New bis(imino)pyridine complexes of iron(II) and iron(III), and their catalytic-activity in the mukaiyama aldol reaction

Article abstract of DOI:10.1055/s-0033-1340081

New iron(II) and iron(III) complexes bearing bis(imino)pyridine ligands were synthesized and successfully applied to the Mukaiyama aldol reaction. The two complexes [FeCl₂ L] (L = bis(imino)pyridine ligand, 55% isolated yield) and [LFe(μCl)

Full text of DOI:10.1055/s-0033-1340081

PAPER
▌57
paper
New Bis(imino)pyridine Complexes of Iron(II) and Iron(III), and Their
Catalytic Activity in the Mukaiyama Aldol Reaction
New Bis(imino)pyridine Complexes of Iron(II) and Iron(III)
Pushkar Shejwalkar,a Nigam P. Rath,a,b Eike B. Bauer*a
a
University of Missouri–St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
Fax +1(314)5165342; E-mail: bauere@umsl.edu
b
Center for Nanoscience, University of Missouri–St. Louis, St. Louis, MO 63121, USA
Received: 02.08.2013; Accepted after revision: 10.10.2013
O
O
OSiR3
R3SiO
Lewis
acid
R1
Abstract: New iron(II) and iron(III) complexes bearing bis(imi-
no)pyridine ligands were synthesized and successfully applied to
the Mukaiyama aldol reaction. The two complexes [FeCl2L]
(L = bis(imino)pyridine ligand, 55% isolated yield) and
[LFe(μCl)3FeCl3] (76%) were obtained employing FeCl2 and FeCl3
iron sources, respectively, and characterized by elemental analyses,
mass spectrometry, IR spectroscopy and, one example, by X-ray
diffraction. The two new iron complexes were subsequently em-
ployed as catalysts in the Mukaiyama aldol reaction after abstrac-
tion of two chlorides by AgSbF6 to obtain the aldol products in 43%
to virtually quantitative yield (CH2Cl2 solvent, room temperature,
3.5 to 16 h reaction time). The impact of the oxidation state of the
iron center on the reaction rate and the diastereomeric ratios of the
products was investigated.
R4
R3
+
R4
R3
R2
R2
R1
1
2
3
O
OH
R4
R3
R2
R1
4
Scheme 1 The Mukaiyama aldol reaction
Accordingly, strong Lewis acids are among the most com-
monly employed catalysts for the reaction. In the original
work, Mukaiyama and co-workers utilized freshly dis-
tilled, oxophilic TiCl4 and employed it in an equimolar
amount.2 This reagent promotes the reaction between cy-
clic silyl enol ethers and both aldehydes and ketones.
Somewhat weaker Lewis or Brønsted acids typically re-
quire aldehydes instead of ketones as coupling partners.15
Silyl enol ethers frequently employed (and also utilized in
our study) include the cyclic enol ether 5, the enol ether 6
derived from acetophenone, and the commercial silyl ke-
tene acetal 1-(tert-butyldimethylsiloxy)-1-methoxyethene
(7, Figure 1). For steric and electronic reasons, these enol
ether substrates exhibit a range of nucleophilicities. By
analogy to Mayr’s nucleophilicity scale,21 5 exhibits the
lowest nucleophilicity, followed by 6; the silyl ketene
acetal 7 exhibits the highest nucleophilicity among these
three substrates. According to Mayr, the influences of ste-
ric effects on the nucleophilicities of comparable silyl
enol ethers are more difficult to assess. We think that 7 is
more nucleophilic than 6 due to electronic reasons. The
challenge is to develop catalyst systems that are capable
of coupling less reactive nucleophiles such as 5 with both
aldehyde and ketone carbonyl groups. A balance needs to
be found between an efficient, yet easy-to-handle and
bench-stable, catalyst system that does not require the
strict exclusion of moisture or air. Accordingly, a number
of catalyst systems that can be employed under aqueous
conditions have been reported.22
Key words: aldol reaction, catalysis, homogeneous catalysis, iron,
ligands, nucleophilic addition
The Mukaiyama aldol reaction is a carbon–carbon bond-
forming reaction between silyl enol ethers 1 or silyl ke-
tene acetals and carbonyl groups (as in 2) to give β-hy-
droxy ketones 4 and β-hydroxy esters, or their silyl-
protected derivatives 3 (Scheme 1).1 First reported by Mu-
kaiyama and co-workers in 1973,2 the reaction is fre-
quently applied in the synthesis of complex organic
molecules and natural products.3 The Mukaiyama aldol
reaction is catalyzed by Brønsted4 or Lewis acids5 based
on metals such as Zr,6 Bi,7 B,8 Ti,9 Pt,10 Re,11 Li,12 In,13
Sc14 and Zn,15 and by Lewis bases.16 Organocatalytic ver-
sions are known as well.1a,17 Depending on the silyl enol
ether substrate employed, one or two new stereocenters
are created in the product. Substituted enol ethers (R1 ≠ H
in Scheme 1) produce two new stereocenters, and give rise
to the formation of syn- and anti-diastereomers. Obtaining
high diastereomeric ratios and enantiomeric excesses is a
goal of current research concerning the Mukaiyama aldol
reaction.4a,18 Mechanistic investigations have revealed
that the reactions proceed through an open transition
state,19 and Newman projections of the transition state al-
low for the prediction of the diastereoselectivity of the re-
action.20 The Lewis acid catalyst coordinates to the
oxygen of the carbonyl group, making the carbon atom
more susceptible to an attack by the silyl enol ether nu-
cleophile.
Iron complexes have also been employed as catalysts in
the Mukaiyama aldol reaction,23 and enantioselective ver-
sions thereof have been reported as well.24 Iron catalysis
has lately emerged as an intensely studied research area.25
SYNTHESIS 2014, 46, 0057–0066
Advanced online publication: 21.11.2013
0
0
3
9
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7
8
8
1
1
4
3
7
-
2
1
0
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DOI: 10.1055/s-0033-1340081; Art ID: SS-2013-M0541-OP
© Georg Thieme Verlag Stuttgart · New York

58
P. Shejwalkar et al.
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OSiMe3
Synthesis and Characterization of the Iron Complexes
t-BuMe2SiO
t-BuMe2SiO
OMe
Ph
We selected specifically the known42 bis(imino)pyridine
ligand 8 (Scheme 2) for the synthesis of the corresponding
iron complexes. Ligand 8 can be synthesized by conden-
sation of 1,6-diacetylpyridine with 2,6-diethylaniline in
ethanol in the presence of a catalytic amount of glacial
acetic acid.43 We also considered bis(imino)pyridine li-
gands with isopropyl substituents in the 2′,6′-positions,
but encountered difficulties with converting these into the
corresponding iron complexes.
5
6
7
Figure 1 Silyl enol ether substrates
Iron catalysts have a number of advantages over other
transition metals typically employed in catalysis. Iron is
inexpensive, abundant, relatively nontoxic and environ-
mentally friendly. Consequently, iron-based complexes
have increasingly been investigated as catalysts in a num-
ber of organic transformations, such as in C–C,26 C–N,27
C–P,28 C–B29 and C–Si30 bond-forming reactions, hetero-
atom–heteroatom bond-forming reactions,31 reductions,32
oxidations33 and polymerizations,34 as well as other func-
tional group interconversions.35
Next, we targeted the synthesis of iron(II) and iron(III)
complexes of ligand 8. We have previously synthesized
iron complexes of the general formula [FeL2(OTf)2],
where L is a phosphinooxazoline ligand and OTf is the
weakly coordinating F3CSO3– counterion;44 however, the
complex [Fe(8)(OTf)2], when synthesized from
[Fe(OTf)2] and ligand 8, only showed at best moderate
catalytic activity in the Mukaiyama aldol reaction. We,
thus, changed to FeCl2 and FeCl3 as the iron source.
