Amine-tunable ruthenium catalysts for asymmetric reduction of ketones

Article abstract of DOI:10.1002/adsc.201300727

A series of efficient ruthenium catalysts has been developed for the asymmetric hydrogenation and transfer hydrogenation of ketones with high reactivities and selectivities. The new chiral bisdihydrobenzooxaphosphole (BIBOP)/diamineruthenium complexes catalyzed the enantioselective hydrogenation of substrates such as aryl and heteroaryl cyclic and alkyl ketones with substrate/catalyst (S/C) ratios of up to 100,000. The opposite sense of enantioselectivity can be obtained by proper selection of a diamine with a given chirality of the phosphine. The usefulness of the new system has been demonstrated in the asymmetric hydrogenation of a complex synthetic intermediate towards cholesteryl ester transfer protein (CETP) inhibitors at S/C 20,000 on large-scale operation.

Full text of DOI:10.1002/adsc.201300727

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DOI: 10.1002/adsc.201300727
Amine-Tunable Ruthenium Catalysts for Asymmetric Reduction
of Ketones
Sonia Rodrꢀguez,a,* Bo Qu,a Keith R. Fandrick,a Frederic Buono,a Nizar Haddad,a
Yibo Xu,a Melissa A. Herbage,a Xingzhong Zeng,a Shengli Ma,a Nelu Grinberg,a
Heewon Lee,a Zhengxu S. Han,a Nathan K. Yee,a and Chris H. Senanayakea
a
Chemical Development US, Boehringer Ingelheim Pharmaceuticals, Inc., 900 Ridgebury Rd, Ridgefield, CT 06877, USA
E-mail: sonia.rodriguez@boehringer-ingelheim.com
Received: September 16, 2013; Revised: November 27, 2013; Published online: February 2, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300727.
Abstract: A series of efficient ruthenium catalysts
has been developed for the asymmetric hydrogena-
tion and transfer hydrogenation of ketones with
high reactivities and selectivities. The new chiral
bisdihydrobenzooxaphosphole (BIBOP)/diamine-
ruthenium complexes catalyzed the enantioselective
hydrogenation of substrates such as aryl and hetero-
aryl cyclic and alkyl ketones with substrate/catalyst
(S/C) ratios of up to 100,000. The opposite sense of
enantioselectivity can be obtained by proper selec-
tion of a diamine with a given chirality of the phos-
phine. The usefulness of the new system has been
demonstrated in the asymmetric hydrogenation of
a complex synthetic intermediate towards cholester-
yl ester transfer protein (CETP) inhibitors at S/C
20,000 on large-scale operation.
heteroaryl cyclic ketones utilized a Ru-BINAP com-
plex with a 1,4-diamine derived from natural mannitol
for the reduction of 4,5,6,7-tetrahydrobenzofuran-4-
one[4b] or h6-arene/N-tosylethylenediamine-rutheni-
AHCTUNGTRENNUNG
um(II) catalysts for the reduction of 4-chromanones.[5]
Some chiral catalysts have been reported to exhibit
turnover numbers (TON, molar ratio of converted
substrate to catalyst) over 100,000 with simple aryl
methyl ketones,[2b,c,3l,6] but most of the reported chiral
catalysts have TON lower than 1000, making these
catalytic systems unsuitable for industrial applica-
tions.[7]
Herein, we disclose a new family of amine-tunable
ruthenium catalysts based on chiral bisdihydroben-
zooxaphosphole ligands (BIBOP ligands) as effective
catalysts for the asymmetric hydrogenation and trans-
fer hydrogenation of a variety of highly challenging
ketones, including heteroaryl cyclic ketones. The reac-
tion is conducted with TON values as high as 100,000
under 400 psi of hydrogen. The hydrogenation of
a sterically hindered synthetic intermediate of a drug
candidate, catalyzed by a Ru-BIBOP complex, was
performed with S/C 20,000 in >99:1 er.
Keywords: asymmetric hydrogenation; asymmetric
transfer hydrogenation; bisdihydrobenzooxaphos-
ACHTUNGTRENNUNG
In the synthesis of potential cholesteryl ester trans-
fer protein (CETP) inhibitors, we needed to perform
The asymmetric reduction of ketones is a key trans- the asymmetric reduction of ketone 1 (Scheme 1).
formation in the pharmaceutical industry for the The reduction of 1 was initially conducted with one
preparation of enantiomerically pure alcohols, partic- equivalent of borane-diethylaniline and 15–20 mol%
ularly
those
bearing
heterocycles.[1]
Chiral
RuCl2(diphosphine)AHCTUNGTR(ENNNUG diamine) complexes pioneered
by Noyori and co-workers catalyze the highly efficient
asymmetric hydrogenation of a wide array of ketones
to afford the corresponding alcohols.[2] Despite the va-
riety of catalysts described in the literature[3] very few
ruthenium catalysts have been able to hydrogenate
cyclic ketones such as 1-tetralones.[3l,4] Furthermore,
examples of the hydrogenation of heteroaryl cyclic
ketones are extremely rare. To the best of our knowl-
Scheme 1. Asymmetric hydrogenation of ketone 1 towards
edge, the only reported asymmetric hydrogenations of CETP inhibitors.
