Synthesis, coordination behavior, and structural features of chiral amino-, pyrazolyl-, and phosphino-substituted ferrocenyloxazolines and their application in asymmetric hydrogenations

Article abstract of DOI:10.1021/om200557c

A highly flexible and modular synthesis of 15 chiral nonracemic ferrocenyloxazolines-all based on a ferrocenylethyl backbone-is described. Starting from N,N-dimethylaminoethyl ferrocene a range of oxazoline derivatives with various oxazoline substituents were prepared in a three-step sequence. After reaction with methyl iodide, the quaternized dimethylamino group was replaced by different amino, pyrazolyl, and phosphinyl groups. Subsequent reduction of the phosphinyl groups gave phosphinooxazoline derivatives. The molecular structures of three palladium [PdCl₂(L)] and three ruthenium [RuCl(p-cymene)(L)]PF₆ complexes of representative ferrocenyloxazoline ligands were studied by X-ray diffraction. In addition, five ruthenium complexes [RuCl₂(PPh₃)(L)] were prepared. These complexes were tested, using a high-throughput screening system, in the asymmetric hydrogenation of three ketones. Enantioselectivities of up to 99% ee were obtained.

Full text of DOI:10.1021/om200557c

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Coordination Behavior, and Structural Features of Chiral
Amino-, Pyrazolyl-, and Phosphino-Substituted Ferrocenyloxazolines
and Their Application in Asymmetric Hydrogenations
Raffael Schuecker,† Afrooz Zirakzadeh,† Kurt Mereiter,‡ Felix Spindler,§ and Walter Weissensteiner*,†
†Institute of Organic Chemistry, University of Vienna, W€ahringer Strasse 38, A-1090 Wien, Austria
‡Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna, Austria
§Catalysis Research, Solvias AG, P.O. Box, CH-4002 Basel, Switzerland
S Supporting Information
b
ABSTRACT: A highly flexible and modular synthesis of 15
chiral nonracemic ferrocenyloxazolines—all based on a ferro-
cenylethyl backbone—is described. Starting from N,N-di-
methylaminoethyl ferrocene a range of oxazoline derivatives
with various oxazoline substituents were prepared in a three-
step sequence. After reaction with methyl iodide, the quater-
nized dimethylamino group was replaced by different amino,
pyrazolyl, and phosphinyl groups. Subsequent reduction of the phosphinyl groups gave phosphinooxazoline derivatives. The
molecular structures of three palladium [PdCl2(L)] and three ruthenium [RuCl(p-cymene)(L)]PF6 complexes of representative
ferrocenyloxazoline ligands were studied by X-ray diffraction. In addition, five ruthenium complexes [RuCl2(PPh3)(L)] were
prepared. These complexes were tested, using a high-throughput screening system, in the asymmetric hydrogenation of
three ketones. Enantioselectivities of up to 99% ee were obtained.
Chart 1
’ INTRODUCTION
For more than three decades chiral ferrocene derivatives have
been employed as ligands for asymmetric catalysts,1 and at
present they are even used in large-scale industrial processes.2
Ferrocenyldiphosphines such as Josiphos-type ligands are cer-
tainly the most prominent examples (Chart 1).3 In addition to a
variety of diphosphines, oxazoline-based ferrocene derivatives
have also been successfully used in a huge number of asymmetric
transformations.4 In 1995 several groups reported independently
on the synthesis of 1-phosphino-2-oxazolinyl-substituted ferro-
cenes5 (Chart 1) and their 1,2,10,20-tetrasubstituted analogues.6
Since that time these so-called FOXAP ligands have found
applications in enantioselective hydrogenations,7 transfer
hydrogenations,7k,8 hydrosilylations,5d,7k,9 cross-coupling reac-
tions,5b,10 Heck reactions,11 alkylations of aldehydes,12 allylic
substitutions,7k,10c,13 and many other transformations.14 The
success of this class of ligand has initiated additional attempts
to develop ferrocenyloxazolines with alternative substitution
patterns, with alternative combinations of functional groups, or
with ligands that have more extended backbones such as
biferrocene,15 ferrocenylmethyl,16 ferrocenylethyl,17 or related
units.18
the structural features of both classes of ligands and, by making
use of the ease and high modularity of both synthesis routes,
attempting to prepare a new class of oxazoline ligand.
In continuation of our previous research in this field,20 in this
contribution we report on a very general and modular synthesis
of a novel class of oxazoline ligand, all of which are based on a
ferrocenylethyl backbone with differently substituted oxazoline
units attached directly to the ferrocene core and additional
amino, pyrazolyl, or phosphino groups connected to the ethyl
side chain. Furthermore, the synthesis of a number of palladium
and ruthenium complexes is described along with some of their
structural features and applications of selected ruthenium com-
plexes in the asymmetric hydrogenation of ketones.
Both classes of compound, i.e., Josiphos-type19 and FOXAP-
type ligands,4e can be prepared in a very straightforward and
modular manner. In both cases the phosphino and the oxazoline
substituents can be varied easily and independently (Scheme 1),
and this feature has given rise to a large number of representatives
of both ligand groups. In this context we considered combining
Received: June 28, 2011
Published: August 05, 2011
r
2011 American Chemical Society
4711
dx.doi.org/10.1021/om200557c Organometallics 2011, 30, 4711–4719
|

Organometallics
ARTICLE
’ RESULTS AND DISCUSSION
Appel23 to oxazolines (R,SFc)-6, (4S,10R,SFc)-7, (4R,10R,SFc)-7,
(4S,10R,SFc)-8, and (4R,10R,SFc)-8. It is clear that the substitution
pattern of the oxazoline unit can be determined by choosing the
appropriate amino alcohol in step 2.
