Synthesis of a P-stereogenic PNPtBu,Ph ruthenium pincer complex and its application in asymmetric reduction of ketones

Article abstract of DOI:10.1002/ejoc.201500389

A novel P-stereogenic PNP^tBu,Ph ruthenium complex has been synthesized and characterized enabling the asymmetric hydrogenation of a wide range of aromatic ketones with excellent catalytic activities (up to 98% conversion) and good levels of enantiodiscrimination (up to 95% ee). P-stereogenic PNP^tBu,Ph Ru complex was for the first time synthesized and characterized and has proven to be an efficient catalyst for the asymmetric reduction of a wide range of ketones

Full text of DOI:10.1002/ejoc.201500389

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500389
Synthesis of a P-Stereogenic PNPtBu,Ph Ruthenium Pincer Complex and Its
Application in Asymmetric Reduction of Ketones
Ismael Arenas,[a] Omar Boutureira,[a] M. Isabel Matheu,[a] Yolanda Díaz,*[a] and
Sergio Castillón*[a]
Keywords: Asymmetric hydrogenation / Ruthenium / Pincer ligands / Chiral alcohols
A novel P-stereogenic PNPtBu,Ph ruthenium complex has
been synthesized and characterized enabling the asymmetric
hydrogenation of a wide range of aromatic ketones with ex-
cellent catalytic activities (up to 98% conversion) and good
levels of enantiodiscrimination (up to 95% ee).
nitroalkenes demonstrated that P-stereogenic pincer ligands
are efficient inductors of enantioselectivity (Figure 1, c).
Herein, we report the design, synthesis and characterization
of a new P-stereogenic PNPtBu,Ph ruthenium complex and
its catalytic activity in the asymmetric hydrogenation of
ketones. The difference between the planar phenyl and the
bulky tert-butyl group at P atoms is envisioned to create an
effective enantiodiscriminating environment for asymmetric
catalysis (Figure 1, d).[13]
Introduction
Pincer ligands are privileged scaffolds widely employed
in a variety of catalytic transformations as a result of their
stability, tunability and high activity when coordinated to
metals.[1] They are indeed highly modulable and several
structural combinations are possible taking into account
some structural features such as the nature of coordinating
groups, central aromatic ring and spacers. Among the great
variety of transition-metal pincer complexes, those with N-
or P-donor groups are particularly interesting since out-
standing results have been reported in a vast number of
organic transformations, including hydrogenation,[2] ac-
ceptorless dehydrogenation,[3] C–H or N–H activation reac-
tions,[4] allylation of aldehydes and imines,[5] hydroamin-
ation[6] and aldol reactions.[7]
Introduction of chirality in pincer ligands is of growing
interest in this field since it might enable ready access to
asymmetric catalytic reactions and would extend the appli-
cation of these ligands in organic synthesis. Different ap-
proaches have been reported to this end, the installation of
chiral substituents at the benzylic positions (Figure 1, a)[8]
or the introduction of donor substituents with predefined
chiral entities (Figure 1, b)[9] being the most common strate-
gies. Alternatively, the use of P-stereogenic phosphines in
chiral pincer metal complexes is still relatively unexplored,
probably due to the lack of a natural chiral pool of P-ste-
reogenic compounds as well as the relative configurational
instability of P atoms.[10,11] Nonetheless, a recent example
by Zhang et al.,[12] describing a P-stereogenic PCP-Pd
complex in the asymmetric addition of diarylphosphines to
Figure 1. Representative examples of chiral pincer metallic com-
plexes.
Results and Discussion
The straightforward synthesis of 3 was accomplished in
two steps, starting from phosphine–borane complex 1
(Scheme 1).[14] Deprotection of these adducts is generally
achieved by treatment with a large excess of an amine,
namely pyrrolidine, morpholine or DABCO[15] and removal
of the corresponding amine–borane byproduct is usually
described to proceed by low-temperature sublimation.[14] In
our hands, the deprotection of the phosphine–borane moi-
ety resulted more reluctant than expected, and low yields
[a] Departament de Química Analítica i Química Orgànica,
Facultat de Química, Universitat Rovira i Virgili,
C/ Marcel·lí Domingo 1, 43007 Tarragona, Spain
E-mail: yolanda.diaz@urv.cat
sergio.castillon@urv.cat
http://www.quimica.urv.es/sintcarb/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500389.