However, iron-based catalyst systems for the Mukaiyama
aldol reaction are still scarce. Mainly iron(II) complexes
have been employed; we are aware of only one report
where an iron(III) salt (FeCl3 in combination with a sur-
factant under aqueous conditions) was utilized for cataly-
sis.36 In all these reports, a variety of silyl enol ethers
(such as those shown in Figure 1) were employed as the
nucleophiles, but the carbonyl coupling partners were, in
all cases, aldehydes. We hypothesized that a very strong
Lewis acid is more efficient when demanding silyl enol
ethers such as 5 are employed. We were, thus, interested
in determining whether a preformed iron(III) complex is a
suitable catalyst for the Mukaiyama aldol reaction and
how it performs compared to a related iron(II) complex.
According to the mechanistic understanding outlined
above, a higher formal charge on the Lewis acid should
improve its efficiency as a catalyst.
Accordingly, the ligand 8 and FeCl2 in n-butanol were
heated for 15 minutes, as previously reported for related
syntheses (Scheme 2).45 The complex [FeCl28] was isolat-
ed as a dark blue solid in 55% yield by concentrating the
solution and precipitation of the product complex by the
addition of pentane. The complex was analyzed by IR and
UV–vis spectroscopy, mass spectrometry and elemental
analysis. Due to the paramagnetic nature of the new com-
plex, characterization by NMR spectroscopy was not at-
tempted.
The coordination of the ligand to the iron center was best
seen by a shift of the νC=N stretching frequency in the IR
spectrum. The free ligand 8 showed an imine C=N stretch
of 1639 cm–1, whereas the coordinated C=N unit in the
product showed a stretch at 1585 cm–1. Such a change of
the imine stretching frequency upon coordination to an
iron center has previously been observed by us46a and oth-
ers.46b The UV–vis spectrum of complex [FeCl28] exhib-
ited a strong absorption at 698 nm and, due to its large
molar absorption coefficient ε of 1164 cm–1·M–1, was ten-
tatively assigned to a metal-to-ligand charge transfer
(MLCT). Such absorptions have been previously reported
for related iron complexes by us46a and others.47 The mass
spectrum gave a molecular ion peak [FeCl28]+ with
m/z = 551, confirming the mononuclear nature of the
complex and the coordination of the ligand. A fragment
[FeCl8]+ with m/z = 516 was also observed in the FAB-
MS. The elemental analysis for the complex was slightly
low on carbon (presumably due to FeCl2 impurities) but
still confirmed the overall molecular formula [FeCl28].
The analysis of the complex by X-ray diffraction is de-
scribed below.
Accordingly, we describe herein the synthesis and charac-
terization of bis(imino)pyridine complexes of iron(II) and
iron(III). The bis(imino)pyridine ligand class is known37
and has previously been employed in the synthesis of iron
complexes.38 Iron complexes of bis(imino)pyridines are
widely utilized as ethylene polymerization catalysts,39 and
also have been used to catalyze cyclization or cyclo-
isomerization reactions;40 however, they have not been
employed in the Mukaiyama aldol reaction to date.
We hypothesized that a bulky, tridentate ligand such as
the bis(imino)pyridine ligand class might keep its corre-
sponding iron complexes soluble even in relatively non-
polar solvents and might prevent the formation of dimers
and oligomers as often observed for iron complexes with
chloro or other potentially bridging ligands.41 After syn-
thesis and characterization of the iron(II) and iron(III)
complexes, we successfully employed them as catalysts in
the Mukaiyama aldol reaction. Furthermore, we investi-
gated the impact of the formal charge on the iron catalyst
on the efficiency and diastereoselectivity of the reaction.
Similarly, we also treated the ligand 8 with FeCl3 (Scheme
2). When a solution of FeCl3 in diethyl ether was added to
a solution of the ligand 8 in diethyl ether, an orange-red
solid precipitated immediately. The mother liquor was de-
canted and the solid precipitate was washed several times
Synthesis 2014, 46, 57–66
© Georg Thieme Verlag Stuttgart · New York

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New Bis(imino)pyridine Complexes of Iron(II) and Iron(III)
59
with diethyl ether to obtain the product as an orange-red plex [FeCl28] were crystals obtained through slow
solid. The ether washings were slightly yellow in color; diffusion of pentane into a dichloromethane solution of
analysis of the combined ether portions used for washing the complex at –20 °C over a course of one week. Crystal-
revealed that half of the ligand 8 had not reacted with the lographic parameters are given in Table 1, and key bond
FeCl3; on the other hand, there was no evidence for unre- lengths and angles are compiled in Table 2. As can be seen
acted FeCl3 starting material, as judged from the color of from the molecular structure (Figure 2), the crystal con-
the washings and the precipitate. Based on instrumental tained a binuclear structure, where two iron centers are
analysis, as detailed below, we came to the conclusion bridged by an oxo ligand. One of the two iron centers
that the isolated material had a dimeric structure best de- bears the ligand 8, whereas the other one is coordinated by
scribed as [8Fe(μCl)3FeCl3], and was isolated in 76% three chloro ligands. The structure is, thus, best described
yield.
as [8ClFe(μO)FeCl3] and the oxidation states of the two
iron centers in the complex increased from iron(II) to
iron(III). The formation of the oxo-bridged dimer must
have taken place during the crystallization process, as the
Again, the coordination of the ligand to the iron center
was best seen by a shift of the νC=N stretching frequency in
the IR spectrum, as described above for the iron(II) com-
plex. The coordinated C=N unit showed a stretch at 1592
cm–1, which is 47 wavenumbers lower than the free li-
gand. The UV–vis spectrum exhibited two strong absorp-
tions: one was observed at 415 nm (ε = 1257 cm–1·M–1)
and the other at 506 nm (ε = 990 cm–1·M–1). Based on the
values of the molar absorption coefficients, we again as-
cribed the transitions to a charge-transfer process. Unfor-
tunately, neither FAB nor EI mass spectrometry gave a
molecular ion peak that corresponded to the proposed mo-
lecular structure; however, the elemental analysis was in
full agreement with the formulation [8Fe(μCl)3FeCl3].
Structurally related iron clusters have been previously re-
ported in the literature,41 and we, thus, consider that the
suggested formulation [8Fe(μCl)3FeCl3] was sufficiently
corroborated by IR spectroscopy, elemental analysis and
literature precedents.
To unequivocally establish the structures of the new iron
complexes, we undertook substantial effort to grow crys-
tals for X-ray diffraction studies; however, only for com-
Figure 2 X-ray structure of [8ClFe(μO)FeCl3]. Crystallographic pa-
rameters are compiled in Table 1, and key bond lengths and angles are
listed in Table 2.
N
N
FeCl2
N
N
N
N
n-BuOH
reflux
15 min
Fe
Cl
Cl
8
[FeCl28], 55%
[O]
one week
in solution
FeCl3
Et2O
r.t.
