
not pursued any further as it is not a preferred solvent in the prolongation of the reaction time led to significant
pharmaceutical industry, mainly for environmental reasons. improvements. Eventually, it was discovered that the order of
Dried and/or degassed solvents were not necessary since we addition of the reaction mixture components played a critical
obtained identical results both with and without paying role. The original order was as follows: 1. substrate, 2.
attention to this aspect.
methanol, 3. catalyst, and 4. acid – after switching the catalyst
and acid, the reaction proceeded to full conversion. Given that
the substrate must be protonated, it is advisable for it to react
with the acid first. Subsequently, the catalyst can be added with
a significantly lower risk of deactivation.
Cl Ru
NTs
Ru
Cl Ru
H2N
NTs
Under optimized conditions, the method was tested on a mini-
library containing six catalysts and twelve substrates (Fig. 1).
HN
Ph
NTs
Ph
H2N
Ph
Ph
Aside from complex
was selected to examine the role of the Ru–Cl bond
autodissociation5c and capability of
to react with hydrogen –
e.g. in the AH of quinolines it was reported that was
catalytically inactive.6i Complex
was chosen as an alternative
to and were studied as
, bearing a different η6-arene.
representatives of the newer tethered complexes, and Rh(III)
analogue was included in order to show the applicability of the
method on another metal. As substrates we tested nine DHIQs
differing by substitutions in positions 1, 6 and 7 ( ), 3,4-
dihydro- -carbolines harmalane (10) and harmaline (11), and
cyclic N-sulfonyl imine 12
Full conversion was achieved with substrates
with complexes and (Table 1).
comparable activity and enantioselectivity, implying that the
Ru–Cl autodissociation is not rate limiting and that can react
with molecular hydrogen under such conditions. Interesting
differences emerged with – the 6-methoxy substituted
substrates and showed lower reactivity than and , which
agrees with our previous ATH study on this set of substrates.13
A B,
, its derivative, the 16e– amido complex
Ph
Ph
C
B
A
B
B
O
Cl Rh
H2N
Cl Ru
Cl Ru
NH
C
NTs
NTs
NTs
A
D
E
NH
Ph
Ph
Ph
Ph
Ph
Ph
F
E
F
D
O
S
O
1: R1 = OMe, R2 = OMe, R3 = Me
2: R1 = H, R2 = H, R3 = Me
3: R1 = OMe, R2 = H, R3 = Me
4: R1 = H, R2 = OMe, R3 = Me
5: R1 = H, R2 = H, R3 = Ph
6: R1 = H, R2 = H, R3 = 4-CF3-Ph
7: R1 = OEt, R2 = OEt, R3 = Me
8: R1 = OMe, R2 = OMe, R3 = i-Pr
9: R1 = H, R2 = H, R3 = i-Pr
10: R = H
R1
1–9
N
β
N
R2
.
R3
1–9
12
1
–
4
in 6 hours
A
–
D
F
A and B showed
N
B
N
H
R
11: R = OMe
10, 11
E
Fig. 1
Complexes A–F and substrates 1–12 used in this study.
1
3
2
4
1-Aryl-DHIQs
5
and
6
were poorly reactive with the exception
Table 1 AH of imines 1–11 catalysed by complexes A–H. Conversion and ee values are given in %a
N
NH
H2, A–H (1 mol%)
H2, A–H (1 mol%)
R
R
NH
N
CF3COOH, MeOH
40 °C
CF3COOH, MeOH
N
H
N
H
R
R
40 °C
R'
R'
1–9
10, 11
Complex
A
B
C
D
E
F
G
H
Imine
1
2
3
4
5
6
7
8
cnv
>99
>99
>99
>99
1
ee
cnv
>99
>99
>99
>99
4
ee
96
85
92
92
n.d.
n.d.
96
98
70
cnv
>99
>99
>99
>99
3
1
98
98
38
ee
96
81
89
90
n.d.
n.d.
97
98
39
cnv
>99
>99
96
>99
24
12
70
90
>99
70
ee
cnv
44
>99
84
>99
0
ee
94
84
90
91
n.d.
n.d.
95
98
61
cnv
>99
>99
>99
>99
>99
>99
98
>99
>99
>99
>99
ee
89
76
90
75
7
cnv
98
>99
>99
>99
2
ee
95
87
92
91
n.d.
n.d.
97
98
82
cnv
95
>99
90
>99
6
ee
96
87
91
91
n.d.
n.d.
97
96
81
96
87
92
93
n.d.
5
97
98
72
97
95
78
59
68
74
11
n.d.
