A chiral iron complex containing a bis(oxazolinyl)phenyl ligand: Preparation and asymmetric hydrosilylation of ketones

Article abstract of DOI:10.1021/om1009186

New chiral pincer iron complexes having bis(oxazolinyl)phenyl ligands have been synthesized by oxidative addition of Fe₂(CO)₉ to 2-bromo-substituted ligands. This chiral pincer iron complex can catalyze the asymmetric hydrosilylation of simple aromatic ketones to give the corresponding alcohols.

Full text of DOI:10.1021/om1009186

Organometallics 2010, 29, 5773–5775 5773
DOI: 10.1021/om1009186
A Chiral Iron Complex Containing a Bis(oxazolinyl)phenyl Ligand:
Preparation and Asymmetric Hydrosilylation of Ketones
Satomi Hosokawa, Jun-ichi Ito, and Hisao Nishiyama*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa,
Nagoya 464-8603, Japan
Received September 24, 2010
Summary: New chiral pincer iron complexes having bis-
(oxazolinyl)phenyl ligands have been synthesized by oxidative
addition of Fe2(CO)9 to 2-bromo-substituted ligands. This
chiral pincer iron complex can catalyze the asymmetric hydro-
silylation of simple aromatic ketones to give the corresponding
alcohols.
ligands for iron atoms. Bis(oxazoline)pyridine (pybox) li-
gands, which are a prominent class of chiral ligands for a
range of transition metals, have been utilized in the generation
of chiral iron alkyl complexes and asymmetric
hydrosilylation.8b
Recently, we reported chiral transition-metal complexes
having chiral meridional NCN-type ligands, namely bis-
(oxazolinyl)phenyl (phebox) ligands, for asymmetric catalytic
reactions.9 The Rh and Ru complexes displayed high cata-
lytic activity and enantioselectivity toward several asym-
metric reactions, including reductive aldol coupling, conjugate
reduction withhydrosilanes, hydrogenation, and cyclopropanation.
Wenowturnourattentiontotheconstruction of iron complexes
having the phebox ligand. Here we report the first synthesis
of both achiral and chiral Phebox-Fe complexes and their
utilization in the asymmetric hydrosilylation of ketones.
Oxidative addition of Fe(0) complexes with alkyl halides
leads to the formation of the corresponding alkyl Fe(II)
complexes.10 In this reaction, Fe2(CO)9 is thought to be a
suitable iron source because it generates the unsaturated iron
species “Fe(CO)4”.11 The initial experiment was carried out
using an achiral bromide ligand precursor, (phebox-dm)Br
(1a).12 The reaction of 1a with Fe2(CO)9 in toluene at 50 °C
for 5 h resulted in the formation of a new phebox-Fe(II)
complex, (phebox-dm)FeBr(CO)2 (2a) (eq 1). Purification by
column chromatography on silica gel afforded 2a in 52%
yield. In contrast, the reaction of 1a with Fe3(CO)12 in toluene
at 50 °C significantly decreased the yield of 2a (5%) and the
reaction of 1a with Fe(CO)5 failed.
Anionic meridional ECE ligands (E = P, N) have been
utilized for the stabilization of transition-metal complexes
that exhibit several types of transformations of organic
molecules.1 Although Ru, Rh, Ir, and Pd complexes with
PCP- and NCN-type ligands have been developed for catalytic
reactions,1 iron complexes with ECE-type meridional ligands
have been limited to 1,3-bis(dimethylphosphinomethyl)phenyl
and 1,3-bis(dimethylaminomethyl)phenyl ligands, reported
by Kaska and van Koten, respectively.2 In contrast, neutral
NNN-type meridional ligands, which have skeletons similar
to those of anionic ECE ligands, have attracted much atten-
tion for use in iron complexes.3-5 In this context, 2,6-bis-
(imino)pyridine ligands have the potential to generate highly
reactive iron complexes for catalytic reactions, such as
polymerization,3 cycloaddition,4 and hydrogenation.5 Other
neutral meridional ligands based on pyridine and two-
electron-donor moieties, such as phosphine,6 NHC,7 and
oxazoline groups,8 have also been reported as supporting
*To whom correspondence should be addressed. E-mail:
hnishi@apchem.nagoya-u.ac.jp.
(1) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750. (b) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (c)
The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M.,
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A. A. H.; van Koten, G.; Vrieze, K. Acta Crystallogr. 1985, C41, 893. (b)
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r
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Published on Web 10/27/2010
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5774 Organometallics, Vol. 29, No. 22, 2010
Hosokawa et al.
The phebox-Fe complex 2a is diamagnetic. The 1H NMR
spectrum of 2a exhibited two singlet signals at δ 1.42 (6H)
and 1.25 (6H), which were assigned to the methyl groups on
the oxazoline rings. Likewise, the AB doublet signals for the
oxazoline CH2 groups were observed at δ 4.43 (2H) and 4.39
(2H). These spectral features indicated that 2a has a Cs-
symmetric structure. In the IR spectrum of 2a, strong
absorptions were observed at 2020 and 1969 cm-1, which
were assigned to the symmetric and asymmetric CO stretch-