Ligand Synthesis. Starting from Ugi’s amine, (R)-1,21 five
oxazoline derivatives with different substituents R were prepared
in a three-step sequence (Scheme 2). In the first step the ethyl
ester (R,SFc)-2 was obtained from (R)-1 by a diastereoselective
ortho-lithiation with t-BuLi and subsequent reaction with diethyl
carbonate.22 In the second step ester 2 was reacted in the
presence of trimethylaluminum10f with five differently substi-
tuted amino alcohols to give amides (R,SFc)-3, (1S,10R,SFc)-4,
(1R,10R,SFc)-4, (1S,10R,SFc)-5, and (1R,10R,SFc)-5, which in the
third step could be transformed according to the methodology of
Methods for the replacement of the dimethylamino group
by other nucleophiles were subsequently explored by study-
ing oxazoline 6. It was found that the most widely used
methodology—quaternization of the amino nitrogen with meth-
yl iodide followed by nucleophilic substitution—worked best.21b
Therefore, 6 was treated with methyl iodide in acetonitrile, and
the crude ammonium salt was subsequently reacted with either
diisopropylamine, pyrrolidine, piperidine, pyrazole, or 3,5-di-
methylpyrazole to give the amino- and pyrazolyl-substituted
ferrocenyloxazoline derivatives (R,SFc)-9ꢀ13.
Scheme 1. Synthesis Routes for Josiphos- and FOXAP-Type
Ligands
Furthermore, the ammonium salt of 6 could be transformed
with diphenylphosphinyllithium to the phosphinyl-substituted
derivative (R,RFc)-14.24 Reduction of this phosphine oxide with
polymethylhydrosiloxane in the presence of titanium isopro-
poxide25 gave the phosphinooxazoline (R,RFc)-17. On the basis
of this reaction sequence the aminooxazolines 7 and 8 were also
transformed into phosphineoxides (4S,10R,RFc)-15, (4R,10R,
RFc)-15, (4S,10R,RFc)-16, and (4R,10R,RFc)-16 and subsequently
to the phosphinooxazolines (4S,10R,RFc)-18, (4R,10R,RFc)-18,
(4S,10R,RFc)-19, and (4R,10R,RFc)-19.
In summary, this highly flexible reaction sequence can be used
to access amino- and pyrazolyl-substituted oxazolines from Ugi’s
amine in four steps, while for the preparation of phosphinooxazolines
Scheme 2
4712
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
Scheme 3. Preparation of Palladium and Ruthenium Complexes 20ꢀ27
Figure 1. Molecular structures of the complexes [PdCl2(L)] (H-atoms omitted for clarity; assignment of absolute configuration: 4 refers to
configuration at C14, 10 to C11).
five steps are needed. Selection of the appropriate amino alcohol in
the second step determines the oxazoline substitution pattern, while
in the fourth step a variety of nucleophiles can be attached to the ethyl
side chain.
of oxazolines (R,SFc)-6, (R,SFc)-12, (4S,10R,RFc)-19, and
(4R,10R,RFc)-19 and ruthenium p-cymene complexes [RuCl(p-
cymene)(L)]PF6 of ligands (R,SFc)-12, (4S,10R,RFc)-19, and
(4R,10R,RFc)-19 were prepared (Scheme 3). The palladium
complexes 20, 21, (SOx)-22, and (ROx)-22 were obtained by
reacting the appropriate ligands (6, 12, 19) with [PdCl2-
(CH3CN)2], while reaction of ligands 12 and 19 with the
ruthenium dimer [RuCl2(p-cymene)]2 and subsequent treat-
ment of the reaction mixture with ammonium hexafluoropho-
sphate gave the cationic ruthenium complexes 23, (SOx)-24, and
(ROx)-24. It should be mentioned that in the latter reaction for
each ligand two diastereomeric complexes with different config-
urations at ruthenium might be expected. In fact, for all three
ligands only one diastereomer was isolated.
From the three palladium complexes 20, (SOx)-22, and (ROx)-
22 [PdCl2(L)] [L = (R,SFc)-6, (4S,10R,RFc)-19, (4R,10R,RFc)-
19] and the three ruthenium complexes 23, (SOx)-24, and (ROx)-
24 [RuCl(p-cymene)(L)]PF6 [L = (R,SFc)-12, (4S,10R,RFc)-19,
(4R,10R,RFc)-19] suitable single crystals were obtained, and their
molecular structures in the solid state were studied by X-ray
Synthesis and Structural Features of Pd and Ru Com-
plexes of Ligands (R,SFc)-6, (R,SFc)-12, (4S,10R,RFc)-19, and
(4R,10R,RFc)-19. Ferrocenyloxazoline derivatives have frequently
been used as ligands in enantioselective transition metal catalysts.
For this purpose the metals Pd and Ru are mainly used (to a
lesser extent Rh and Ir have been applied),4 but coordination
compounds of other metals such as Os,26 Pt,27 Cu,28 Au,29 Zn,30
and Hg31 have also been explored. Palladium complexes of
ferrocenyloxazolines have been used as catalyst precursors
for a number of reactions including allylic alkylations,7k,10c,13
Heck reactions,11 aza Claisen rearrangements,32 Grignard cross-
coupling,5b,10 and Suzuki reactions,33 while ruthenium com-
plexes have found applications in hydrogenations,7 hydrogen
transfer,7k,8 and hydrosilylation reactions.5d,7k,9
In order to study the complexation behavior of our newly
synthesized ligands, palladium dichloride complexes [PdCl2(L)]
4713
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
Figure 2. Molecular structures of complexes [RuCl(p-cymene)(L)]PF6 (H-atoms omitted for clarity; assignment of absolute configuration: 4 refers to
configuration at C14, 10 to C11).
diffraction. Details of these X-ray crystallography studies are
given in Table S1 of the Experimental Section (see Supporting
Information). The absolute configuration of each compound was
determined from the X-ray anomalous dispersion effects and was
consistent with the chemical evidence. Views of the molecular
structures of these compounds are shown in Figures 1 and 2, and
selected geometrical data are given in Tables 1 and 2.
In all complexes the ligands are bidentate, forming square-
planar palladium dichloride complexes and three-legged piano
stool-type ruthenium complexes. All palladium complexes co-
crystallized with one or two equivalents of CHCl3, which in each
the phosphino-phosphorus shows a stronger trans-influence than
the oxazoline nitrogen. Interestingly, the arrangements of the
respective oxazoline nitrogen N1 and atoms directly connected
to it (Pd1, C13, C14) deviate significantly from planarity, a trend
that is especially pronounced in complex (SOx)-22 (N 0.228 Å
above the plane defined by Pd1, C13, C14). In complexes 20 and
(ROx)-22 the PdCl2 unit is located above the ferrocene Cp1 ring
(exo position), an arrangement that places the side chain methyl
carbon C12 below Cp1 (Cp1 = C1 through C5, Cp2 = C6 through
C10).