83
96
66
93
88
26
2
0
9
2
3
>99
>99
39
>99
>99
>99
>99
35
>99
>99
31
27
23
69
90
96
99
84
95
93
98
99
88
99
99
99
99
>99
99
98
9
10
11
97
95
>99
98
97
92
95
91
96
93
96
90
71
a Amount of substrate n = 44 μmol, concentration of substrate c = 88 mM, catalyst loading 1 mol%, TFA-to-substrate molar ratio A/S = 1, p(H2) = 15 bar, 40 °C, 6 h. Order
of addition: imine-solvent-acid-catalyst.
2 | Chem. Commun., 2015, 00, 1-3
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of Rh(III) complex
COMMUNICATION
F
, delivering nearly racemic products, which
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DOI: 10.1039/C5CC06712J
is in agreement with previously reported findings from ATH.3
Otherwise, maximum conversions (26% and 24%) were
Cl Ru
H2N
Cl Ru
OH
NTs
NTs
achieved with combinations 6-A and 5-D, respectively. 1-Aryl-
H2N
substituted DHIQs thus require different reaction conditions
and most likely different, more reactive catalysts. Experiments
towards efficient extension of our methodology to these
substrates are currently underway.
Ph
Ph
Ph
Ph
G
H
Fig. 2
Complexes G and H.
6,7-Diethoxy-substituted imine
described above, except with
7
D
performed similarly like those
and we observed lower
In the context of this study, we were interested in both AH and
ATH with to evaluate its eligibility for its use as a modular
substitute for . Monitoring the ATH of imines 10 and 12
by H NMR,18 we observed enhanced reactivity in comparison
to complex
(Table 2, Fig. S1‡). To probe whether the hydroxyl
group of was responsible for this, we synthesized complex
(Fig. 2, Scheme S1‡), which does not contain any heteroatom on
the η6-arene. Surprisingly, ATH using
revealed similar or only
slightly lower performance than , except for N-sulfonyl imine
12, which can form an O-H···O=S hydrogen bond with that
E
G
reactivity. Imines
1, showed quite dissimilar performance: just like
dimethoxy derivative
complexes apart from
8
and
9
, bearing an isopropyl group in position
, the 6,7-
gave very high conversions with all
and . On the contrary, exhibited
A
1–4, 8–
7
1
8
D
A
E
9
G
H
sluggish reactivity and surprisingly, only the tethered complex
afforded full conversion out of the Ru(II) series. β-
D
A
–
E
H
Carbolines 10 and 11 behaved in line with the previously
described substrates. Apparently, the structurally more
complex imines (containing bulkier substituents in positions 1,
G
G
may increase the reactivity. Therefore, the hydroxyl group was
not the key parameter responsible for the enhanced reactivity
6 or 7, or having the
using tethered catalysts
β
-carboline scaffold) are less reactive when
and . Eventually we attempted the
D
E
of
of results in Fig. S1‡) suggest the hydrogenations with
first order, while and are closer to zero-order kinetics.
G
. The reaction kinetics (one example in Fig. 3, complete set
AH of N-sulfonyl imine 12 – unfortunately, it was not soluble
under the reaction conditions in methanol, and did not react in
dichloromethane.
Good-to-excellent enantioselectivity was achieved in most
cases (Table 1). Lower ee values were typically obtained with
A
to be
G
H
Systematic studies to clarify this phenomenon are currently
underway in our laboratories.
Both
(Table 1), and their activity was comparable or higher than that
of Ru(II) complexes . Particularly, the poorly reactive imine
was hydrogenated with high conversions. The AH method was
G and H were also tested in AH under optimised conditions
complexes
enantioselectivity.
One reaction was performed on a ten-fold scale with complex
and substrate (0.44 mmol of ; all other components scaled
D and F, and imine 9 gave only moderate
A–E
A
9
1
1
thus extended to a complex containing a hydroxyl, and has the
potential to be operable with heterogenized catalysts derived
up accordingly) to show the synthetic utility of these reactions.
The product was obtained at 92% yield, 97% ee and >95% purity.
In the course of this project, we also synthesized a complex
from
The ees delivered by
obtained with in both AH (Table 1) and ATH (Table 2) for all
substrates, again with the exception of , in which case we
G
, which are being developed in our laboratories.
bearing a 4-hydroxybutyl group at the η6-arene (
G
, see Fig. 2).