ing vibrations, respectively. The intensity ratio of these peaks
implied a cis arrangement of the CO ligands. The wavenum-
bers of these peaks are close to those of the related phebox-Ru
analogue (phebox)Ru(CO)2Br, (2037, 1959 cm-1).13
Reaction of the chiral isopropyl phebox precursor [(S,S)-
phebox-ip]Br(1b) withFe2(CO)9 at 50 °C for 3 h also produced
the chiral phebox-Fe complex 2b in 69% yield (eq 1). In
the 1H NMR spectrum of 2b, there are four doublet signals for
methyl protons of the isopropyl group at δ 0.43, 0.45, 0.49, and
0.83, due to the lack of symmetry elements in the molecule.
The molecular structure of 2a was determined by X-ray
analysis using crystals obtained from a CH3CN/CH2Cl2
solution.14 The ORTEP diagram shows pseudo-octahedral
geometry with the meridionally coordinated phebox ligand
Figure 1. ORTEP diagram of 2a at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Selected bond
lengths (A): Fe(1)-C(1) = 1.930(2), Fe(1)-C(17) = 1.792(2),
Fe(1)-C(18) = 1.839(2), Fe(1)-N(1) = 2.0122(17), Fe(1)-
N(2) = 2.0060(18), Fe(1)-Br(1) = 2.4924(5), C(17)-O(3) =
1.086(3), C(18)-O(4)=1.134(3). Selected angles (deg): N(1)-Fe-
(1)-N(2)=157.42(7), Br(1)-Fe(1)-C(17)=177.48(7), C(1)-Fe-
(1)-C(18)=175.82(9).
˚
˚
(Figure 1). The Fe-C1 bond length of 1.930(2) A is compa-
rable to that of [C6H3(CH2NMe2)2]FeCl2 (1.937(2) A).
2a
˚
This bond length of the phebox-Fe complex 2a is slightly
13
˚
shorter than that of the phebox-Ru complexes (1.96-2.00 A).
Similarly, the Fe-N bond lengths (2.0122(17) and 2.0060(18)
Asymmetric reduction of ketone is one of the most versa-
tile methods for preparation of optically active secondary
alcohols.16 In this context, the iron-catalyzed asymmetric
reduction of carbonyl compounds is thought to be a envi
ronmentally benign method.17,18 Next, we examined the
catalytic activity of the phebox-Fe complex in the hydro-
silylation of simple aromatic ketones. Initially, the phebox-Fe
complex 2b was subjected to hydrosilylation of 4-phenyla-
cetophenone (3a) using HSi(OEt)2Me as a hydrogen source
(Table 1). The reduction of 3a with 1.2 equiv of HSi(OEt)2Me
in the presence of 5 mol % of 2b in toluene at 50 °C yielded
the corresponding (R)-alcohol 4a in 12% yield with 24% ee
(entry 1). The use of 1.5 equiv of HSi(OEt)2Me increased the
yield and enantioselectivity (entry 2). In contrast, other
hydrosilanes, HSiPh2Me, HSiCl3, and H2SiPh2 were not
effective as hydrogen sources (entries 3-5). When the reac-
tion using HSi(OEt)2Me was performed at 80 °C, the yield
was improved to 91% without a decrease in enantioselec-
tivity (entry 6). It was found that the reaction rate and
enantioselectivity were significantly influenced by additives.
Use of a strong base (2 mol %), namely NaO-t-Bu and
NaOMe, increased the reaction rate but decreased the
enantioselectivity of 4a (entries 7 and 8). On the other hand,
the reaction in the presence of Na(acac) (2 mol %) was
completed within 1 h to give 4a in 99% yield with 49% ee
(entry 9). The catalytic reaction in the presence of Na(acac)
also proceeded at 50 °C to give 4a in 86% yield with the same
enantioselectivity (entry 10). Furthermore, the reaction sol-
vent affected the enantioselectivity. The reaction in THF
gave a result similar to that in toluene (entry 11). On the other
hand, reduction of 3a in hexane quantitatively provided the
(R)-alcohol 4a with 66% ee (entry 12).
˚
A) are also shorter than those of the phebox-Ru complexes
˚
(2.10-2.13 A). The Br ligand is coordinated to the position
vertical to the phebox plane, and the two CO ligands are
oriented in a cis arrangement. This CO configuration is
similar to that of (PCP)RuCl(CO)2.15 The Fe(1)-C(18)
˚
bond length of 1.839(2) A is longer than the Fe(1)-C(17)
˚
bond length of 1.792(2) A, probably due to the trans
influence of a phenyl fragment.
(12) Motoyama, Y.; Okano, M.; Narusawa, H.; Makihara, N.; Aoki,
K.; Nishiyama, H. Organometallics 2001, 20, 1580.
(13) Ito, J.; Ujiie, S.; Nishiyama, H. Organometallics 2009, 28, 630.
(14) Crystallographic data for 2a: C19H21BrCl2FeN2O4, Mr =
˚
548.04, temperature 143 K, triclinic, P1, a = 8.555(2) A, b =
˚
˚
11.224(3) A, c = 12.970(3) A, R = 69.062(5)°, β = 82.751(5)°, γ =
3
69.821(5)°, V = 1091.8(5) A , Z = 2; Fcalcd = 1.667 Mg/m3, μ = 2.793
˚
mm-1, 7847 reflections collected, 4993 independent reflections (R(int) =
0.0251), 4993/0/266 data/restraints/parameters, goodness of fit on F2
1.060, final R indices (I > 2σ(I)) R1 = 0.0333 and wR2 = 0.0881, R
indices (all data) R1 = 0.0370-a3nd wR2 = 0.0901, largest difference
˚
peak/hole 0.0901 and -0.379 A
.
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Santos, E. N.; Fogg, D. E. Organometallics 2004, 23, 4047. (b) Jia, G.;
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Several ketones, e.g., 2-acetylanthracene (3b) and 2-acet-
ylnaphthalene (3c), were found to give the corresponding
secondary alcohols 4b (49% ee) and 4c (53% ee). In contrast,
4-methoxyphenyl methyl ketone (3d) and 4-tolyl methyl
ketone (3e) resulted in low enantioselectivities (21-38% ee).