N
N
N
N
N
N
Fe
Cl
Fe
Cl
O
Cl
Cl
Cl
Fe
Cl
Fe
Cl
Cl
Cl
[8Fe(μCl)3FeCl3]
76%
Cl
Scheme 2 Synthesis of iron(II) and iron(III) complexes of ligand 8
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 57–66

60
P. Shejwalkar et al.
PAPER
original complex [FeCl28] exhibited an iron(II) center and es to obtain oxo-bridged iron(III) dimers have been
its molecular formula has been established by MS and el- described previously.48
emental analysis. Considering the long crystallization
The O–FeCl3 fragment of the structure is best described as
process, it appears reasonable to assume that the complex
slightly distorted tetrahedral. The Cl–Fe(2)–Cl and Cl–
was oxidized in solution over time. Further analysis of the
Fe(2)–O bond angles range from 102.73(12)° to
oxidized material by NMR spectroscopy and ESI-MS
115.5(5)°, and are, thus, close to 109°, as expected for an
failed; due to the paramagnetism of the sample, no NMR
ideal tetrahedron. The metal center of the 8ClFe–O frag-
spectra could be recorded, and the ESI-MS (for the spec-
ment of the complex can be best described as square pyra-
trum, see the Supporting Information) did not exhibit
midal. The N(2)–Fe(1)–N(3) and N(2)–Fe(1)–N(1) bond
peaks that can be assigned to the oxidized material or frag-
angles are 73.39(16)° and 72.84(16)°, respectively, and
ments thereof. Air oxidations of similar iron(II) complex-
the O(1)–Fe(1)–N and O(1)–Fe(1)–Cl(1) bond angles
range from 100.48(16)° to 113.10(12)°. Thus, the three ni-
trogen atoms of the ligand 8 and the chloro ligand occupy
the base of the pyramid, whereas the bridging oxo unit oc-
Table 1 Crystallographic Parameters
cupies the apex. The Fe–Cl bond lengths for both iron
centers do not significantly differ and range from
2.2022(15) to 2.261(3) Å. The Fe–N bond lengths were
determined to range from 2.102(4) to 2.191(4) Å. Iron(III)
complexes are paramagnetic; bond lengths around 2.2 Å
are typical for high-spin iron(II) complexes, which is lon-
ger by about 0.2 Å than those observed for iron(II) low-
spin complexes.49 The Fe–O–Fe angle was determined to
be 146.0(2)°, which is at the lower end of the 145° to 180°
typically observed for that unit.50a
Empirical formula
C29H35Cl4Fe2N3O
Formula weight
Temperature
Wavelength
695.10
100(2) K
0.71073 Å
monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
α = 90°
The X-ray structure is similar to that reported for the thia-
zole complex [(L)4ClFe(O)FeCl3], where L is a thiazole
ligand.50b In this complex, the Fe–Cl bond lengths range
from 2.215(2) to 2.387(2) Å and the Fe–N bond lengths
from 2.164(5) to 2.190(4) Å. They are, thus, comparable
to the corresponding values in the complex
[8ClFe(μO)FeCl3] and in other, related oxo-bridged iron
dimers.50a,c
a = 12.3617(10) Å
b = 21.4167(15) Å
c = 11.8660(9) Å
3108.3(4) Å3
4
β = 98.340(5)°
γ = 90°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
1.485 Mg/m3
1.305 mm–1
The bond lengths for the C=N imine bond for the coordi-
nated ligand are 1.280(6) and 1.294(6) Å; the C(imine)–
C(pyridyl) bond lengths C(1)–C(2) and C(6)–C(7) are
1.492(7) and 1.478(7) Å, respectively. These values are
very close to those found in free, closely related bis(imi-
no)pyridine ligands.51 Thus, extensive electron delocal-
ization from the iron(II) center to the ligand does not
appear to take place. Such a formal reduction of the ligand
by the iron center has been reported for related iron(0)
species, and resulted in significant elongation of the C=N
bond and a concomitant reduction of the C(imine)–C(pyr-
idyl) bond length.51
1432
Crystal size
0.349 × 0.193 × 0.172 mm3
1.665 to 25.371°
Theta range for data collection
Index ranges
–14 ≤ h ≤ 14, –25 ≤ k ≤ 25,
–14 ≤ l ≤ 14
Reflections collected
56258
Independent reflections
5588 [Rint = 0.1017]
In summary, our synthetic efforts resulted in the isolation
of two new iron complexes [FeCl28] and
[8Fe(μCl)3FeCl3] that differ in the oxidation state of iron.
It is interesting to note that both complexes did not behave
as straightforwardly as assumed. The iron(II) complex
[FeCl28] appears to oxidize (in solution) over time to an
oxo-bridged dimeric structure [8ClFe(μO)FeCl3]. The
formation of the complex [8Fe(μCl)3FeCl3] was not ex-
pected as a complex of the formula [FeCl38] was antici-
pated. It has previously been observed that the
employment of iron chlorides as sources for the synthesis
of iron complexes can result in the formation of clusters.41
These findings might be of interest in the application of in
Completeness to theta = 25.000° 98.9%
Absorption correction
Max and min transmission
Refinement method
semiempirical from equivalents
0.7452 and 0.6162
full-matrix least squares on F2
5588/45/375
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.071
R1 = 0.0577, wR2 = 0.1281
R1 = 0.0954, wR2 = 0.1495
Largest difference peak and hole 0.711 and –0.848 e·Å–3
Synthesis 2014, 46, 57–66
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New Bis(imino)pyridine Complexes of Iron(II) and Iron(III)
61
lyze the catalytically active species, an ESI-MS was re-
corded (see the Supporting Information), but it did not
exhibit peaks that could be unambiguously assigned to a
structure that resulted from chloride abstraction. Treat-
ment of the two complexes with AgBF4 gave activated
species that were not as efficient in catalysis as those gen-
erated by AgSbF6. We ascribe this to the low solubility of
AgBF4 in dichloromethane, which might result in incom-
plete chloride abstraction.
Table 2 Key Bond Lengths and Angles for Complex
[8ClFe(μO)FeCl3]
Key bond lengths (Å)
1.756(3)
Key bond angles (°)
Fe(1)–O(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–N(1)
Fe(1)–Cl(1)
Fe(2)–O(1)
Fe(2)–Cl(2)
Fe(2)–Cl(3)
Fe(2)–Cl(4)
N(1)–C(1)
N(3)–C(7)
C(1)–C(2)
C(6)–C(7)
N(1)–C(14)
N(3)–C(8)
O(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
O(1)–Fe(1)–N(1)
N(2)–Fe(1)–N(1)
N(3)–Fe(1)–N(1)
O(1)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(1)
N(3)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(1)
O(1)–Fe(2)–Cl(2)
O(1)–Fe(2)–Cl(3)
O(1)–Fe(2)–Cl(4)
101.95(15)
104.50(16)
73.39(16)
100.48(16)
72.84(16)
141.29(16)
113.10(12)
144.95(11)
97.00(11)
99.82(12)
112.31(13)
112.1(4)
2.102(4)
2.172(4)
2.191(4)
2.2152(14)
1.774(3)
2.2022(15)
2.239(4)
2.261(3)
1.280(6)
1.294(6)
1.492(7)
1.478(7)
1.445(7)
1.443(6)
The solutions of the activated iron complexes were found
to be catalytically active in the Mukaiyama aldol reaction,
when added to a solution of the silyl and aldehyde sub-
strates. The results are compiled in Tables 3 and 4 (room
Table 3 Isolated Yieldsa
Me3SiO
Ar
O
H
OSiMe3
O
syn
[Fe] (5 mol%)
CH2Cl2, r.t.