G
and were very similar to those
H
A–F
Such
a
functionalization offers the possibility of its
9
immobilization – unlike heterogenization via the DPEN ligand,
which has been shown on many examples,7 the arene ligand has
been utilized much less often.14
observed a significant increase. This means that replacing the
isopropyl group of p-cymene with butyl or hydroxybutyl had no
negative effect on ee. A similar observation was made earlier on
a complex bearing a 2-hydroxyethoxy group on the η6-arene.19
In conclusion, we present a simple method for the AH of cyclic
imines catalysed by Ru(II) and Rh(III) half-sandwich complexes
using trifluoroacetic acid for substrate activation. The catalysts
were used in the standard Ru-chloride forms, which are easily
accessible and non-air sensitive. In contrast to other reported
approaches to imine AH with hydrogen gas, this method does
not require air-sensitive additives nor inert atmosphere. New
We employed the [4+2] cycloaddition reaction5g,15,16 of a
terminal alkyne (5-hexyn-1-ol) with a 1,3-diene (isoprene) to
conveniently synthesize cyclohexadiene 13 (Scheme S1‡). In the
original procedure,5g the authors suggested that along with 1,4-
substituted cyclohexa-1,4-diene (13a), only analogous 1,3-
substituted cyclohexa-1,4-diene (13c) was formed (>80:20
ratio). However, with the aid of 2D NMR spectroscopy (Fig. S2‡),
we identified that three regioisomers 13a, 13b (being a 1,4-
substituted cyclohexa-1,3-diene) and 13c were formed in a ratio
of 74:19:7. As these could not be separated, the mixture served
for the synthesis of Ru(II) dimer (14a),17 which further afforded
hydroxybutyl-arene-functionalized
Ru(II)
catalyst
was
synthesized – such functionalization gives the possibility of
modular heterogenization. This complex (and its congener
lacking the hydroxyl group) showed enhanced reactivity in
imine ATH and performed similarly to the existing catalysts
under the newly-developed AH conditions. Asymmetric
hydrogenation of imines is still a discussed topic. By this work,
we wish to add one more option to the highly versatile
collection of possibilities offered by the Noyori-Ikariya class of
catalysts.
G
by complexation with the (S,S)-TsDPEN ligand (Scheme S1‡).
As expected, dimer 14a contained around 7% of the meta-
substituted analogue (14b) originating from 13c. In the spectra
of
G this could no longer be resolved due to signal broadening.
Complex
G was further characterized by single-crystal X-ray
diffraction (Figs. S3, S4).‡
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Table 2 ATH of imines 1–4, 8–10 and 12 catalysed by complexes A, G and Ha
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Lett., 2007, , 255–257; (e) T. Ohkuma,DNO.I:U10ts.1u0m39i,/CM5C. C06712J
Watanabe, K. Tsutsumi, N. Arai and K. Murata, Org. Lett.,
2007, , 2565–2567; (f) K. E. Jolley, A. Zanotti-Gerosa, F.
9
ee (%)
G
TONb
G
TOF (h–1)b
9
A
H
A
H
A
G
H
Hancock, A. Dyke, D. M. Grainger, J. A. Medlock, H. G.
Nedden, J. J. M. Le Paih, S. J. Roseblade, A. Seger, V.
Sivakumar, I. Prokes, D. J. Morris and M. Wills, Adv. Synth.
Catal., 2012, 354, 2545–2555; (g) T. Touge, T. Hakamata, N.
Hideki, T. Kobayashi, N. Sayo, T. Saito, Y. Kayaki and T.
Ikariya, J. Am. Chem. Soc., 2011, 133, 14960–14963.
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13209; (b) C. Li, C. Wang, B. Villa-Marcos and J. Xiao, J. Am.
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Kayaki and T. Ikariya, Organometallics, 2009, 28, 802–809;
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1
2
3
4
8
9
10
12
93
85
90
87
93
50
92
92
94
86
91
88
95
70
93
92
92
85
90
87
95
68
93
92
142 174 154 178 195 175
108 138 135 136 155 157
110 138 126 146 163 162
125 134 114 160 153 145
78
29
160 150 111 207 198
82 68 32 102 83
6
148 200 146 183 209 187
37 76 39 45 97 46
a Amount of substrate n = 55 μmol, concentration of substrate c = 75 mM, catalyst
loading 0.5 mol%, hydrogen source HCOOH/Et3N (5:2), 30 °C. b Turnover number
calculated after 50 min. c Turnover frequency calculated at 20% conversion.
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,
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8
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Fig. 3
ATH of imine 1 catalysed by complexes A, G and H using HCOOH/Et3N (5:2) in
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This work was financially supported by the Czech Science
Foundation (P106/12/1276 and 15-08992S), grant for long-term
conceptual development of the Institute of Microbiology (RVO:
61388971) and the National Program of Sustainability (NPU I
(LO-2015) MSMT – 34870/2013). The research was conducted
within the infrastructure built up from the support of the
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Notes and references
‡ See the Electronic Supplementary Information.
§ Fan et al.12 found that the addition of trifluoroacetic acid
promoted the AH of quinolines with Ir(III)-triflate complexes.
However, Ir(III) are much more reactive and their activity is
further enhanced by the triflate counteranion. Therefore, we
could not employ these reaction conditions in our work.
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