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I. Arenas, O. Boutureira, M. I. Matheu, Y. Díaz, S. Castillón
SHORT COMMUNICATION
and oxidation of the free ligand were always detected. Other absence of base, no traces of alcohol were detected (entry
common procedures such as treatment with a strong acid 5). In order to increase the enantioselectivity, we next per-
followed by basic aqueous workup,[16] or the use of poly- formed the reaction at lower temperatures. Decreasing the
mer-supported amines[17] did not provide the unprotected reaction temperature improved the enantioselectivity (up to
PNPtBu,Ph ligand 2 in reasonable yields and/or purity. Li- 77%), at the expense of the conversion, which was slightly
gand 2 was finally obtained in good yield (78%) by reaction eroded (entries 6–7). To our delight, at temperatures as low
with excess of diethylamine and purification by preparative as –40 °C, complete conversions and 87% ee were achieved
TLC inside a glove-box. The use of alumina instead of silica by increasing the equivalents of base with respect to the Ru
plates and careful anhydrous handling were crucial in order complex (entry 8).
to obtain reproducible experiments. The structure of 2 was
confirmed by 1H, 13C and 31P-NMR spectroscopy (see Sup-
Table 1. Optimization of reaction conditions for the asymmetric
porting Information).[14] The resulting ligand was treated
with [RuHCl(PPh3)3(CO)] in refluxing benzene to afford Ru
complex 3 in 85% yield as a pale-yellow solid. The CO
streatching band in the IR spectrum was observed at
1890.3 cm–1 and the 31P-NMR spectrum showed two dou-
blets at δ = 80.3 and 64.3 ppm with JPP = 267.6 Hz, corre-
sponding to the two magnetically different phosphorus
hydrogenation of acetophenone (4).[a]
Entry
Solvent
T [°C]
Conv. [%][b]
ee [%][c]
1
atoms. In the H-NMR spectrum, the diastereotopic pro-
1
2
3
benzene
THF
room temp.
room temp.
room temp.
room temp.
room temp.
0
98
98
96
98
–
98
91
98
rac
20
32
64
–
71
77
87
tons at the P-CH2-pyridine arm resonated at δ = 4.60, 4.14
and 3.96–3.86 ppm. The chemical shift for the hydride (δ =
–14.52 ppm, JHP = 23.6 and 15.2 Hz) was consistent with a
trans-disposition with respect to the chlorine atom, in
agreement with other [(PNP)RuHCl(CO)] complexes.[18]
These data strongly suggested the coordination of the
PNPtBu,Ph ligand in a meridional fashion around the octa-
hedral Ru center.
MeOH
EtOH
EtOH
EtOH
EtOH
EtOH
4
5[d]
6
7
–20
–40
8[e]
[a] General conditions: 0.0025 mmol of Ru complex 3,
0.0025 mmol of tBuOK, 0.5 mmol of acetophenone, EtOH(5 mL),
H2 (30 atm), 16 h. [b] Conversion was calculated by1H-NMR spec-
troscopy. [c] Enantiomeric excess was calculated by GC. Absolute
configurations were determined to be (S) (see the Supporting Infor-
mation). [d] Control reaction carried out without KOtBu. [e]
0.0025 mmol of Ru complex 3 and 0.025 mmol of KOtBu.
With the optimal reaction conditions in hand, we next
evaluated the scope of complex 3 with a wide range of
structurally diverse aromatic ketones (Scheme 2). Electron-
donating groups in para-position delivered the correspond-
ing alcohols (5b–5c) with good ee’s (85%). For the particu-
lar case of the aryl methyl ketones, a slight increase of the
ee (up to 90%) of alcohols (5d–5e) was observed when the
substituent was located in meta or ortho-positions with re-
spect to that in para-position. Ketones bearing electron-
withdrawing groups were completely hydrogenated to
alcohols (5f–5g) albeit with lower ee values and the same
trend was observed with a more electrophilic ketone to give
alcohol 5h. When the methyl group of acetophenone was
replaced by either iPr or aliphatic 4-, 5- and six-membered
Scheme 1. Synthesis of P-stereogenic PNPtBu,Ph ruthenium complex
3.