+
Ar
H
Me3SiO
O
H
5
104.47(15)
Ar
Cl(2)–Fe(2)–Cl(3) 109.4(3)
Cl(2)–Fe(2)–Cl(4) 102.73(12)
Cl(3)–Fe(2)–Cl(4) 115.5(5)
anti
Entry Product
Isolated yield (%), (syn/anti ratiob)
[FeCl28]c [8Fe(μCl)3FeCl3]d
Fe(1)–O(1)–Fe(2)
146.0(2)
O
Me3SiO
H
1
88 (62:38) 63 (83:17)
situ catalyst systems where FeCl2 or FeCl3 is combined
with a ligand and the resulting solution is subsequently
applied in catalysis. It appears that it cannot always be as-
sumed that a mononuclear iron complex results when iron
chlorides and ligands are mixed in solution.
Me3SiO
O
H
2
3
60 (50:50) 59 (71:29)
91 (62:38) 55 (83:17)
Me3SiO
O
H
Application of the Iron Complexes in the Mukaiyama
Aldol Reaction
Me3SiO
Me3SiO
O
H
We next investigated the new iron complexes [FeCl28]
and [8Fe(μCl)3FeCl3] for their application as catalysts in
the Mukaiyama aldol reaction. The two complexes were
not found to be catalytically active alone, and we decided
to activate them through chloride abstraction. According-
ly, we pretreated dichloromethane solutions of the two
complexes with two equivalents of AgSbF6 which should,
ideally, afford dications of the general formula [Fe8]2+
and [8Fe(μCl)3FeCl]2+. The exact nature of the two acti-
4e
no reaction 86e (50:50)
72 (50:50) 62 (71:29)
O2N
O
H
5
MeO
a Typical reaction conditions: aldehyde (0.36 mmol) and silyl enol
vated species in solution could not be determined; howev- ether (0.54 mmol) in CH2Cl2 (2 mL) catalyzed by [Fe] (0.018 mmol,
activated with 0.036 mmol AgSbF6) at r.t. The products were isolated
er, the chloride abstraction was evident from precipitation
of a white powder after the addition of AgSbF6, presum-
ably AgCl, which was removed immediately before cata-
lytic runs. After the complexes were treated with AgSbF6,
a color change was also observed, and the UV–vis spectra
of the complexes changed dramatically (for the UV–vis
spectra, see the Supporting Information). To further ana-
by filtration through a short pad of silica gel.
b Determined by 1H NMR spectroscopy, based on the 3JHH coupling
constant of the H–C–O proton (anti: ~4.5 Hz, syn: <1 Hz).
c 14–16 h reaction time.
d 3.5 h reaction time.
e This was the only product where the yield could not accurately be
reproduced and a yield range from 10% to 90% was observed.
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62
P. Shejwalkar et al.
PAPER
determined by 1H NMR spectroscopy. The 2-methylbenz-
aldehyde substrate (Table 3, entry 2) gave somewhat low-
er yields, presumably due to steric effects exerted by the
methyl group in the ortho position to the reacting alde-
hyde unit. A comparison of the two complexes [FeCl28]
and [8Fe(μCl)3FeCl3] is most instructional (Table 3). It
appeared that the iron(III) precursor complex
[8Fe(μCl)3FeCl3], after activation with AgSbF6, catalyzed
the reaction more rapidly (3.5 h reaction time) than the ac-
tivated iron(II) precursor complex [FeCl28] (14–16 h re-
action time). As hypothesized, a higher oxidation state of
the iron center renders the complex more reactive. How-
ever, the isolated yields were somewhat higher when the
iron(II) precursor was employed as the catalyst. We tenta-
tively ascribe the lower isolated yield to the fact that the
activated iron complexes themselves also decompose the
silyl enol ether substrate. The silyl nucleophiles were al-
ways employed in a slight excess, but at the end of the re-
action, only aldehyde starting material and product was
observed by GC.
Table 4 Isolated Yieldsa
O
OSiMe2t-Bu
t-BuMe2SiO
O
[Fe]
+
Ar
H
R
Ar
R
CH2Cl2, r.t.
6 R = Ph
7 R = OMe
Entry Ar
R
Product
Isolated
yield (%)
t-BuMe2SiO
O
1b
4-Tol
Ph
75
43
Ph
t-BuMe2SiO
O
2b
4-MeOC6H4 Ph
Ph
MeO
t-BuMe2SiO
O
3b
4c
5c
6c
7c
4-EtC6H4
4-Tol
Ph
75
Ph
The reaction of benzaldehyde with (trimethylsiloxy)cy-
clopentene 5 gave a mixture of the desired silyl-protected
coupling product, the desilylated coupling product (i.e.,
the corresponding alcohol, exemplified in Scheme 1,
compound 4) and cyclopentanone, as assessed by NMR
(1H, 13C{1H}; for the spectra, see the Supporting Informa-
tion) and IR spectroscopy. Further workup efforts did not
yield the desired coupling product in sufficient yield. The
reaction between (trimethylsiloxy)cyclopentene 5 and
both furfural and heptanal gave a multicomponent mix-
ture, as assessed by 1H NMR spectroscopy (for the spec-
tra, see the Supporting Information), and the desired
product was not observed.
t-BuMe2SiO
O
OMe
quant.d
quant.d
quant.d
quant.d
OMe
t-BuMe2SiO
t-BuMe2SiO
O
OMe
4-MeOC6H4 OMe
MeO
O
OMe
4-EtC6H4
2-Tol
OMe
OMe
t-BuMe2SiO
O
When the cyclic silane 5 was added to a solution of the
precursor complex [FeCl28] activated with AgSbF6, the
silane decomposed over the course of several hours to cy-
clopentanone and siloxanes such as Me3Si–O–SiMe3, as
assessed by GC/MS and NMR spectroscopy (1H,
13C{1H}; for the spectra, see the Supporting Information).
Similar experiments were performed with the silanes 6
and 7, and the decomposition products were analyzed by
GC/MS (for the chromatograms, see the Supporting Infor-
mation). In these two cases, starting material was still
present in the reaction mixture. For the silyl enol ether 6,
the acetophenone decomposition product was observed in
addition to t-BuMe2Si–OH and other siloxanes. For the si-
lyl ketene acetal 7, besides some unreacted 7, only t-
BuMe2Si–OH was observed on GC/MS analysis. It ap-
pears reasonable to assume that the iron(III) complex de-
composes the silane substrate more rapidly than the
iron(II) complex, resulting in the lower isolated yields.