Communication
Organometallics, Vol. 29, No. 22, 2010 5775
Table 1. Enantioselective Hydrosilylation of 3a Catalyzed by 2b
entry
silane (amt (equiv))
amt of 2b (mol %)
additive
solvent
temp (°C)/time (h)
yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
HSi(OEt)2Me (1.2)
HSi(OEt)2Me (1.5)
HSiPh2Me (1.5)
HSiCl3 (1.5)
H2SiPh2 (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
HSi(OEt)2Me (1.5)
5
5
5
5
5
2
2
2
2
2
2
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
50/24
50/24
50/24
50/24
50/24
80/24
80/1
80/1
80/1
50/24
50/24
50/24
12
40
trace
trace
2
91
70
85
99
24
48
6
49
36
32
49
49
48
66
NaO-t-Bu
NaOMe
Na(acac)
Na(acac)
Na(acac)
Na(acac)
9
10
11
12
86
89
99
hexane
a Isolated yield. b Determined by HPLC.
Scheme 1. Asymmetric Reduction of Ketone Catalyzed by Fe and Ru Complexes Containing (S,S)-phebox-ip Ligands
Hydrosilylation with the (S,S)-phebox-ip-Fe catalysts
afforded the alcohol with the R absolute configuration in
toluene and hexane. On the other hand, we previously
reported that asymmetric hydrogenation catalyzed by
(S,S)-phebox-ip-Ru catalysts afforded alcohols with the S
absolute configuration (Scheme 1).13 These results illustrate
that both enantiomers of a secondary alcohol in the asym-
metric reduction of ketones can be obtained by using a single
chiral source.19 Such a reversal of absolute configuration of
the products upon changing from an Rh atom to Ir was
reported in the asymmetric hydrosilylation of ketones.20
Recently, Chirik et al. reported that hydrosilylation of
acetophenone derivatives using [(S,S)-pybox-ip]Fe(CH2SiMe3)2
gave the R alcohol.8b Thus, the (S,S)-phebox-Fe and
(S,S)-pybox-Fe systems have similar prochiral face discrimi-
nation properties for ketones. As several types of Fe silyl
hydride complexes have been synthesized by the reaction with
hydrosilane,21 we assumed that such an active Fe-H inter-
mediate can be generated by the initial reaction of 2b with
Na(acac) and hydrosilane.
In summary, we have described the synthesis and structur-
al characterization of the first chiral iron complexes with
bis(oxazolinyl)phenyl ligands resulting from the oxidative
addition of phebox-Br to Fe2(CO)9. The phebox-Fe com-
plex was utilized in the enantioselective hydrosilylation of
ketones with HSi(OEt)2Me with asymmetric induction. Further
investigation of this chiral phebox Fe complex is ongoing.
Acknowledgment. This research was partially sup-
ported by a Grant-in-Aid for Scientific Research from the
JapanSocietyforthePromotionofScience(Nos. 22245014,
20750073) and the G-COE in chemistry (Nagoya).
ꢀ
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Fisch, K. J. Organomet. Chem. 1991, 412, C11. (c) Gutsulyak, D. V.;
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Supporting Information Available: Text, figures, and a CIF
file giving preparation details and spectral data for 2a,b, crystal
data for 2a, and data of catalytic hydrosilylation. This material
is available free of charge via the Internet at http://pubs.acs.org.