The catalytic performance of P-stereogenic PNPtBu,Ph Ru rings, complete conversions and good ee values (up to 91%)
complex 3 was evaluated in the asymmetric hydrogenation were observed (5i–5l). On the other hand, ketones contain-
of ketones as a model reaction. The optimization of reac- ing benzo-annulated rings were reduced with moderate con-
tion conditions was carried out using acetophenone (4)[19] version, probably as a result of an increase in the steric
(Table 1). Preliminar experiments established the use of hindrance to give alcohol 5m. As for heteroaromatic methyl
0.5 mol-% of complex 3 as the more appropriate option due ketones, 2-acetylpyridine was fully hydrogenated with lower
to a better overall reaction time vs. conv. vs. ee ratio. ee (5n). On the contrary, ketones bearing π-excedent hetero-
Among the various solvents evaluated at room temperature, cycles such as 2-furyl methyl ketone or 2-acetyltiophene
EtOH was found to be optimal in terms of conversion (up were converted to alcohols (5o–5p) with better enantio-
to 98%) and enantioselectivity (up to 64%) towards alcohol selectivity (up to 86%). Replacement of the phenyl ring by
5a (entries 1–4). When the reaction was carried out in the naphthalene led to the corresponding alcohols with good
2
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Synthesis of a P-Stereogenic PNPtBu,Ph Ruthenium Pincer Complex
ee values (5q–5r). Finally, the hydrogenation of ethyl benzo-
ylformate proceeded with total chemoselectivity towards
the ketone moiety although with a moderate ee (4s).
Experimental Section
Synthesis of Ru Complex 3: A Schlenk flask was charged under
argon with [RuHCl(PPh3)3(CO)] (118 mg, 0.12 mmol), 2,6-Bis-P,P-
[(tert-butylphenylphosphanyl)methyl]pyridine (65 mg, 0.14 mmol)
and benzene (6.0 mL). The mixture was heated at 80 °C for 3 h and
then filtered. The filtrate was concentrated to dryness and then
the resulting yellow oil was dissolved in THF (1 mL) and pentane
(10 mL). The precipitated solid was filtered, washed with pentane
(3ϫ 2 mL) and dried under vacuum to afford Ru complex 3
1
(63 mg, 85%). H NMR (CD2Cl2, 400 MHz): δ = 7.82–7.78 [m, 4
H, ortho-P(Ph)2], 7.64 (t, J = 7.6 Hz, 1 H, pyridine-H4), 7.51–7.48
[m, 6 H, meta,para-P(Ph)2], 7.37 (dd, J = 7.6, 2.8 Hz, 2 H, pyridine-
H3,5), 4.60 (dd, J = 16.0, 10.0 Hz, 1 H, -CHH-P), 4.14 (dd, J =
16.0, 10.4 Hz, 1 H, -CHH-P), 3.96–3.86 (m, 2 H, -CHH-P), 1.13
{d, J = 14.4 Hz, 9 H, P[C(CH3)3]2}, 0.91 {d, J = 14.0 Hz, 9 H,
P[C(CH3)3]2}, –14.52 (dd, J = 23.6, 15.2 Hz, Ru-H) ppm. 13C
NMR (CD2Cl2, 100.6 MHz): δ = 210.2 (s, Ru-CO), 162.5 (dd, J =
8.1, 3.3 Hz, Py-C6), 162.3 (dd, J = 8.2, 3.9 Hz, Py-C2), 137.9 (s,
Py-C4), 136.6 [d, J = 39.2 Hz, ipso-P(Ph)2], 134.0 [dd, J = 10.1,
1.6 Hz, ortho-P(Ph)2], 132.7 [dd, J = 10.1, 1.1 Hz, ortho-P(Ph)2],
131.8 [dd, J = 34.7, 2.3 Hz, ipso-P(Ph)2], 130.2 [d, J = 2.2 Hz, para-
P(Ph)2], 130.0 [d, J = 2.2 Hz, para-P(Ph)2], 128.3 [d, J = 9.2 Hz,
meta-P(Ph)2], 128.2 [d, J = 9.3 Hz, meta-P(Ph)2], 121.1 (d, J =
9.8 Hz, Py-C5), 120.3 (d, J = 10.2 Hz, Py-C3), 40.4 (d, J =
22.1 Hz, -CH2P-), 38.7 (d, J = 20.8 Hz, -CH2P-), 34.7 {dd, J =