OMe
a Typical reaction conditions: aldehyde (0.36 mmol) and silyl enol
ether (0.54 mmol) in CH2Cl2 (2 mL) catalyzed by [Fe] (0.018 mmol,
activated with 0.036 mmol AgSbF6) at r.t. The products were isolated
by filtration through a short pad of silica gel.
b Utilizing the [8Fe(μCl)3FeCl3] precursor, 3.5 h reaction time.
c Utilizing the [FeCl28] precursor, 14–16 h reaction time.
d Virtually quantitative mass recovery; the products contained up to 5
mol% silane side products, as assessed by NMR spectroscopy.
temperature in CH2Cl2, catalyst loading 5 mol%). As can
be seen from the tables, the two silyl enol ethers 5 and 6,
as well as the silyl ketene acetal 7, were successfully em-
ployed as nucleophiles to couple with a variety of elec-
tron-rich and electron-poor aromatic aldehydes. The
products were obtained in 43% to virtually quantitative
yields isolated after filtration through a short pad of silica
gel to remove the catalyst.52
The oxidation state of the iron center also had an impact
on the diastereomeric ratios. As can be seen from Table 3,
the syn/anti ratios for the products ranged from 50:50 to
62:38 when iron(II) precursor complex [FeCl28] was em-
ployed as catalyst.53 These values indicate virtually no
simple diastereoselection between the two achiral sub-
strates. The syn/anti ratios ranged from 50:50 to 83:17
For the cyclic silyl enol ether substrate 5, both iron precur-
sor complexes were tested (Table 3), and syn/anti mix-
tures ranging from 50:50 to 83:17 were obtained, as
Synthesis 2014, 46, 57–66
© Georg Thieme Verlag Stuttgart · New York

PAPER
New Bis(imino)pyridine Complexes of Iron(II) and Iron(III)
63
when the iron(III) precursor complex [8Fe(μCl)3FeCl3] structurally. To the best of our knowledge, this is the first
was employed. Thus, the increased oxidation state of the time that a preformed iron(III) coordination compound
iron center resulted, on average, in slightly higher diaste- has been applied in the Mukaiyama aldol reaction. After
reomeric ratios. We ascribe this increase in selectivity to activation of the catalyst through chloride abstraction, the
the fact that the iron(III) center with the higher oxidation corresponding silyl-protected β-hydroxy carbonyl com-
state might create a tighter, more compact transition state, pounds were obtained in 43% to quantitative yields (room
resulting in better diastereoselection. The syn/anti ratios temperature, 3.5 to 16 h reaction time, 5 mol% catalyst
did not change when the reactions were performed at loading). The oxidation state of the iron center has an im-
–76 °C. Overall, the diastereoselectivities leave room for pact on the diastereoselectivity of the reaction; the reac-
improvement and are in the range of other Mukaiyama al- tion time was shorter (3.5 h) and the syn/anti ratios for the
dol reactions that are catalyzed by metal-centered, achiral products were somewhat higher when the iron(III) precur-
Lewis acids.18 The only report of iron(III)-catalyzed sor complex was employed. Ligand modification studies
Mukaiyama aldol reactions also determined syn/anti ra- to improve the diastereoselectivities and to potentially
tios around 85:15.36
employ chirality are currently underway.
For the other two, more reactive silyl substrates 6 and 7,
we observed no significant differences between the two
iron complexes, and only the results for one of the two
precursor complexes are listed (Table 4). The isolated
yields ranged from 43% to quantitative. We have em-
ployed the silyl ketene acetal 7 previously in Mukaiyama
aldol reactions; as already noted, it is highly nucleophilic
and it is, thus, not surprising that the yields are virtually
quantitative (Table 4, entries 4–7) for that substrate, as the
workup consists of only filtration through a short pad of
silica gel.
Solvents were treated as follows: Et2O, distilled from Na/benzophe-
none; CH2Cl2, distilled from CaH2; n-BuOH, distilled from Mg. Sil-
ica gel, CHCl3, AgSbF6 and FeCl3 (all Aldrich), FeCl2 (Strem) and
other materials were used as received. The ligand 843 and the silyl
enol ethers (cyclopent-1-en-1-yloxy)trimethylsilane (5)54 and tert-
butyl(dimethyl)[(1-phenylvinyl)oxy]silane (6)55 were prepared ac-
cording to the literature. All reactions were carried out under nitro-
gen employing standard Schlenk techniques, and the workup was
carried out in the air. NMR spectra were obtained at r.t. on a Bruker
Avance 300 MHz or a Varian Unity Plus 300 MHz instrument and
referenced to a residual solvent signal; all assignments are tentative.
GC/MS spectra were recorded on a Hewlett Packard GC/MS Sys-
tem Model 5988A. UV–vis spectra were recorded on a Varian Cary
50 Bio spectrophotometer. Exact masses were obtained on a JEOL
MStation [JMS-700] mass spectrometer. IR spectra were recorded
on a Thermo Nicolet 360 FT-IR spectrometer. Elemental analyses
were performed by Atlantic Microlab Inc., Norcross, GA, USA.
The iron complexes described herein provide another tun-
able platform for iron-based catalyst systems to be em-
ployed in the Mukaiyama aldol reaction. After activation,
the complexes are capable of coupling secondary (Table
3) and primary (Table 4) silyl enol ethers with aldehydes.
The bis(imino)pyridine ligand class is tunable, and it is
possible to employ chirality through the ligand. Still, the
iron complexes described herein perform better than
FeCl3 itself under the reaction conditions of Tables 3 and
Complex [FeCl28]
A solution of FeCl2 (0.025 g, 0.200 mmol) in n-BuOH (3 mL) was
added to a suspension of ligand 8 (0.085 g, 0.200 mmol) in n-BuOH
(3 mL) at once, and the mixture was heated at 80 °C. The solution
changed color from yellow to blue instantaneously. The reaction
4 in dichloromethane as a solvent. When FeCl3 was pre- mixture was stirred at 80 °C for another 15 min, then was concen-
trated to about 1 mL under high vacuum. A blue solid precipitated.
Anhyd pentane (5 mL) was added; the suspension was allowed to
stir for 10 min and the mother liquor was decanted. The solid was
washed several times with anhyd pentane (3 mL at a time), until the
supernatant was almost colorless. The blue solid was dried under
high vacuum for 2 d. The complex was obtained as a blue solid
(0.061 g, 0.111 mmol) in 55% isolated yield.
IR (neat solid): 1621 (m), 1585 (s, C=N), 1446 (s), 1372 (s) cm–1.
treated with AgSbF6 and employed in the reaction be-
tween cyclic silane 5 and 4-ethylbenzaldehyde (Table 3,
entry 1), a plethora of products formed, of which the de-
sired coupling product was a minor one, and thus is syn-
thetically of no value. The ligand provides a beneficial
effect on the catalytic performance of the iron(III) center.
The diastereoselectivities in Table 3 are not very high, but
it could be possible to improve them through ligand mod- MS (FAB, 4-NBA): m/z (%) = 551 (25) [M]+, 516 (100) [M – Cl]+,
426 (10) [8 + H]+.
ification (i.e., by placing bulky substituents in close prox-
HRMS–FAB: m/z [FeCl28]+ calcd for C29H3535Cl2N3Fe: 551.1557;
found: 551.1543; m/z [FeCl8]+ calcd for C29H3535ClN3Fe: 516.1869;
found: 516.1865.
imity to the coordinating nitrogen atoms). All efforts to
successfully employ a ketone substrate as carbonyl com-
pound utilizing the catalysts described herein have failed
thus far. However, through ligand modification, it might
be possible to make the iron(III) catalyst even more reac-
tive (e.g., through the introduction of more electron-
withdrawing substituents on the ligand).
