Ruthenium complexes of the general formula [RuCl2(PHOX) 2] and their catalytic activity in the Mukaiyama aldol reaction

Article abstract of DOI:10.1016/j.tetlet.2014.03.111

New ruthenium phosphinooxazoline (PHOX) complexes were synthesized and applied in the Mukaiyama aldol reaction. Four ruthenium complexes of the general formula [RuCl₂(PHOX)₂] were synthesized from [RuCl ₂(dmso)₄

Full text of DOI:10.1016/j.tetlet.2014.03.111

Tetrahedron Letters 55 (2014) 3033–3037
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ruthenium complexes of the general formula [RuCl2(PHOX)2]
and their catalytic activity in the Mukaiyama aldol reaction
Nichole Curvey a, Andria K. Widaman a, Nigam P. Rath b, Eike B. Bauer a,
⇑
a University of Missouri—St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
b Center for Nanoscience, University of Missouri—St. Louis, St. Louis, MO 63121, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
New ruthenium phosphinooxazoline (PHOX) complexes were synthesized and applied in the Mukaiyama
aldol reaction. Four ruthenium complexes of the general formula [RuCl2(PHOX)2] were synthesized from
[RuCl2(dmso)4] and the corresponding PHOX ligands through thermal ligand exchange. Two of the com-
plexes were characterized structurally. Achiral PHOX ligands gave the ruthenium complexes as single iso-
mers, whereas chiral PHOX ligands gave a mixture of isomers and also some incomplete substitution.
After activation by chloride abstraction, one of the new ruthenium complexes was applied as catalyst
in the Mukaiyama aldol reaction to give silyl-protected b-hydroxyl alcohols in 74–92% isolated yields
(room temperature, 18–24 h reaction time, 1 mol % catalyst loading).
Received 6 March 2014
Revised 23 March 2014
Accepted 24 March 2014
Available online 2 April 2014
Keywords:
Mukaiyama aldol reaction
Homogeneous catalysis
Ruthenium phosphinooxazoline complexes
Transition metal Lewis acids
Ó 2014 Elsevier Ltd. All rights reserved.
Transition-metal catalyzed organic transformations are an
essential part of the toolbox of synthetic organic chemists and con-
sequently are the subject of intense research. Well-characterized
ruthenium complexes play a prominent role in organometallic
catalysis; they can be thoroughly characterized using conventional
analytical techniques and tend to be configurationally stable.1 Pre-
formed ruthenium complexes have some advantages compared to
catalyst systems formed in situ from ligands and a metal source,
because rational ligand and metal complex design are facilitated.
For in situ systems, the exact nature of the catalytically active
species in solution is in many cases not known.
As part of our continuing research program centered around
ruthenium complexes,2 we are in continuous search for well-de-
fined ruthenium complexes to be applied as catalysts. Cationic
ruthenium complexes can serve as gentle Lewis acids in solution,
and are consequently able to exhibit catalytic activity in cases
where Lewis acids are the catalytically active species.3 We have
previously identified the Mukaiyama aldol reaction,4 which is cat-
alyzed by Lewis acids, as a valuable target for investigations.2g,5
Transition metal based catalyst systems for the reaction are
known,2g,4,5 but there is room for improvement with respect to iso-
lated yields, substrate scope, and stereoselectivities. Often air- and
water-sensitive metal complexes—for example, based on tita-
nium—are employed as catalysts,6 hampering routine applications
for synthetic organic chemistry. Ruthenium(II) complexes tend to
be stable under ambient conditions, which makes their employ-
ment in catalysis practical. Nevertheless, ruthenium-catalyzed
Mukaiyama aldol reactions are rare2g and we envisioned easy-to-
synthesize, well-defined, and relatively easy to handle ruthenium
complexes as an interesting alternative for the title reaction. We
have previously employed phosphinooxazoline (PHOX) ligands
(1–4 in Scheme 1) in the synthesis of iron complexes.7 Phosphino-
oxazolines are a bidentate, P–N donating ligand class, that are not
C2 symmetric and have been previously employed in transition
metal catalysis.8 We decided to test whether simple ruthenium
PHOX complexes could be employed as catalysts in the Mukaiyama
aldol reaction and our preliminary findings are reported herein.