In the palladium complexes of ligand 19 gross overall structur-
al changes are observed on changing the configuration at the
oxazoline position 4 from R to S. In complex (ROx)-22 the
palladium dichloride unit is located above Cp1 (exo arrange-
ment), whereas in complex (SOx)-22 this unit is found below Cp1
(endo arrangement). This change also results in a reorientation of
the ethyl side chain.
case show close contacts (nonclassical CꢀH Cl bonds) with
3 3 3
one or both chlorides of the palladium dichloride unit. The atoms
coordinated to palladium form an approximately square-planar
arrangement that deviates only slightly from planarity (rms
deviations from planarity between 0.008 and 0.064 Å). The
NꢀPdꢀN or NꢀPdꢀP bond angles are within a narrow range
of 93.9ꢀ94.9° (Table 1). As expected, in complex 20 the trans-
influence caused by the oxazoline nitrogen is stronger than that of
the amino nitrogen, while in the case of the phosphino-oxazolines
In all piano stool-type ruthenium half-sandwich complexes 23,
(SOx)-24 and (ROx)-24, [RuCl(p-cymene)(L)]PF6 [L = (R,SFc)-
12, (4S,10R,RFc)-19, and (4R,10R,RFc)-19], the coordinated
4714
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
Table 1. Geometric Parameters of Complexes [PdCl2(L)]
complex 20 [L = (R,SFc)-6]
complex (SOx)-22 [L = (4S,10R,RFc)-19]
complex (ROx)-22 [L = (4R,10R,RFc)-19]
Distances (Å)
2.372
PdꢀCl(trans-N(CH3)2), (tr-P)
PdꢀCl(tr. Ox)
2.299
2.378
2.310
2.281
2.291
PdꢀN(N(CH3)2), (P)
PdꢀN(Ox)
2.138
2.241
2.234
2.027
2.045
2.044
ClꢀHCCl3
(tr-Ox) 2.48
(tr-P) 2.48/2.94
(tr-P) 2.65/2.95
(tr-P) 2.75/3.07
Bond Angles (deg)
NꢀPdꢀN, NꢀPdꢀP
ClꢀPdꢀCl
94.2
87.5
94.9
89.3
93.9
90.0
Tilt Angle (deg)
4.4
Cp1/Cp2
1.8
3.8
Normal Distance from Mean Plane (Å)
Cp1ꢀCHCH3
ꢀ0.783
ꢀ0.063
1.327
1.829
ꢀ0.682
ꢀ0.052
1.645
Cp1ꢀCHCH3
0.376
Cp1ꢀN(CH3)2, ꢀPPh2
Cp1ꢀN(Ox)
ꢀ0.786
ꢀ0.262
ꢀ1.820
ꢀ3.602
ꢀ3.014
0.228
0.633
0.705
Cp1ꢀPd
2.332
2.508
Cp1ꢀCl(tr. Ox)
4.375
4.578
Cp1ꢀCl(tr. N(CH3)2), (tr-P)
N(Ox) pl of Pd, NCH2, CdN
3.429
3.456
ꢀ0.158
ꢀ0.150
Table 2. Geometric Parameters of Complexes [RuCl(p-cymene)(L)]PF6
complex (SOx)-24 [L = (4S,10R,RFc)-19]
complex 23 [L = (R,SFc)-12]
molecule 1
molecule 2
complex (ROx)-24 [L = (4R,10R,RFc)-19]
Distance (Å)
2.40
RuꢀCl
2.392
2.117
2.161
3.269
5.780
2.40
2.37
2.14
3.30
5.89
2.379
2.349
2.143
3.415
5.623
RuꢀN(pz), ꢀPPh2
RuꢀN(Ox)
ClꢀCH3(Cy)
ClꢀCH(Cy)
2.41
2.11
5.86
3.32
Angles (deg)
92.7
NꢀRuꢀN, NꢀRuꢀP
ClꢀRuꢀN(Py), ClꢀRuꢀP
ClꢀRuꢀN(Ox)
91.4
84.4
82.3
96.9
82.3
80.5
94.4
85.6
82.5
84.7
82.0
Tilt Angle (deg)
Cp1/Cp2
3.7
1.8
2.7
1.8
Cp1/cymene
105.2
26.6
19.7
24.2
Normal Distance from Mean Plane (Å)
Cp1ꢀCHCH3
ꢀ0.509
0.133
ꢀ0.47
0.11
ꢀ0.50
0.06
1.79
0.92
2.47
0.69
0.05
ꢀ0.376
0.042
Cp1ꢀCHCH3
Cp1ꢀN(Py), ꢀPPh2
Cp1ꢀN(Ox)
2.199
1.87
1.767
0.209
0.92
0.458
Cp1ꢀRu
1.264
2.81
2.273
Cp1ꢀCl
3.002
2.16
0.892
N(Ox) pl of Ru, NCH2, CdN
ꢀ0.074
ꢀ0.06
ꢀ0.135
ruthenium and chloride Cl1 are positioned above Cp1, while the
side-chain methyl carbon C12 is located below Cp1. Interestingly,
reaction of the dimer [RuCl2(p-cymene)]2 with ligand (R,SFc)-12
and NH4PF6 leads to the formation of complex 23 with the
4715
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
Table 3. Results Obtained in the Hydrogenation of Ketones
with Complexes 25ꢀ27 [RuCl2(PPh3)(17ꢀ19)]a
[RuCl2
%
abs
entry substrate (PPh3)(L)]
solvent
base conv eeb confc
1
2
3
4
5
6
7
8
9
ACP
ACP
ACP
ACP
ACP
ACP
ACP
ACP
ACP
25
toluene, H2O K2CO3
99 95
99 99
99 98
99 44
99 49
99 94
99 97
99 96
99 43
99 47
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
S
S
S
R
R
Figure 3. Ketones used as substrates in catalytic hydrogenations.
(ROx)-26 toluene, H2O K2CO3
(ROx)-27 toluene, H2O K2CO3
(SOx)-26 toluene, H2O K2CO3
(SOx)-27 toluene, H2O K2CO3
R configuration at ruthenium, while with both ligands 19 com-
plexes with the S configuration at ruthenium were obtained.