UV/Vis (CH2Cl2): λmax (ε) = 698 (1164) nm.
Anal. Calcd for C29H35Cl2N3Fe: C, 63.06; H, 6.34. Found: C, 62.29;
H, 6.34.
Complex [8Fe(μCl)3FeCl3]
A solution of FeCl3 (0.038 g, 0.234 mmol) in Et2O (4 mL) was add-
ed to a solution of ligand 8 (0.100 g, 0.234 mmol) in anhyd Et2O (6
mL) dropwise over 5 min. An orange-red solid precipitated almost
immediately. The reaction mixture was stirred at r.t. for another 15
min, then the supernatant was decanted. The orange-red solid was
washed with Et2O (5 mL) several times, until the supernatant did not
show a yellow coloration, and then dried under high vacuum for 2
Herein, we have demonstrated for the first time that
iron(II) and iron(III) complexes of a bis(imino)pyridine li-
gand are catalytically active in the Mukaiyama aldol reac-
tion. The two iron(II) and iron(III) complexes [FeCl28]
and [8Fe(μCl)3FeCl3] of the bis(imino)pyridine ligand 8
were synthesized and characterized, one of them also
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 57–66

64
P. Shejwalkar et al.
PAPER
d. The product was obtained as an orange-red solid (0.067 g, 0.09
mmol) in 76% isolated yield.
IR (neat solid): 1626 (m), 1592 (s, C=N), 1370 (s), 1265 (s) cm–1.
based on the Laue symmetry using equivalent reflections. Crystal
data and intensity data collection parameters are listed in Table 1.
Structure solution and refinement were carried out using the
SHELXTL-PLUS software package.57 The structure was solved by
direct methods and refined successfully in the space group P21/c.
Full-matrix least-squares refinement was carried out by minimizing
Σ w(Fo2 – Fc2)2. The non-hydrogen atoms were refined anisotropi-
cally to convergence. All hydrogen atoms were treated using the ap-
propriate riding model (AFIX m3). The disorder in the FeCl3 unit
was resolved with partial occupancy Cl atoms and the disordered at-
oms were refined with geometrical and displacement parameter re-
straints and constraints. The structure refinement converged to the
residual values of R1 = 5.8% and wR2 = 14.9%. The final structure
refinement parameters are listed in Table 1.
MS (FAB, 4-NBA): m/z (%) = 551 (35) [FeCl28]+, 516 (100)
[FeCl8]+, 426 (15) [8 + H]+.
HRMS–FAB: m/z [FeCl28]+ calcd for C29H3535Cl2N3Fe: 551.1557;
found: 551.1578.
UV/Vis (CH2Cl2): λmax (ε) = 415 (1257), 506 (990) nm.
Anal. Calcd for C29H35Cl6N3Fe2: C, 46.44; H, 4.70. Found: C,
46.24; H, 4.81.
Catalysis Experiment (Table 3, Entry 1); Typical Procedure
The complex [FeCl28] (0.010 g, 0.018 mmol) was dissolved in
CH2Cl2 (0.5 mL), and in another flask AgSbF6 (0.012 g, 0.036
mmol) was dissolved in CH2Cl2 (0.5 mL). The solution of AgSbF6
was added to the solution of the metal complex and stirred at r.t. for
1 h. In a third flask, 4-ethylbenzaldehyde (0.049 g, 0.362 mmol) and
the silane 5 (0.085 g, 0.543 mmol) were mixed in CH2Cl2 (1 mL)
and stirred at r.t. for 3.5 h. The solution of the activated catalyst was
then filtered through Celite® directly into the solution of the sub-
strates, and the color of the solution turned to reddish. The reaction
mixture was stirred overnight, then filtered through a pad of silica
gel (3.5 cm). The silica gel was washed with CH2Cl2 (about 2 mL)
and the filtrate was concentrated. The yellow residue that was ob-
tained was 95% pure (1H NMR spectroscopy), and any further pu-
rification gave only decomposition products, and elution with
extremely nonpolar solvents like hexanes also yielded small
amounts of silyl impurities (about 5%). The product was obtained
in 95% purity as a yellow oil (0.093 g, 0.319 mmol) in 88% yield,
CCDC 950588 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment
We would like to thank the University of Missouri–St. Louis College
of Arts and Sciences Research Award for financial support. Fun-
ding from the National Science Foundation for the purchase of the
NMR spectrometer (CHE-9974801), the purchase of the Apex II
diffractometer (MRI, CHE-0420497) and the purchase of the mass
spectrometer (CHE-9708640) is acknowledged.
Supporting Information for this article is available online at
1
as a mixture of diastereomers (62:38) as determined by H NMR
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
r
itmnatr
spectroscopy.
1H NMR (300 MHz, CDCl3): δ = 7.23–7.09 (m, 6 H, 4 H + 2 H′,
Ar), 5.30 [d, 3JHH = 1.0 Hz, 1 H, CHOSi(CH3)3], 5.17 [d, 3JHH = 4.5
Hz, 0.6 H, CH′OSi(CH3)3], 2.68–2.59 (m, 4 H, CH2CH3 + CHCO,
CH2′CH3 + CH′CO), 2.29–1.99 (m, 11 H, CH2 + CH2′), 1.27–1.20
(m, 5 H, CH3 + CH3′), 0.06 (s, 5 H, OSiCH3′), 0.01 (s, 9 H,
OSiCH3).
13C{1H} NMR (75.4 MHz, CDCl3): δ = 223.4 (C′=O), 220.1 (C=O),
144.2, 142.8, 141.2, 138.9, 130.5, 128.1, 127.7, 127.5, 126.7, 126.6,
125.6, 75.2 [CHOSi(CH3)3], 72.5 [C′HOSi(CH3)3], 57.5 (CHCO),
55.5 (C′HCO), 39.8, 38.9, 28.7, 27.2, 24.8, 21.0, 20.6, 15.7, 15.6,
0.2 (SiCH3), 0.1 (SiC′H3).
References
(1) (a) Pansare, S. V.; Paul, E. K. Chem. Eur. J. 2011, 17, 8770.
(b) García, J. M.; Oiarbide, M.; Palomo, C. Angew. Chem.
Int. Ed. 2011, 50, 8790. (c) Palomo, C.; Oiarbide, M.;
García, J. M. Chem. Eur. J. 2002, 8, 37.
(2) (a) Mukaiyama, T.; Narasaka, K.; Banno, K. Chem. Lett.
1973, 1011. (b) Mukaiyama, T.; Banno, K.; Narasaka, K.
J. Am. Chem. Soc. 1974, 96, 7503.
(3) (a) Bielitza, M.; Pietruszka, J. Chem. Eur. J. 2013, 19, 8300.
(b) Abe, H.; Sato, A.; Kobayashi, T.; Ito, H. Org. Lett. 2013,
15, 1298. (c) Yadav, J. S.; Reddy, A. B.; Shankar, K. S.
Synthesis 2013, 45, 1034. (d) Sharpe, R. J.; Jennings, M. P.
J. Org. Chem. 2011, 76, 8027. (e) Brodmann, T.; Lorenz,
M.; Schäckel, R.; Simsek, S.; Kalesse, M. Synlett 2009, 174.
(4) (a) Cheon, C. H.; Yamamoto, H. Org. Lett. 2010, 12, 2476.