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Table 1. Evaluation of commercial Ru complexes for the asymmetric hydrogenation of 1. Enantiomeric ratios were deter-
mined by chiral HPLC; The R absolute configuration of 2 was determined by chemical correlation to the final product.
Entry
Ru complex
Conversion [%][a]
er[b]
1
2
3
4
5
6
7
8
RuCl2[(R)-BINAP][(R)-daipen]
RuCl2[(R)-Xyl-BINAP][(R)-daipen]
11
23
6
100
96
0
–
5:95
–
98:2
8:92
–
RuCl2[(R)-BINAP]
RuCl2[(S)-Tol-BINAP][(S)-i-Pr-BIMAH]
RuBr2[(S)-Xyl-Skewphos](ampy)
RuCl2[(R)-Xyl-Phanephos][(S,S)-dpen]
RuCl2[(S)-Xyl-P-Phos][(R)-daipen]
RuCl2[(S)-Paraphos][(R,R)-dpen]
RuCl2A[(S,S)-DIOP][(S)-i-Pr-BIMAH]
(R)-RUCYTM-Xyl-BINAP
RuCl2[(S)-Tol-BINAP](ampy)
ACHTUNGTREN[NUNG (R,R)-dpen]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1
–
R
100
98
10
100
79:21
26:74
17:83
98:2
9
CHTUNGTRENNUNG
10
11[c]
AHCTUNGTRENNUNG
[a]
Molar conversion as determined by HPLC.
Recorded as R:S.
EtOH as solvent.
[b]
[c]
(1R,2S)-cis-1-amino-2-indanol to afford an 84% yield nomethylpyrimidine,
9,
and
8-aminoquinoline
of (S)-2 in 96:4 er on a multi-kilogram scale. Although (amqui), 12. Interestingly, the inverse enantioselectivi-
the reaction performed well on scale-up, a greener ty was observed when changing the R substituent in
and more efficient catalytic method was desired. To the BIBOP ligand from hydrogen (3a) or methyl (3b)
this end, the hydrogenation of ketone 1 was evaluated to methoxy (3c) with 8 as diamine, going from 9:91
using 32 commercially available RuCl2(diphosphine)- and 6:94 to >99:1 er, respectively. Inverse selectivity
ACHTUNGTRENNUNG(diamine) complexes (S/C 50) with t-BuOK in isopro- effects were also observed with a given phosphine
pyl alcohol (IPA) (see the Supporting Information). complex 3 and different amines, that is, 3c provided
This initial screen identified RuCl2[(S)-tol-BINAP] opposite enantioselectivities with diamines 4 and 5
[(S)-i-Pr-BIMAH]
methanamine]-1H-benzimidazole} as a suitable cata-
lyst for the reduction of 1 providing (R)-2 in quantita- alysts for the reduction of 1, we synthesized the corre-
tive yield and 98:2 er (Table 1). Additional screening sponding RuCl2A(BIBOP)(diamine) complexes by re-
of solvents revealed RuCl2[(S)-tol-BINAP](ampy) action of 3 with the corresponding diamines in tolu-
{i-Pr-BIMAH=2-[a-(isopropyl)- than 8–12.
To evaluate the reactivity of the most selective cat-
C
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(ampy=2-aminomethylpyridine) in ethanol as anoth- ene at 1108C. Isomeric mixtures of at least four iso-
er active catalytic system, affording (R)-2 in quantita- mers were observed by 31P NMR spectroscopy when
tive yield and 98:2 er. However, upon catalyst loading using diamines 8, 9 and 12.[9] As reported for other 2-
optimization studies, complete conversion was not aminomethylpyridine complexes, no differences in re-
achieved at S/Cꢀ1000, which was not economically activity or enantioselectivity were observed regardless
feasible for scale-up due to the high cost of the cata- of the isomeric ratio of the pre-catalyst mixture.[3k,6b]
lysts.