In complexes (RRu)-[RuCl(p-cymene)((R,SFc)-12)]PF6 (23)
and (SRu)-[RuCl(p-cymene)((4R,10R,RFc)-19)]PF6 ((ROx)-24)
the p-cymene units are oriented in such a way that their
respective methyl carbon (C28 and C43) is positioned in proxi-
mity to chloride Cl1. In contrast to the palladium and ruthenium
complexes described so far, complex (SRu)-[RuCl(p-cymene)-
((4S,10R,RFc)-19)]PF6 ((SOx)-24) crystallizes with two mole-
cules in the asymmetric unit, and these differ mainly in the
orientation of their p-cymene units. In one molecule the
p-cymene methyl carbon C43 is located close to chloride Cl1,
while in the second molecule the p-cymene unit is rotated around
the Ruꢀarene bond by approximately 180°, which brings the
isopropyl methine carbon C400 in proximity to Cl10.
Superposition of the molecular structures of complexes (ROx)-
24 with both molecules of (SOx)-24 showed that, unlike the case
of the analogous palladium complexes, a change in the config-
uration of the oxazoline carbon 4 from 4R to 4S does not cause
significant structural changes to the molecular backbone.
Synthesis of Neutral, Five-Coordinate Ruthenium Com-
plexes of Ligands 17ꢀ19 and Their Use in Catalytic Asym-
metric Hydrogenations of Ketones. Reaction of phosphino-
oxazoline ligands 17ꢀ19 with [RuCl2(PPh3)3] in toluene at rt
led to the isolation of green complexes 25ꢀ27 of molecular
formula [RuCl2(PPh3)(L)] (Scheme 3). These complexes were
screened in catalytic hydrogenations of three ketones using a
customized Symyx high-throughput screening system (for sub-
strates see Figure 3). Typical results are listed in Table 3 (for
detailed reaction conditions see Supporting Information).
All hydrogenations were carried out with isolated and fully
characterized catalyst precursors. Reactions were run under
hydrogen gas at a pressure of 25 bar, at a substrate to catalyst
ratio of 25:1, and at a temperature of 25 °C. Different combina-
tions of solvents and bases were tested. Either 2-propanol in
combination with KOt-Bu or toluene/water (9:1) with base
K2CO3 or NaOH was applied. The former solvent/base system is
typically used in transfer hydrogenation reactions,34 while the
latter conditions were found to give excellent results in the
hydrogenation of ketones using ruthenium-FOXAP-based cata-
lysts, even on a large industrial scale.7h
25
toluene, H2O NaOH
(ROx)-26 toluene, H2O NaOH
(ROx)-27 toluene, H2O NaOH
(SOx)-26 toluene, H2O NaOH
(SOx)-27 toluene, H2O NaOH
10 ACP
11 ACP
12 ACP
13 ACP
14 ACP
15 ACP
25
i-PrOH
KOt-Bu 96 75
KOt-Bu 95 94
(ROx)-26 i-PrOH
(ROx)-27 i-PrOH
(SOx)-26 i-PrOH
(SOx)-27 i-PrOH
KOt-Bu 94 89
KOt-Bu 95 58
KOt-Bu 95 62
16 4-Me-ACP 25
toluene, H2O K2CO3 100 88
17 4-Me-ACP (ROx)-26 toluene, H2O K2CO3 100 97
18 4-Me-ACP (ROx)-27 toluene, H2O K2CO3 100 97
19 4-Me-ACP (SOx)-26 toluene, H2O K2CO3
20 4-Me-ACP (SOx)-27 toluene, H2O K2CO3
88 20
99 28
21 4-Me-ACP 25
toluene, H2O NaOH 100 80
22 4-Me-ACP (ROx)-26 toluene, H2O NaOH
23 4-Me-ACP (ROx)-27 toluene, H2O NaOH
24 4-Me-ACP (SOx)-26 toluene, H2O NaOH
25 4-Me-ACP (SOx)-27 toluene, H2O NaOH
99 95
99 90
99 20
99 27
26 4-Me-ACP 25
i-PrOH
KOt-Bu 99 56
KOt-Bu 98 88
KOt-Bu 100 79
KOt-Bu 98 35
KOt-Bu 99 49
27 4-Me-ACP (ROx)-26 i-PrOH
28 4-Me-ACP (ROx)-27 i-PrOH
29 4-Me-ACP (SOx)-26 i-PrOH
30 4-Me-ACP (SOx)-27 i-PrOH
31 PBK
32 PBK
33 PBK
34 PBK
35 PBK
36 PBK
37 PBK
38 PBK
39 PBK
40 PBK
25
toluene, H2O NaOH
59 68
74 85
61 85
34 50
98 50
(ROx)-26 toluene, H2O NaOH
(ROx)-27 toluene, H2O NaOH
(SOx)-26 toluene, H2O NaOH
(SOx)-27 toluene, H2O NaOH
25
i-PrOH
KOt-Bu 100 74
KOt-Bu 100 99
KOt-Bu 100 93
KOt-Bu 100 38
KOt-Bu 100 45
(ROx)-26 i-PrOH
(ROx)-27 i-PrOH
(SOx)-26 i-PrOH
(SOx)-27 i-PrOH
a Reaction conditions: 41.67 μmol of substrate in 500 μL of solvent,
1.67 μmol of catalyst precursor (S/C = 25:1); p(H2) 25 bar, T 25 °C,
reaction time 16 h. b Determined by ACP: GC, Supelco ChiralDex 110;
4-Me-ACP: HPLC, Daicel Chiralcel OB-H; PBK: HPLC, Daicel
Chiralcel OD-H. ACP: GC of authentic sample; 4-Me-ACP: HPLC
(ref 35); PBK: HPLC of authentic sample.