Syntheses of the ruthenium complexes
To keep the synthesis of a catalytically active compound simple,
we targeted a ruthenium(II) complex that bears, besides the
counterions, only PHOX ligands. We speculated that the PHOX li-
gands could prevent the formation of chloro-bridged dimers or
oligomers, which might be inert and catalytically inactive. Due to
the bidentate nature of the PHOX ligands, a configurationally
saturated complex would have the general formula [RuX2(PHOX)2].
Ruthenium(II) dichloro complexes featuring P–N donating,
bidentate ligands are known and have been employed as catalysts,9
but to the best of our knowledge, complexes of the general
formula [RuX2(PHOX)2] were unknown and have not been utilized
as catalysts in the title reaction. The readily accessible10 precursor
⇑
Corresponding author. Tel.: +1 314 516 5311; fax: +1 314 516 5342.
E-mail address: bauere@umsl.edu (E.B. Bauer).
[RuCl2(dmso)4] (dmso = dimethylsulfoxide) appeared to be
a
http://dx.doi.org/10.1016/j.tetlet.2014.03.111
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

3034
N. Curvey et al. / Tetrahedron Letters 55 (2014) 3033–3037
Ph2
P
Ph2 Cl
[RuCl2(dmso)4]
toluene, 2 d reflux
P
Ru
N
N
N
Cl
O
Ph2P
O
O
1
2
82%,
[RuCl2(1)2]
one isomer
Ph2
P
Cl
Ru
Cl
Ph2
P
[RuCl2(dmso)4]
N
toluene ,2 d reflux
N
N
Ph2P
O
O
O
O
95%,
2
[RuCl2( )2]
one isomer
X-Ray
[RuCl2(dmso)4]
toluene, 7 d reflux
Ph2
P
N
Ph
Ph2 Cl
PPh2
Ph
P
O
N
Ph2P
O
Ru
Cl
N
Ph
Ru
N
N
3
(R)-
Cl
O
O
P
Ph
76%,
Ph
Cl
Ph2
mixture of isomers
3
[RuCl2((R)- )2]
Ph2
P
Ph2 Cl
Ph
[RuCl2(dmso)4]
toluene
N
P
O
Ph2 Cl
O
Ru
PPh2
Ph2
P
Ph
Ph2P
O
P
N
N
P
N
N
Ru
Cl
Cl
4
(R)-
5 d reflux
O
O
Ru
Cl
Ph
N
Cl
N
O
Ph
Ph
Ph2
O
Ph
75%,
mixture of isomers
Ph
4
[RuCl2((R)- )2]
Scheme 1. Synthesis of the ruthenium complexes.
suitable precursor for the synthesis of a corresponding ruthenium
PHOX complex and has previously been employed in thermal li-
gand exchange reactions.11 Accordingly, when the known7 PHOX
ligands 1 and 2 were refluxed with [RuCl2(dmso)4] in toluene for
2 days, the corresponding ruthenium complexes of the general for-
mula [RuCl2(PHOX)2] (PHOX = 1,2; Scheme 1) were isolated by
recrystallization from CH2Cl2/hexanes as red–orange, air-stable
solids in 82% and 95% yields (for deviating behavior of the chiral
PHOX ligands (R)-3 and (R)-4 see below). The complexes were
characterized by NMR (1H, 13C{1H}, 31P{1H}), IR, HRMS, and by ele-
mental analysis (Experimental details and characterization data
are given in the Supporting information). The coordination of the
PHOX ligands was best established by NMR and IR. The coordi-
nated PHOX ligands gave signals in the 31P{1H} NMR spectra
around 45 ppm, compared to about ꢀ5 ppm for the free ligands.
Upon coordination, some of the OCHH0 protons of the oxazoline
rings of the ligands 1 and 2 become diastereotopic and gave sepa-
rate signals in the 1H NMR spectra. The coordination of the nitro-
gen atom of the ligands was evidenced by IR; the free ligands
showed only one set of signals, establishing the magnetic equiva-
lency of the two PHOX ligands in each complex.