At this point, the reaction was evaluated using
Screening of the pre-made RuCl2(MeO-BIBOP)-
AHCTUNGTREG(NNUN diamine) catalysts along with optimization of the re-
BIBOP ligands, which we had previously developed action conditions for solvent, pressure and tempera-
for rhodium-catalyzed hydrogenation of functional- ture, allowed the synthesis of (R)-2 using
ized olefins.[8] RuCl
G
G
ACHTUNGTNER(NUNG ampy), 13, at S/C 20,000 under
CTHUNGTRENNUNG
new catalytic system, the hydrogenation of 1 was ex-
Hydrogenation of ketone 1 was monitored by FT-
plored in situ with several diamines (Table 2). The IR. Different amounts of purified product alcohol 2
highest enantioselectivities (over 99:1 er) were ob- (up to 100 mo%) were added at the beginning of the
served with 2-aminomethylpyridine (ampy), 8, 2-ami- reaction and the decay of ketone 1 was recorded. A
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Amine-Tunable Ruthenium Catalysts for Asymmetric Reduction
Table 2. Evaluation of Ru-BIBOP/diamine complexes in the reduction of 1.[a]
[a]
Conditions: H2 (400 psi), 4–5 mol% precatalyst 3, 4–5 mol% diamine, t-BuOK, i-PrOH, 258C, 15 h, molar
conversion as determined by HPLC; er recorded as R:S.
first order simulation fits with the observed experi- isopropoxide in IPA at 808C, a 93% yield of (R)-2
mental data indicating a first order dependence of the and >99:1 er was obtained (Scheme 2).
reaction rate with the ketone concentration under the
The new Ru-BIBOP catalysts also proved to be
reaction conditions. The reaction rates with or with- more selective than 33 commercially available com-
out added product 2 were comparable, indicating plexes in the reduction of 4,5,6,7,8-tetrahydro-5-qui-
a minor inhibiting effect of the product.
nolinone, 14 (see the Supporting Information and
The prepared catalysts were also efficient for asym- Table 3). Evaluation of commercial complexes al-
metric transfer hydrogenation applications.[3g,10] When lowed up to 92:8 er by using RuCl2[(S)-Xyl-P-Phos]
1 was treated with 0.2 mol% 13 and 10 mol% sodium [(R)-daipen] (entry 7). The highest enantioselectivities
of >97:3 er were obtained when using complex 3c
Scheme 2. Asymmetric hydrogenation of ketone 1.
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Sonia Rodrꢀguez et al.
Table 3. Asymmetric hydrogenation of ketone 14.
Entry
Ru complex
Conversion [%][a]
er[b]
1
2
3
4
5
6
7
8
RuCl2[(R)-BINAP][(R)-daipen]
RuCl2[(R)-Xyl-BINAP][(R)-daipen]
RuCl2[(R)-BINAP]ACTHUGNETRNNU[G (S,S)-dpen]
RuCl2[(S)-BINAP][(S)-i-Pr-BIMAH]
RuBr2[(S)-Xyl-Skewphos][(R)-daipen]
98
86:14
68:32
12:88
60:40
87:13
56:44
92:8
49:51
49:51
35:65
1:99
10:90
5:95
3:97
59:41
90:10
100
100
100
100
100
100
100
100
96
100
100
100
100
100
100
RuCl2[(R)-Xyl-Phanephos]
RuCl2[(S)-Xyl-P-Phos][(R)-daipen]
RuCl2[(S)-Paraphos][(R,R)-dpen]
ACHTUNGTREN[NUNG (S,S)-dpen]
U
9
RuCl2ACTHNGUETRNNU[G (S,S)-DIOP][(S)-i-Pr-BIMAH]
10
11
12
13
14
15
16
(R)-RUCYTM-Xyl-BINAP
3c+4
3c+5
3c+6
3c+7
3c+8
3c+12
[a]
Molar conversion determined by HPLC.
Recorded as R:S.
[b]
with (R,R)-dpen, 4, or (S)-daipen, 7 (entries 11 and thol, 29, was obtained in 98% yield and 6:94 er when
14). A remarkable inversion of the stereoselectivity using 27 at S/C 100,000 under the same conditions.
was observed when using 3c in combination with 12
In order to analyze the inverse stereoselectivity ob-
(entry 16) instead of diamines 4–7 (entries 11–14). Al- served in the reduction of 1-tetralone, 28, between
though some scattered examples of inversion of ste- catalysts 26 and 27, computational studies utilizing
reoselectivity can be found in the literature, no gener- density functional theory (DFT) using the Gaussian
al evaluation has been undertaken.[11]
09 program[13] (B3LYP/LANL2DZ[14]) were conducted
The reduction of other heterocycles to provide iso- (Figure 1). Transition state optimizations were per-
quinolines 16–17, benzothiophene 18, benzofuranone formed with the lowest energy binding mode for the
19, and pyrazole 20 also resulted in high selectivities Ru hydride complexes derived from 26 and 27 accord-
(Table 4). Complete inversion of enantioselectivity ing to Noyoriꢂs postulated mechanism.[2f] The complex
was also observed in these cases using pre-made com- derived from pre-catalyst 26, containing the dpen
plexes 26 and 27. These catalysts also provided high ligand 4, is C2 symmetrical and utilizing the axial hy-
stereoselectivities of 1-tetralols 21–22. In the case of drogen on the amine to complex the ketone orients
indanones, high enantioselectivities but low isolated the more sterically demanding side of the prochiral
yields were obtained. Aryl alkyl ketones such as ace- ketone away from the arene of the ligand framework.