For all ketones used as substrates [acetophenone (ACP),
4-methylacetophenone (4-Me-ACP), and phenyl benzyl ketone
(PBK)] catalyst systems and reaction conditions were identified
that transformed the substrates with quantitative or nearly
quantitative conversion into products with an enantiomeric
excess ranging from 97% ee (4-Me-ACP) to 99% ee (ACP and
PBK).
c
On using acetophenone as the substrate, all solvent/base
systems tested gave excellent results (Table 3, entries 1ꢀ3,
6ꢀ8, 12), with the toluene/H2O/K2CO3 system performing
slightly better than the toluene/H2O/NaOH system (99%
versus 97% ee; entries 2 and 7). The best conditions for
the hydrogenation of 4-Me-ACP also involved the use of
toluene/H2O. As with acetophenone the influence of the base
was found to be rather small (97% versus 95% ee; entries 17 and
22). Only in the case of phenyl benzyl ketone did the i-PrOH/
KOt-Bu system prove to be superior to the toluene/H2O system
(entries 37 and 32), giving quantitative conversion and a product
enantiomeric excess of 99% ee (entry 37).
4716
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
As one might expect, the product enantioselectivity depends
strongly on the ligand substitution pattern as well as on the
relative configuration. Ligands 17, 18, and 19 differ only in their
oxazoline unit in that the oxazoline unit of ligand 17 is unsub-
stituted, whereas ligands 18 bear an isopropyl and ligands 19 a
phenyl group at the oxazoline position 4. For each type of ligand
(18 and 19) two diastereomers with either an R or S configura-
tion at the oxazoline carbon 4 were tested [e.g., (4R,10R,RFc)-18
and (4S,10R,RFc)-18].
excellent enantiomeric excess [ACP: 99% ee (entry 2), 4-Me-
ACP: 97% ee (entry 17), PBK: 99% ee (entry 37)]. In all cases
the highest enantioselectivities were obtained with the isopropyl-
substituted ligand (4R,10R,RFc)-18. Replacement of the isopro-
pyl with a phenyl substituent [(4R,10R,RFc)-19] resulted in either
the same or only slightly lower product enantiomeric excess,
while the use of ligands with an S-configured oxazoline unit led
not only to a significant drop in the product enantiomeric excess
but also to a change in the product absolute configuration. It is
worth mentioning that a change in the oxazoline substituent from
isopropyl to phenyl resulted in less marked changes in the
hydrogenation results than a change in the absolute configura-
tion at the oxazoline carbon 4.
With respect to the ligand substitution pattern and relative
configuration, the results obtained in the hydrogenation test
reactions show highly consistent trends. For all three substrates
tested the highest enantioselectivities were obtained with the
isopropyl-substituted ligand (4R,10R,RFc)-18 with an R config-
uration at the oxazoline unit (entries 2, 17, 37). The phenyl-
substituted ligand (4R,10R,RFc)-19, which also has an R config-
uration at the oxazoline unit, gave identical conversions, and in
this case the product enantiomeric excess either was identical
(4-Me-ACP, entries 17 and 18) or only dropped to a very small
extent (e.g., 99% versus 98% ee; entries 2 and 3). However, the
corresponding S-configured ligands (4S,10R,RFc)-18 and (4S,10R,
RFc)-19 led not only to a significant drop in the product
enantiomeric excess but also to a change in the product absolute
configuration as compared to ligands with an R configuration
(e.g., entries 2 and 3 versus 4 and 5; 17/18 versus 19/20; 37/38
versus 39/40). Surprisingly, on using acetophenone as the
substrate, the unsubstituted oxazoline ligand (R,RFc)-17 also
gave excellent results (99% conversion and 95% ee; entry 1).
As mentioned above, when ligands with R configuration at the
oxazoline unit and R configuration at ferrocene were used [e.g.,
ligand (4R,10R,RFc)-18 in the form of complex (ROx)-26], all
three substrates (ACP, 4-Me-ACP, and PBK) were transformed
into product alcohols with an S absolute configuration (Table 3).
The same dependence of ligand and product configuration
was reported for the original FOXAP ligands (Chart 1).8b
For example, when acetophenone was hydrogenated under
comparable conditions with the use of complex [RuCl2(PPh3)-
((S,SFc)-A)] [ligand A: S configuration at the oxazoline unit and
S configuration at ferrocene], a product with an R absolute
configuration was obtained. These observations suggest for the
hydrogenation of acetophenone and similar substrates that for
both ligands of type A and ligands of type 18 the combination of
S configuration at the oxazoline unit and S configuration at
ferrocene induces an R absolute configuration of the product.
As already pointed out, changing the configuration of ligands
18 and 19 at the oxazoline unit from R to S while leaving the R
ferrocene configuration identical [e.g., (4R,10R,RFc)-18 f
(4S,10R,RFc)-18; complexes (ROx)-26 f (SOx)-26, Table 3,
entries 2/4] leads to a change of product configuration from S
to R. A similar behavior has been described for FOXAP ligand
(S,SFc)-B and its diastereomer (S,RFc)-B (Chart 1).7i In the iridium-
catalyzed enantioselective hydrogenation of 2-methylquinoline
both diastereomers of B led to product of the same absolute
configuration. As a consequence, like for ligands 18 and 19, a
change of the oxazoline configuration of ligand B [(S,SFc)-B f
(R,SFc)-B] results in a change of product configuration. It is
obvious that with respect to enantioselective induction in the
hydrogenation of acetophenone and similar substrates FOXAP
ligands and our newly developed analogues act in a highly
comparable manner.
’ SUMMARY
A very general and highly modular synthesis of ferrocenyl
oxazolines, all of which are based on a ferrocenylethyl backbone,
has been developed. Starting from N,N-dimethylaminoethyl
ferrocene (Ugi’s amine) differently substituted ferrocenyloxazo-
lines were built up in a three-step sequence. After quaternization
of the dimethylamino nitrogen the former dimethylamino group
could be replaced by a variety of nucleophiles bearing amino,
pyrazolyl, or phosphino substituents. The synthetic route not
only allows the easy variation of the type of side-chain substit-
uents but also provides easy access to differently substituted
oxazoline units. Three palladium and three ruthenium complexes
of representative amino-, pyrazolyl-, and phosphino-substituted
ligands were studied by X-ray diffraction. A comparison of the
molecular structures of palladium dichloride complexes of dia-
stereomeric phosphinooxazoline ligands [(4S,10R,RFc)-19 and
(4R,10R,RFc)-19] revealed that changing the configuration at the
oxazoline unit at position 4 leads to significant overall changes of
the complex geometry in the solid state. In the ruthenium arene
complexes, besides the position of the oxazoline substituent, such
gross overall changes were not observed.