15.1, 5.0 Hz, P[C(CH3)3]2}, 31.7 {dd, J = 23.8, 2.2 Hz, P[C(CH3)3]2},
26.7 {d, J = 4.6 Hz, P[C(CH3)3]2}, 25.8 {d, J = 5.3 Hz, P[C(CH3)3]2}
ppm. 31P NMR (CD2Cl2, 162 MHz): δ = 80.3 (d, J = 267.6 Hz,
PtBuPh), 64.3 (d, J = 267.4 Hz, PtBuPh) ppm. IR (KBr, neat): ν
˜
= 1969.9 (νRuH), 1890.3 (νCO) cm–1. C28H36ClNOP2Ru (601.07):
calcd. C 55.95, H 6.04, N 2.33; found C 55.39, H 5.59, N 2.23.
Typical Procedure for the Asymmetric Hydrogenation of Ketones:
An autoclave was charged inside the glove-box with catalyst 3
(0.0025 mmol), tBuOK (0.025 mmol), EtOH (5 mL) and ketone
(0.5 mmol). The autoclave was then removed from the glove-box
and immediately pressurized to the appropriate H2 pressure and
temperature. After the desired reaction time, the autoclave was de-
1
pressurized. Conversion was calculated by H-NMR and enantio-
meric excess by GC (Chirasil-Dex CB or β-ciclodex 120).
Scheme 2. Substrate scope. General conditions: 0.0025 mmol of Ru
complex 3, 0.025 mmol of tBuOK, 0.5 mmol of ketone, EtOH
(5 mL), H2 (30 atm), 16 h, –40 °C. Conversion was calculated by
1H-NMR spectroscopy. Enantiomeric excess was calculated by GC.
Absolute configurations were determined to be (S) (see Supporting
Information). Results average of at least two independent runs.[a]
Reaction time: 24 h.[b] Reaction time: 96 h.
Acknowledgments
The authors thank Ministerio de Ciencia e Innovación (MICINN)
(project number CTQ-2011-22872-BQU) for financial support.
I. A. thanks MICINN for a research fellowship, and Serveis de
Recursos Científics (URV) for support. O. B. thanks the European
Commission (Marie Curie CIG) and Ministerio de Ciencia e Inno-
vación, Spain (for a Juan de la Cierva Fellowship).
Conclusions
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We have synthesized and characterized the first P-
stereogenic PNPtBu,Ph Ru complex which has proven to be
an efficient catalyst for the asymmetric reduction of a
variety of aromatic and heterocyclic ketones. Although the
enantioselectivities obtained are not superior to those re-
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that this catalyst, active in very mild operating conditions,
could be of significant interest in the chemoselective re-
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groups.
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SHORT COMMUNICATION
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Published Online: ᭿
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Synthesis of a P-Stereogenic PNPtBu,Ph Ruthenium Pincer Complex
Ru-Chiral Pincer Complexes
P-stereogenic PNPtBu,Ph Ru complex was
for the first time synthesized and charac-
terized and has proven to be an efficient
catalyst for the asymmetric reduction of a
wide range of ketones
I. Arenas, O. Boutureira, M. I. Matheu,
Y. Díaz,* S. Castillón* ...................... 1–5
Synthesis of a P-Stereogenic PNPtBu,Ph
Ruthenium Pincer Complex and Its Appli-
cation in Asymmetric Reduction of
Ketones
Keywords: Asymmetric hydrogenation / Ru-
thenium / Pincer ligands / Chiral alcohols
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