tophenone, p-bromoacetophenone and 3-acetylpyri- This, in turn, positions the Re-face of the ketone for
dine were also suitable substrates providing 23–25 in hydride delivery producing the observed S-enantio-
relatively lower enantiomeric ratios of 90:10, 95:5 and mer of the substrate. In contrast, the transition state
96:4, respectively, when using the dpen complex 26.[12] corresponding to the Ru hydride complex derived
Interestingly, in the case of the aryl methyl ketones, from 27, containing the aminoquinoline ligand, posi-
no inversion of enantioselectivity was observed when tions the ketone on the opposite side of the complex
using the amqui complex 27.
as compared to the dpen catalyst. The arene unit of
1-Tetralone, 28, was selected as an inexpensive the substrate is then positioned away from the steri-
commercially available ketone to assess the reactivity cally demanding tert-butyl fragment of the ligand.
of the newly developed catalysts (Scheme 3). (S)- This orientation exposes the Si-face of the prochiral
1,2,3,4-Tetrahydro-1-naphthol, 21, was obtained in ketone for hydride transfer generating the R-configu-
99% yield and 99:1 er when using 26 at S/C 100,000 ration of the secondary alcohol.
under 400 psi of hydrogen and 608C. Analogously, the
In summary, we have developed a series of novel
opposite enantiomer (R)-1,2,3,4-tetrahydro-1-naph- and efficient ruthenium catalysts for asymmetric hy-
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Amine-Tunable Ruthenium Catalysts for Asymmetric Reduction
Table 4. Scope of the asymmetric hydrogenation.[a]
Scheme 3. Asymmetric hydrogenation of 1-tetralone, 28.
[a]
Reaction conditions: 0.1–2 mol% RuCl2(MeO-BIBOP)-
ACHTUNGTRENNUNG(diamine), 0.2 equiv. t-BuOK, i-PrOH, room tempera-
ture, 20 h, 400 psi of hydrogen; The enantiomeric ratios
were determined by chiral HPLC and refer to S/R ratios.
drogenation and transfer hydrogenation of ketones
with high reactivities and excellent selectivities. The
new chiral BIBOP/diamine-Ru complexes catalyzed
the enantioselective hydrogenation of traditionally
challenging substrates such as aryl and heteroaryl
cyclic ketones as well as aryl and heteroaryl alkyl ke-
tones with S/C up to 100,000. In addition, the opposite
sense of enantioselectivity can be obtained by proper
selection of a diamine with a given chirality of the
phosphine. The usefulness of the new system has been
demonstrated on the asymmetric hydrogenation of
a complex synthetic intermediate towards CETP in-
hibitors at S/C 20,000 on large scale operation.
Figure 1. DFT (B3LYP/LANL2DZ) calculated transition
states.
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COMMUNICATIONS
Sonia Rodrꢀguez et al.
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Experimental Section
General Procedure for Hydrogenation
Hydrogenation of 28 illustrates the typical reaction proce-
dure. To a mixture of 1-tetralone, 28, (10.0 g, 66.4 mmol)
and
RuCl2[(2R,2’R,3S,3’S)-MeO-BIBOP]
(amqui),
27,
(1.0 mg, 0.001 mmol, 0.002 mol%) were added isopropyl al-
cohol (40 mL) and a 1M solution of t-BuOK in tert-butyl al-
cohol (1.33 mL,1.33 mmol, 0.02 equiv.). The autoclave reac-
tor was first purged with nitrogen, then with hydrogen, and
then the reaction mixture was stirred at 608C under 400 psi
of hydrogen for 20 h. After venting the hydrogen gas, the
solvent was removed under reduced pressure. The residue
was purified by silica-gel column chromatography with 0–
50% ethyl acetate in hexane as eluent to give (R)-1,2,3,4-tet-
rahydro-1-naphthol, 29, as a colorless oil; yield: 9.6 g (98%);
96:4 er. The er of 1,2,3,4-tetrahydro-1-naphthol was deter-
mined by HPLC analysis [column, Chiralcel OJ-3, 4.6ꢃ
150 mm; eluent, heptane/isopropyl alcool (95:5); flow rate,
1 mLminÀ1; column temperature, 258C]: retention time (tR)
of (R)-1,2,3,4-tetrahydro-1-naphthol, 7.45 min (95.8%); tR of
(S)-1,2,3,4-tetrahydro-1-naphthol, 5.75 min (4.2%); [a]D:
À31.2 (c=2.0, MeOH); literature[15] [a]D23: À33.28 (c=1.0,
CHCl3), >99% ee (R).
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