Five-coordinate, neutral Ru(II) complexes [RuCl2(PPh3)(L)]
of five phosphineoxazoline ligands were prepared and tested
using a high-throughput screening system in the hydrogenation
of three ketones. Three ligands delivered products with quanti-
tative conversion and with excellent enantiomeric excess of
97ꢀ99% ee. The product enantiomeric excess and the product
absolute configuration were found to be highly dependent on the
relative configuration of the ligands used.
At present applications of the newly developed ligands in
ruthenium-catalyzed transfer hydrogenation and hydrogenation
of ketones are being explored.
’ ASSOCIATED CONTENT
S
Supporting Information. Text giving full experimental
b
descriptions and spectroscopic data for all newly synthesized
ligands and complexes. Crystallographic and geometric data
for three palladium and three ruthenium complexes of the
type [PdCl2(L)] and [RuCl(p-cymene)(L)]PF6, respectively,
are given in the supporting text and in CIF format. This mate-
rial is available free of charge via the Internet at http://pubs.acs.
org.
’ AUTHOR INFORMATION
In summary, for all three substrates tested, reaction conditions
could be identified that gave products in high conversion and
Corresponding Author
*E-mail: Walter.Weissensteiner@univie.ac.at.
4717
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
’ ACKNOWLEDGMENT
Hidai, M.; Uemura, S. J. Organomet. Chem. 1997, 545ꢀ546, 381.
(e) Nishibayashi, Y.; Segawa, K.; Takada, H.; Ohe, K.; Uemura, S.
Chem. Commun. 1996, 847.
The authors thank the Austrian Science foundation FWF
(project P23376-N19) as well as Solvias AG for their strong
support of this work and UMICORE for a generous gift of metal
complexes.
(10) (a) Shen, Q.; Ogata, T.; Hartwig, J. F. J. Am. Chem. Soc. 2008,
130, 6586. (b) Adams, H.; Bawa, R. A.; McMillan, K. G.; Jones, S.
Tetrahedron: Asymmetry 2007, 18, 1003. (c) Manoury, E.; Fossey, J. S.;
Aiet-Haddou, H.; Daran, J. C.; Balavoine, G. G. A. Organometallics 2000,
19, 3736. (d) Chung, K. G.; Miyake, Y.; Uemura, S. J. Chem. Soc., Perkin
Trans. 1 2000, 2725. (e) Chung, K. G.; Miyake, Y.; Uemura, S. J. Chem.
Soc., Perkin Trans. 1 2000, 15. (f) Ahn, K. H.; Cho, C. W.; Baek, H. H.;
Park, J.; Lee, S. J. Org. Chem. 1996, 61, 4937.
’ REFERENCES
(1) Ferrocenes: Ligands, Materials and Biomolecules; Stepnicka, P.,
Ed.; Wiley: Chichester, 2008.
(11) (a) Kilroy, T. G.; Malone, Y. M.; Guiry, P. J. Catal. Fine Chem.
2004, 3, 104. (b) Kiely, D.; Guiry, P. J. J. Organomet. Chem. 2003,
687, 545. (c) Tu, T.; Hou, X. L.; Dai, L. X. Org. Lett. 2003, 5, 3651. (d)
Kiely, D.; Guiry, P. J. Tetrahedron Lett. 2003, 44, 7377. (e) Tu, T.; Deng,
W. P.; Hou, X. L.; Dai, L. X.; Dong, X. C. Chem.—Eur. J. 2003, 9, 3073.
(f) Kilroy, T. G.; Hennessy, A. J.; Connolly, D. J.; Malone, Y. M.; Farrell,
A.; Guiry, P. J. J. Mol. Catal. A: Chem. 2003, 196, 65. (g) Kiely, D.; Guiry,
P. J. Tetrahedron Lett. 2002, 43, 9545. (h) Hennessy, A. J.; Connolly,
D. J.; Malone, Y. M.; Guiry, P. J. Tetrahedron Lett. 2000, 41, 7757. (i)
Deng, W. P.; Hou, X. L.; Dai, L. X.; Dong, X. W. Chem. Commun.
2000, 1483. (j) Hennessy, A. J.; Malone, Y. M.; Guiry, P. J. Tetrahedron
Lett. 2000, 41, 2261. (k) Hennessy, A. J.; Malone, Y. M.; Guiry, P. J.
Tetrahedron Lett. 1999, 40, 9163.
(2) Asymmetric Catalysis on Industrial Scale: Challenges, Approaches
and Solutions; Blaser, H. U.; Schmid, E., Eds.; Wiley-VCH: Weinheim,
2004.
(3) (a) Phosphorus Ligands in Asymmetric Catalysis; B€orner, A., Ed.;
Wiley-VCH: Weinheim, 2008. (b) Handbook of Homogeneous Hydro-
genation; de Vries, J. G.; Elsevier., C. J., Eds.; Wiley-VCH: Weinheim,
2007. (c) Zhang, W.; Chi, Y.; Zhang, X. Acc. Chem. Res. 2007, 40, 1278.
(d) Li, Y. M.; Kwong, F. Y.; Yu, W. Y.; Chan, A. S. C. Coord. Chem. Rev.
2007, 251, 2119. (e) Gomez Arrayꢀas, R.; Adrio, J.; Carretero, J. C. Angew.
Chem., Int. Ed. 2006, 45, 7674. (f) Barbaro, P.; Bianchini, C.;
Giambastiani, G.; Parisel, S. L. Coord. Chem. Rev. 2004, 248, 2131. (g)
Atkinson, R. C. J.; Gibson, V. C.; Long, N. J. Chem. Soc. Rev. 2004,
33, 313. (h) Colacot, T. J. Chem. Rev. 2003, 103, 3101. (i) Blaser, H. U.;
Brieden, W.; Pugin, B.; Spindler, F.; Studer, M.; Togni, A. Top. Catal.
2002, 19, 3.
(12) Garabatos-Perera, J. R.; Butensch€on, H. J. Organomet. Chem.
2009, 694, 2047.