To unequivocally establish the structures of the two complexes
that were isolated as single isomers, the X-ray structure of
[RuCl2(2)2] was determined (Fig. 1, top; details for the structure
determination are given in the Supporting information). The struc-
ture revealed that the two chloro ligands take trans positions and
the phosphorus atoms of the PHOX ligands are located cis to each
other. The Ru-P, Ru-N, and Ru-Cl bond lengths are comparable to
related ruthenium complexes;2d,g,13 the bond angles show that
the structure of the complex can be best described as slightly dis-
torted octahedral, which is in accordance with the spectroscopic
data. By analogy, we assigned to the complex [RuCl2(1)2] the same
structure (Scheme 1).
The syntheses of the ruthenium complexes bearing the known7
PHOX ligands (R)-3 and (R)-4 proceeded differently. After two days
in refluxing toluene, coordination of only one PHOX ligand was
observed (31P{1H} NMR) and 7 days were required for the reactions
to go to completion. For (R)-4, after five days a small amount of
coordinated dmso was still observed in the isolated material (as
judged from small singlets between 2.5 and 3 ppm in the 1H
NMR), suggesting incomplete substitution to a species of the for-
mula [RuCl2((R)-4)(dmso)2]. The two complexes [RuCl2((R)-3)2]
and [RuCl2((R)-4)2] exhibited multiple signals in the 31P{1H}
NMR, suggesting a mixture of isomers in the isolated materials,
whereas the other spectroscopic data were similar to those of the
complexes of the achiral ligands 1 and 2.
showed mC
@
stretches around 1630 cmꢀ1
, which shifted to
N
1605–1618 cmꢀ1 upon coordination as previously observed for
other PHOX metal complexes.7 The general formulas [RuCl2(1)2]
and [RuCl2(2)2] were also established by HRMS and elemental
analysis.
For complexes of the general formula [RuCl2(PHOX)2], several
coordination geometries are possible,12 as displayed in Scheme 1.
The ligands can take cis or trans arrangements, and in addition,
the phosphorus and nitrogen donors of each of the two coordi-
nated ligands also can assume cis or trans positions. The number
of isomers in the isolated material can be easily assessed by
31P{1H} NMR, as different isomers give different chemical shifts
for the coordinated phosphorus atoms. Accordingly, we deter-
mined that the two complexes [RuCl2(1)2] and [RuCl2(2)2] were
isolated as single isomers, as they exhibited only one signal in
the 31P {1H} NMR. Furthermore, the 1H and 13C{1H} NMR spectra
The 31P{1H} NMR of the isolated material [RuCl2((R)-3)2] con-
tained two doublets and one singlet as the major signals; thus,
by analogy with complex [RuCl2(2)2], the two structures shown
in Scheme 1 might be the two isomers present in the isolated
material, which would be in accordance with the spectroscopic
data. The 31P{1H} NMR of the isolated material [RuCl2((R)-4)2]
contained three singlets as the major signals, which is spectroscop-
ically in best agreement with the three structures shown in

N. Curvey et al. / Tetrahedron Letters 55 (2014) 3033–3037
3035
similar to related complexes2d,g,13 and to [RuCl2(2)2]. The two dmso
ligands are coordinated through the sulfur atoms to the ruthenium
center, as previously reported for related ruthenium complexes.13
Currently we do not have a satisfactory explanation as to why
the two achiral PHOX ligands 1 and 2 give only one isomer in their
respective complexes and the chiral ligands (R)-3 and (R)-4 give a
mixture of isomers. We speculated that the steric congestion cre-
ated by the phenyl and benzyl substituents hampers the coordina-
tion of two ligands to the ruthenium center, making their
coordination more difficult. These findings are important for cata-
lyst systems formed in situ from a ruthenium source and bidentate
ligands, as coordination of two bidentate ligands to a ruthenium
center cannot readily be assumed.
Catalytic investigations with the ruthenium complex
[RuCl2((R)-3)2]
For further catalytic investigations, we decided to use complex
[RuCl2((R)-3)2], because albeit a mixture of isomers, we decided
to test for enantioselectivity in catalytic applications. The complex
[RuCl2((R)-3)2] itself did not exhibit catalytic activity in the title
reaction, presumably due to lack of an open coordination site. Thus,
the complex was activated by treatment with two equivalents
Table 1
Isolated yields.