(b) Cheon, C. H.; Yamamoto, H. Tetrahedron 2010, 66,
4257. (c) Ratnikov, M. O.; Tumanov, V. V.; Smit, W. A.
Angew. Chem. Int. Ed. 2008, 47, 9739.
(5) Qiu, R.; Chen, Y.; Yin, S.-F.; Xu, X.; Au, C.-T. RSC Adv.
2012, 2, 10774.
(6) Röttger, D.; Erker, G.; Grehl, M.; Fröhlich, R.
Organometallics 1994, 13, 3897.
(7) (a) Ollevier, T.; Desyroy, V.; Debailleul, B.; Vaur, S. Eur. J.
Org. Chem. 2005, 4971. (b) Le Roux, C.; Gaspard-
Iloughmane, H.; Dubac, J.; Jaud, J.; Vignaux, P. J. Org.
Chem. 1993, 58, 1835.
(8) Ishihara, K.; Kondo, S.; Yamamoto, H. J. Org. Chem. 2000,
65, 9125.
MS (EI): m/z (%) = 216 (35) [M – Si(CH3)3 + H]+, 200 (60) [M –
OSi(CH3)3 + H]+, 135 (100) [M – Si(CH3)3 – C5H7O + H]+.
X-ray Structure Determination of [8ClFe(μO)FeCl3]
Crystals of appropriate dimension were obtained by slow vapor dif-
fusion of pentane into a CH2Cl2 solution of complex [FeCl28]. A
crystal with approximate dimensions 0.17 × 0.19 × 0.35 mm3 was
mounted on a MiTeGen cryoloop in a random orientation. Prelimi-
nary examination and data collection were performed using a Bruk-
er Kappa Apex II CCD (charge-coupled device) detector system
single-crystal X-ray diffractometer equipped with an Oxford Cryo-
stream LT device. All data were collected using graphite monochro-
mated Mo Kα radiation (λ = 0.71073 Å) from a fine-focus sealed
tube X-ray source. Preliminary unit cell constants were determined
with a set of 36 narrow frame scans. Typical data sets consist of
combinations of ω and Φ scan frames with typical scan width of
0.5° and counting time of 15 seconds/frame at a crystal-to-detector
distance of 4.0 cm. The collected frames were integrated using an
orientation matrix determined from the narrow frame scans. Apex
II and SAINT software packages56 were used for data collection and
data integration. Analysis of the integrated data did not show any
decay. Final cell constants were determined by global refinement of
xyz centroids of 9423 reflections from the complete data set. Col-
lected data were corrected for systematic errors using SADABS56
(9) Mikami, K.; Matsukawa, S.; Kayaki, Y.; Ikariya, T.
Tetrahedron Lett. 2000, 41, 1931.
(10) Kiyooka, S.; Matsumoto, S.; Kojima, M.; Sakonaka, K.;
Maeda, H. Tetrahedron Lett. 2008, 49, 1589.
(11) Nishiyama, Y.; Kaiba, K.; Umeda, R. Tetrahedron Lett.
2010, 51, 793.
Synthesis 2014, 46, 57–66
© Georg Thieme Verlag Stuttgart · New York

PAPER
New Bis(imino)pyridine Complexes of Iron(II) and Iron(III)
65
(12) (a) Sudha, R.; Sankararaman, S. J. Chem. Soc., Perkin
Trans. 1 1999, 383. (b) Reetz, M. T.; Fox, D. N. A.
Tetrahedron Lett. 1993, 34, 1119.
(13) Han, J. S.; Kim, S. B.; Mukaiyama, T. Bull. Korean Chem.
Soc. 1994, 15, 529.
(14) Olmos, A.; Alix, A.; Sommer, J.; Pale, P. Chem. Eur. J.
2009, 15, 11229.
(15) (a) Mlynarski, J.; Jankowska, J. Adv. Synth. Catal. 2005,
347, 521. (b) Evans, D. A.; Kozlowski, M. C.; Tedrow, J. S.
Tetrahedron Lett. 1996, 37, 7481.
(30) Blom, B.; Enthaler, S.; Inoue, S.; Irran, E.; Driess, M. J. Am.
Chem. Soc. 2013, 135, 6703.
(31) García Mancheño, O.; Dallimore, J.; Plant, A.; Bolm, C.
Org. Lett. 2009, 11, 2429.
(32) (a) Berkessel, A.; Reichau, S.; von der Höh, A.; Leconte, N.;
Neudörfl, J.-M. Organometallics 2011, 30, 3880. (b) Yang,
J.; Tilley, T. D. Angew. Chem. Int. Ed. 2010, 49, 10186.
(c) Flückiger, M.; Togni, A. Eur. J. Org. Chem. 2011, 4353.
(d) Cardoso, J. M. S.; Royo, B. Chem. Commun. 2012, 48,
4944.
(16) (a) Denmark, S. E.; Beutner, G. L.; Wynn, T.; Eastgate, M.
D. J. Am. Chem. Soc. 2005, 127, 3774. (b) Denmark, S. E.;
Beutner, G. L.; Wynn, T. J. Am. Chem. Soc. 2002, 124,
13405.
(17) Mukaiyama, T.; Matsui, S.; Kashiwagi, K. Chem. Lett. 1989,
993.
(18) (a) García-García, P.; Lay, F.; García-García, P.; Rabalakos,
C.; List, B. Angew. Chem. Int. Ed. 2009, 48, 4363.
(b) Heathcock, C. H.; Davidsen, S. K.; Hug, K. T.; Flippin,
L. A. J. Org. Chem. 1986, 51, 3027.
(19) Lee, J. M.; Helquist, P.; Wiest, O. J. Am. Chem. Soc. 2012,
134, 14973.
(33) (a) Fernandes, R. R.; Lasri, J.; Guedes da Silva, M. F. C.; da
Silva, J. A. L.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L.
J. Mol. Catal. A: Chem. 2011, 351, 100. (b) Moyer, S. A.;
Funk, T. W. Tetrahedron Lett. 2010, 51, 5430. (c) Balogh-
Hergovich, E.; Speier, G. J. Mol. Catal. A: Chem. 2005, 230,
79. (d) Hosseini-Monfared, H.; Näther, C.; Winkler, H.;
Janiak, C. Inorg. Chim. Acta 2012, 391, 75. (e) Comba, P.;
Wadepohl, H.; Wunderlich, S. Eur. J. Inorg. Chem. 2011,
5242. (f) Shaikha, T. M.; Hong, F. E. Adv. Synth. Catal.
2011, 353, 1491. (g) Wu, M.; Miao, C.-X.; Wang, S.; Hu, X.;
Xia, C.; Kühn, F. E.; Sun, W. Adv. Synth. Catal. 2011, 353,
3014. (h) Schröder, K.; Join, B.; Amali, A. J.; Junge, K.;
Ribas, X.; Costas, M.; Beller, M. Angew. Chem. Int. Ed.
2011, 50, 1425. (i) Gelalcha, F. G.; Anilkumar, G.; Tse, M.
K.; Brückner, A.; Beller, M. Chem. Eur. J. 2008, 14, 7687.
(j) Bruijnincx, P. C. A.; Buurmans, I. L. C.; Gosiewska, S.;
Moelands, M. A. H.; Lutz, M.; Spek, A. L.; van Koten, G.;
Klein Gebbink, R. J. M. Chem. Eur. J. 2008, 14, 1228.