(13) (a) Zhao, X.; Liu, D.; Xie, F.; Zhang, W. Tetrahedron 2009,
65, 512. (b) Liu, D.; Xie, F.; Zhang, W. Tetrahedron Lett. 2007, 48, 7591.
(c) Geisler, F. M.; Helmchen, G. J. Org. Chem. 2006, 71, 2486. (d) Tu,
T.; Hou, X. L.; Dai, L. X. J. Organomet. Chem. 2004, 689, 847. (e) You,
S. L.; Hou, X. L.; Dai, L. X.; Yu, Y. H.; Xia, W. J. Org. Chem. 2002, 67, 684.
(f) You, S. L.; Luo, Y. M.; Deng, W. P.; Hou, X. L.; Dai, L. X. J.
Organomet. Chem. 2001, 637ꢀ639, 845. (g) Lee, S.; Koh, J. H.; Park, J. J.
Organomet. Chem. 2001, 637ꢀ639, 99. (h) Lautens, M.; Hiebert, S.;
Renaud, J. L. J. Am. Chem. Soc. 2001, 123, 6834. (i) You, S. L.; Hou, X. L.;
Dai, L. X.; Cao, B. X.; Sun, J. Chem. Commun. 2000, 1933. (j) Longmire,
J. M.; Wang, B.; Zhang, X. Tetrahedron Lett. 2000, 41, 5435. (k) Zhang,
W.; Shimanuki, T.; Kida, T.; Nakatsuji, Y.; Ikeda, I. J. Org. Chem. 1999,
64, 6247. (l) Ahn, K. H.; Cho, C. W.; Park, J.; Lee, S. Bull. Korean Chem.
Soc. 1997, 18, 789. (m) Ahn, K. H.; Cho, C. W.; Park, J.; Lee, S.
Tetrahedron: Asymmetry 1997, 8, 1179. (n) Zhang, W.; Kida, T.;
Nakatsuji, Y.; Ikeda, I. Tetrahedron Lett. 1996, 37, 7994. (o) Koch, G.;
Pfaltz, A. Tetrahedron: Asymmetry 1996, 7, 2213.
(14) For a comprehensive summary of reactions see ref 4.
(15) (a) Kim, S. G.; Cho, C. W.; Ahn, K. H. Tetrahedron 1999,
55, 10079. (b) Kim, S. G.; Cho, C. W.; Ahn, K. H. Tetrahedron:
Asymmetry 1997, 8, 1023.
(16) (a) Yuan, Y.; Raabe, G.; Bolm, C. J. Organomet. Chem. 2005,
690, 5747. (b) Sebesta, R.; Toma, S.; Salisova, M. Eur. J. Org. Chem.
2002, 692.
(4) (a) Hargaden, G. C.; Guiry, P. J. Chem. Rev. 2009, 109, 2505. (b)
Miyake, Y.; Nishibayashi, Y.; Uemura, S. Synlett 2008, 1747. (c)
McManus, H. A.; Guiry, P. J. Chem. Rev. 2004, 104, 4151. (d) McManus,
H. A.; Guiry, P. J. Chem. Rev. 2004, 104, 4151. (e) Sutcliffe, O. B.; Bryce,
M. R. Tetrahedron: Asymmetry 2003, 14, 2297. (f) Dai, L. X.; Tu, T.;
You, S. L.; Deng, W. P.; Hou, X. L. Acc. Chem. Res. 2003, 36, 659.
(g) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336.
(5) (a) Richards, C. J.; Damalidis, T.; Hibbs, D. E.; Hursthouse,
M. B. Synlett 1995, 74. (b) Richards, C. J.; Hibbs, D. E.; Hursthouse,
M. B. Tetrahedron Lett. 1995, 36, 3745. (c) Nishibayashi, Y.; Uemura, S.
Synlett 1995, 79. (d) Nishibayashi, Y.; Segawa, K.; Ohe, K.; Uemura, S.
Organometallics 1995, 14, 5486.
(6) (a) Park, J.; Lee, S.; Ahn, K. H.; Cho, C. W. Tetrahedron Lett.
1995, 36, 7263. (b) Zhang, W.; Adachi, Y.; Hirao, T.; Ikeda, I.
Tetrahedron: Asymmetry 1996, 7, 451.
(7) (a) Wang, Y.; Liu, D.; Meng, Q.; Zhang, W. Tetrahedron:
Asymmetry 2009, 20, 2510. (b) Wang, D. S.; Zhou, J.; Wang, D. W.;
Guo, Y. L.; Zhou, Y. G. Tetrahedron Lett. 2010, 51, 525. (c) Lu, W. J.;
Hou, X. L. Adv. Synth. Catal. 2009, 351, 1224. (d) Legault, C. Y.;
Charette, A. B.; Cozzi, P. G. Heterocycles 2008, 76, 1271. (e) Palmer,
A. M.; Nettekoven, U. Tetrahedron: Asymmetry 2007, 18, 2381. (f) Li, X.;
Li, Q.; Wu, X.; Gao, Y.; Xu, D.; Kong, L. Tetrahedron: Asymmetry 2007,
18, 629. (g) Tellers, D. M.; Bio, M.; Song, Z. J.; McWilliams, J. C.; Sun, Y.
Tetrahedron: Asymmetry 2006, 17, 550. (h) Naud, F.; Malan, C.;
Spindler, F.; Ruggeberg, C.; Schmidt, A. T.; Blaser, H. U. Adv. Synth.
Catal. 2006, 348, 47. (i) Lu, S. M.; Han, X. W.; Zhou, Y. G. Adv. Synth.
Catal. 2004, 346, 909. (j) Rouzaud, J.; Jones, M. D.; Raja, R.; Johnson,
B. F. G.; Thomas, J. M.; Duer, M. J. Helv. Chim. Acta 2003, 86, 1753.
(k) Bolm, C.; Xiao, L.; Kesselgruber, M. Org. Biomol. Chem. 2003, 1, 145.
(l) Blankenstein, J.; Pfaltz, A. Angew. Chem., Int. Ed. 2001, 40, 4445.
(8) (a) Madrigal, C. A.; Garcia-Fernandez, A.; Gimeno, J.; Lastra,
E. J. Organomet. Chem. 2008, 693, 2535. (b) Nishibayashi, Y.; Takei, I.;
Uemura, S.; Hidai, M. Organometallics 1999, 18, 2291. (c) Arikawa, Y.;
Ueoka, M.; Matoba, K.; Nishibayashi, Y.; Hidai, M.; Uemura, S. J.