Me3SiO
O
H
Ar
OSiMe3
O
syn
[Ru] (1 mol%)
+
H
Ar
Me3SiO
Ar
O
CH2Cl2, rt, 18 - 24 h
H
5
6
7
Entrya
Product
Isolated yield/%
Figure 1. Molecular Structure of [RuCl2(2)2] depicted with 50% probability ellip-
soids (top) and of [RuCl2((R)-3)(dmso)2] depicted with 30% probability ellipsoids
(bottom); H atoms are omitted for clarity. Key bond lengths (Å) and angles (°) for
[RuCl2(2)2]: Ru(1)-N(1), 2.1902(10); Ru(1)-P(1), 2.2794(3); Ru(1)-Cl(1) 2.4389(3);
N(1)-Ru(1)-N(2), 92.63(4); N(1)-Ru(1)-P(1), 80.60(3); P(1)-Ru(1)-P(2), 106.874(12);
(syn:anti)b
Me3SiO
O
H
85
1
(67:33)b
N(1)-Ru(1)-Cl(2),
87.80(3);
P(1)-Ru(1)-Cl(2),
92.787(12);
Cl(2)-Ru(1)-
Cl(1),177.842(11). For [RuCl2((R)-3)(dmso)2]: Ru(1)-N(1), 2.140(6); Ru(1)-S(1),
2.278(2); Ru(1)-P(1), 2.290(2); Ru(1)-Cl(1), 2.400(2); N(1)-Ru(1)-S(2), 174.72(17);
N(1)-Ru(1)-P(1), 82.30(18); S(1)-Ru(1)-P(1), 92.87(8), N(1)-Ru(1)-Cl(1), 89.30(16),
S(1)-Ru(1)-Cl(1), 172.18(9); Cl(1)-Ru(1)-Cl(2), 87.48(8).
Me3SiO
O
H
81
2
3
(78:22)b
MeO
Me3SiO
O
H
Scheme 1, which all would give individual singlets in the 31P{1H}
NMR.
88
(70:30)b
OMe
Heating of solutions of the mixture of isomers resulted in a
slight shift of the relative ratios (assessed by 31P{1H} NMR), as pre-
viously reported for related P,N-donating bidentate ligands,12 but
our efforts never resulted in bulk material that contained only
one isomer. Also, efforts to isolate a single isomer from the reaction
mixture by chromatography or fractional crystallization failed. In
the MS, fragments of the general formula [RuCl(PHOX)2]+ were ob-
served. The mixture of isomers isolated for the two complexes also
contained small amounts of incompletely substituted ruthenium
complexes bearing only one PHOX ligand, as reflected by the ele-
mental analyses for [RuCl2((R)-3)2] and [RuCl2((R)-4)2], which were
somewhat low on carbon.
During the synthesis of [RuCl2((R)-3)2], extensive attempts were
undertaken to obtain an X-ray structure, but only a single crystal
could be obtained from crude, incompletely substituted material,
which was analyzed by X-ray (Fig. 1, bottom). It turned out that
the crystal indeed contained a complex that can be described
as octahedral [RuCl2((R)-3)(dmso)2] with bond lengths and angles
Me3SiO
O
H
81
4
5
6
(69:31)b
O2N
Me3SiO
O
O
H
H
74c
81c
Me3SiO
a
General conditions: Aldehyde or ketone (0.32 mmol) and silyl enol ether
(1.28 mmol) in CH2Cl2 (1 mL) catalyzed by [RuCl2((R)-3)2] (0.0032 mmol, activated
with 0.0064 mmol AgSbF6) at room temperature for 18–24 h. The products were
isolated chromatographically.
Determined by 1H NMR of the isolated material through the JHH coupling
b
3
constant of the H–C–O proton (anti: ꢁ4.5 Hz; syn: ꢁ1 Hz).
c
Only the syn diastereomer was observed after isolation.

3036
N. Curvey et al. / Tetrahedron Letters 55 (2014) 3033–3037
AgSbF6 to selectively remove the chloro ligands. After filtration, the
activated complex was employed as catalyst in the Mukaiyama al-
dol reaction (Tables 1 and 2) when added to a solution of the silyl
and aldehyde substrates (room temperature in CH2Cl2, catalyst
loading 1 mol %, 18–24 h).