Organomet. Chem. 1999, 572, 163. (d) Du, X. D.; Dai, L. X.; Hou, X. L.;
Xia, L. J.; Tang, M. H. Chin. J. Chem. 1998, 16, 90. (e) Sammakia, T.;
Stangeland, E. L. J. Org. Chem. 1997, 62, 6104.
(17) Bueno, A.; Rosol, M.; Garcia, J.; Moyano, A. Adv. Synth. Catal.
2006, 348, 2590.
(18) (a) Fukuzawa, S.; Yahara, Y.; Kamiyama, A.; Hara, M.; Kikuchi,
S. Org. Lett. 2005, 7, 5809. (b) Locatelli, M.; Cozzi, P. G. Angew. Chem.,
Int. Ed. 2003, 42, 4928.
(19) (a) Togni, A.; Breutel, C.; Schnyder, A.; Spindler, F.; Landert,
H.; Tijani, A. J. Am. Chem. Soc. 1994, 116, 4062. (b) Hayashi, T.; Mise,
T.; Fukushima, M.; Kagotani, M.; Nagashima, N.; Hamada, Y.;
Matsumoto, A.; Kawakami, S.; Konishi, M.; Yamamoto, K.; Kumada,
M. Bull. Chem. Soc. Jpn. 1980, 3, 1138.
(20) (a) Schuecker, R.; Weissensteiner, W.; Mereiter, K.; Lotz, M.;
Spindler, F. Organometallics 2010, 29, 6443. (b) Lotz, M.; Schuecker, R.;
Mereiter, K.; Knochel, P. Organometallics 2010, 29, 6503. (c) Schuecker,
R.; Mereiter, K.; Spindler, F.; Weissensteiner, W. Adv. Synth. Catal. 2010,
352, 1063. (d) Espino, G.; Xiao, L.; Puchberger, M.; Mereiter, K.;
Spindler, F.; Manzano, B. R.; Jalon, F. A.; Weissensteiner, W. Dalton
Trans. 2009, 2751. (e) Wang, Y.; Sturm, T.; Steurer, M.; Arion, V. B.;
Mereiter, K.; Spindler, F.; Weissensteiner, W. Organometallics 2008,
(9) (a) Takei, I.; Nishibayashi, Y.; Ishii, Y.; Mizobe, Y.; Uemura, S.;
Hidai, M. Chem. Commun. 2001, 2360. (b) Takei, I.; Nishibayashi, Y.;
Arikawa, Y.; Uemura, S.; Hidai, M. Organometallics 1999, 18, 2271.
(c) Nishibayashi, Y.; Takei, I.; Uemura, S.; Hidai, M. Organometallics
1998, 17, 3420. (d) Nishibayashi, Y.; Segawa, K.; Arikawa, Y.; Ohe, K.;
4718
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719

Organometallics
ARTICLE
27, 1119. (f) Steurer, M.; Wang, Y.; Mereiter, K.; Weissensteiner, W.
Organometallics 2007, 26, 3850. (g) Wang, Y.; Weissensteiner, W.;
Spindler, F.; Arion, V. B.; Mereiter, K. Organometallics 2007, 26, 3530.
(21) (a) Gokel, G.; Hoffmann, P.; Kleimann, H.; Klusacek, H.;
Marquarding, D.; Ugi, I. Tetrahedron Lett. 1970, 1771. (b) Marquarding,
D.; Klusacek, H.; Gokel, G.; Hoffmann, P.; Ugi, I. J. Am. Chem. Soc. 1970,
92, 5389.
(22) (a) Cabou, J.; Brocard, J.; Pelinski, L. Tetrahedron Lett. 2005,
46, 1185. (b) Bertogg, A.; Togni, A. Organometallics 2006, 25, 622.
(23) (a) Richards, C. J.; Mulvaney, A. W. Tetrahedron: Asymmetry
1996, 7, 1419. (b) Vorbr€uggen, H.; Krolokliewicz, K. Tetrahedron Lett.
1981, 22, 4471.
(24) Sawamura, M.; Hamashima, H.; Sugawara, M.; Kuwano, R.; Ito,
Y. Organometallics 1995, 14, 4549.
(25) Coumbe, T.; Lawrence, N. J.; Muhammad, F. Tetrahedron Lett.
1965, 35, 625.
(26) Carmona, D.; Medrano, R.; Dobrinovich, I. T.; Lahoz, F. J.;
Ferrer, J.; Oro, L. A. J. Organomet. Chem. 2006, 691, 5560.
(27) Bauer, W.; Polborn, K.; Beck, W. J. Organomet. Chem. 1999,
579, 269.
(28) Imai, Y.; Zhang, W.; Kida, T.; Nakatsuji, Y.; Ikeda, I. Chem. Lett.
1999, 243.
(29) Kodama, H.; Ohto, K.; Oohara, N.; Nakatsui, K.; Kaneda, Y.
Pat. Appl. US 7390915, 2008.
(30) Gerard, S.; Pressel, Y.; Riant, O. Tetrahedron: Asymmetry 2005,
16, 1889.
(31) (a) Li, H.; Wei, K.; Du, C.; Zhu, Y.; Xu, C.; Wu, Y. J. Organomet.
Chem. 2007, 692, 1033. (b) Anderson, C. E.; Donde, Y.; Douglas, C. J.;
Overman, L. E. J. Org. Chem. 2005, 70, 648.
(32) Uozumi, Y.; Kato, K.; Hayashi, T. Tetrahedron: Asymmetry
1998, 9, 1065.
(33) Indolese, A. F.; Wasescha, D.; Eisele, T.; Bucher, C. Chimia
2005, 59, 22.
(34) (a) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (b)
Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004,
248, 2201. (c) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97.
(35) Kureshy, R. I.; Ahmad, I.; Pathak, K.; Khan, N.-U. H.; Abdi,
S. H. R.; Prathap, J. K.; Jasra, R. V. Chirality 2007, 19, 352–357.
4719
dx.doi.org/10.1021/om200557c |Organometallics 2011, 30, 4711–4719