Further investigations to improve the performance of the catalyst
system described herein are planned.
Acknowledgments
As can be seen from the tables, the cyclic silyl enol ether 6 (Ta-
ble 1) as well as the silyl ketene acetal 8 (Table 2) were successfully
employed as nucleophiles to couple with a variety of electron-rich
and electron-poor aromatic aldehydes. The products were obtained
in 74% to 92% isolated yields after chromatographic workup. The
cyclic silyl enol ether substrate 6 generated two new stereocenters
during the reaction, and mostly syn/anti mixtures ranging around
70:30 were obtained, as determined by 1H NMR (Table 1).14 The
silyl ketene acetal 8 could also be coupled with cyclohexanone (Ta-
ble 2). Cyclohexanone is a ketone and, compared to the aldehyde
substrates in Scheme 1, is sterically more demanding.
Overall, the isolated yields in Tables 1 and 2 are comparable to
those obtained for other catalytic systems.15 The syn/anti ratios in
Table 1 are comparable to other Lewis acid catalyst systems known
for the Mukaiyama aldol reaction,16 albeit there are some catalysts
known that produce higher ratios.4c Also, no enantioselectivity was
observed in the products, as assessed by optical rotation measure-
ments. However, the relatively simple ruthenium complex
[RuCl2((R)-3)2] employed in the study can be tuned through mod-
ification of the ligands in order to improve performance. In general,
the system presented herein is one of the rare examples of ruthe-
nium-based catalysts to be employed in the title reaction, and a
catalyst load of only 1 mol % was employed. The ruthenium com-
plexes [RuCl2(PHOX)2] are easy to synthesize and stable under
ambient conditions. The catalytic experiments were performed
under ambient conditions, and air and water were not rigorously
excluded, which makes use of the catalyst system in synthetic
organic chemistry attractive.
We would like to thank the University of Missouri – St. Louis
College of Arts and Sciences – Research Award and the National
Science Foundation (NSF CHE-1300818) for financial support.
Further funding from the National Science Foundation for the
purchase of the NMR spectrometer (CHE-9974801), the purchase
of the ApexII diffractometer (MRI, CHE-0420497), and the purchase
of the mass spectrometer (CHE-9708640) is acknowledged.
Supplementary data
Supplementary data (crystallographic data, excluding structure
factors) for the structures in this Letter have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication CCDC 982742 ([RuCl2((R)-3)(dmso)2]) and CCDC
982743 ([RuCl2(2)2]). Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK, (fax: +44 (0)1223 336033 or e-mail: deposit@ccdc.cam.a-
c.uk). Experimental details and characterization data for the new
metal complexes and for the catalysis products in Tables 1 and 2,
1H and 13C{1H} NMR spectra for all new complexes and catalysis
products, experimental details and crystallographic data for the
X-ray determinations are available electronically. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.tetlet.2014.03.111.
References and notes
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ligands, one single isomer was isolated as assessed by NMR and
X-ray diffraction. Chiral PHOX ligands gave a mixture of isomers
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abstraction, the new ruthenium complex [RuCl2((R)-3)2] exhibited
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protected b-hydroxyl alcohols in 74% to 92% isolated yields (room
temperature, 18–24 h reaction time, 1 mol % catalyst loading).
Table 2
Isolated yields.
t
OSiMe2 Bu
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R
O
O
[Ru] (1 mol%)
+
OMe
R
H
OMe
18 - 24 h, rt, CH2Cl2
5
8
9
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O
Entrya
Product
Isolated yield/%
R
H
tBuMe2SiO
tBuMe2SiO
O
O
O
1
2
3
85
92
82
H
H
OMe
OMe
O
O
O
OMe
tBuMe2SiO
a
General conditions: Aldehyde or ketone (0.206 mmol) and silyl enol ether
(0.412 mmol) in CH2Cl2 (1 mL) catalyzed by [RuCl2((R)-3)2] (0.0021 mmol, activated
with 0.0041 mmol AgSbF6), at room temperature for 18–24 h. The products were
isolated by column chromatography.

N. Curvey et al. / Tetrahedron Letters 55 (2014) 3033–3037
3037
3
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