(k) Oddon, F.; Girgenti, E.; Lebrun, C.; Marchi-Delapierre,
C.; Pécaut, J.; Ménage, S. Eur. J. Inorg. Chem. 2012, 85.
(l) Liu, S.-G.; Zhou, X.-T.; Ji, H.-B. Catal. Commun. 2013,
37, 60. (m) Clemente-Tejeda, D.; Lopez-Moreno, A.;
Bermejo, F. A. Tetrahedron 2013, 69, 2977. (n) Lenze, M.;
Bauer, E. B. Chem. Commun. 2013, 49, 5889. (o) Lenze, M.;
Martin, E. T.; Rath, N. P.; Bauer, E. B. ChemPlusChem
2013, 78, 101. (p) Christmann, M. Angew. Chem. Int. Ed.
2008, 47, 2740.
(20) Mahrwald, R. Chem. Rev. 1999, 99, 1095.
(21) Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36,
66.
(22) (a) Mei, Y.; Averill, D. J.; Allen, M. J. J. Org. Chem. 2012,
77, 5624. (b) Mei, Y.; Dissanayake, P.; Allen, M. J. J. Am.
Chem. Soc. 2010, 132, 12871. (c) Gu, Y.; Ogawa, C.;
Kobayashi, J.; Mori, Y.; Kobayashi, S. Angew. Chem. Int.
Ed. 2006, 45, 7217. (d) Kobayashi, S.; Nagayama, S.;
Busujima, T. J. Am. Chem. Soc. 1998, 120, 8287.
(23) (a) Lenze, M.; Sedinkin, S. L.; Rath, N. P.; Bauer, E. B.
Tetrahedron Lett. 2010, 51, 2855. (b) Tanabe, K. K.; Cohen,
S. M. Angew. Chem. Int. Ed. 2009, 48, 7424. (c) Jankowska,
J.; Paradowska, J.; Rakiel, B.; Mlynarski, J. J. Org. Chem.
2007, 72, 2228. (d) Jankowska, J.; Paradowska, J.;
Mlynarski, J. Tetrahedron Lett. 2006, 47, 5281. (e) Bach, T.;
Fox, D. N. A.; Reetz, M. T. J. Chem. Soc., Chem. Commun.
1992, 1634.
(34) Tondreau, A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H. M.;
Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. J. Am. Chem.
Soc. 2010, 132, 15046.
(24) (a) Plancq, B.; Ollevier, T. Aust. J. Chem. 2012, 65, 1564.
(b) Ollevier, T.; Plancq, B. Chem. Commun. 2012, 48, 2289.
(25) (a) Gopalaiah, K. Chem. Rev. 2013, 113, 3248. (b) Sun, X.;
Li, J.; Huang, X.; Sun, C. Curr. Inorg. Chem. 2012, 2, 64.
(c) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Chem. Rev. 2011, 111,
1293. (d) Czaplik, W. M.; Mayer, M.; Cvengroš, J.; Jacobi
von Wangelin, A. ChemSusChem 2009, 2, 396. (e) Sarhan,
A. A. O.; Bolm, C. Chem. Soc. Rev. 2009, 38, 2730.
(f) Bauer, E. B. Curr. Org. Chem. 2008, 12, 1341.
(g) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem. Int. Ed.
2008, 47, 3317. (h) Correra, A.; Garcia Mancheño, O.;
Bolm, C. Chem. Soc. Rev. 2008, 37, 1108. (i) Sherry, B. D.;
Fürstner, A. Acc. Chem. Res. 2008, 41, 1500. (j) Bolm, C.;
Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217.
(26) (a) Chen, Y.; Li, K.; Liu, X.; Zhu, J.; Chen, B. Synlett 2013,
24, 130. (b) Hata, T.; Iwata, S.; Seto, S.; Urabe, H. Adv.
Synth. Catal. 2012, 354, 1885. (c) Gülak, S.; Jacobi von
Wangelin, A. Angew. Chem. Int. Ed. 2012, 51, 1357. (d) Lin,
M.; Chen, X.-l.; Wang, T.; Yan, P.; Xu, S.-x.; Zhan, Z.-p.
Chem. Lett. 2011, 40, 111. (e) Midya, G. C.; Paladhi, S.;
Dhara, K.; Dash, J. Chem. Commun. 2011, 47, 6698.
(f) Mayer, M.; Czaplik, W. M.; Jacobi von Wangelin, A.
Adv. Synth. Catal. 2010, 352, 2147.
(35) (a) Lin, Y.-Y.; Wang, Y.-J.; Lin, C.-H.; Cheng, J.-H.; Lee,
C.-F. J. Org. Chem. 2012, 77, 6100. (b) Bézier, D.;
Venkanna, G. T.; Misal Castro, L. C.; Zheng, J.; Roisnel, T.;
Sortais, J.-B.; Darcel, C. Adv. Synth. Catal. 2012, 354, 1879.
(c) Enthaler, S. Eur. J. Org. Chem. 2011, 4760. (d) Wang,
C.; Li, X.; Wu, F.; Wan, B. Angew. Chem. Int. Ed. 2011, 50,
7162.
(36) Aoyama, N.; Manabe, K.; Kobayashi, S. Chem. Lett. 2004,
33, 312.
(37) Cámpora, J.; Ángeles Cartes, M.; Rodríguez-Delgado, A.;
Marcos Naz, A.; Palma, P.; Pérez, C. M. Inorg. Chem. 2009,
48, 3679.
(38) (a) Tondreau, A. M.; Stieber, C. E.; Milsmann, C.;
Lobkovsky, E.; Weyhermüller, T.; Semproni, S. P.; Chirik,
P. J. Inorg. Chem. 2013, 52, 635. (b) Tondreau, A. M.;
Milsmann, C.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem.
2011, 50, 9888.
(39) (a) Wang, S.; Li, B.; Liang, T.; Redshaw, C.; Lib, Y.; Sun,
W.-H. Dalton Trans. 2013, 42, 9188. (b) Smit, T. M.;
Tomov, A. K.; Britovsek, G. J. P.; Gibson, V. C.; White, A.
J. P.; Williams, D. J. Catal. Sci. Technol. 2012, 2, 643.
(c) Görl, C.; Englmann, T.; Alt, H. G. Appl. Catal., A 2011,
403, 25. (d) Appukuttan, V. K.; Liu, Y.; Son, B. C.; Ha, C.-
S.; Suh, H.; Kim, I. Organometallics 2011, 30, 2285.
(e) Tondreau, A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H.
M.; Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. J. Am.
Chem. Soc. 2010, 132, 15046. (f) Long, Z.; Wua, B.; Yang,
(27) Bonnamour, J.; Bolm, C. Org. Lett. 2011, 13, 2012.
(28) Han, W.; Mayer, P.; Ofial, A. R. Adv. Synth. Catal. 2010,
352, 1667.
(29) Haberberger, M.; Enthaler, S. Chem. Asian J. 2013, 8, 50.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 57–66

66
P. Shejwalkar et al.
PAPER
5319. (b) Li, F.; Wang, M.; Gao, A.; Chen, H.; Sun, L.
P.; Li, G.; Liu, Y.; Yang, X.-Y. J. Organomet. Chem. 2009,
694, 3793. (g) Bryliakov, K. P.; Talsi, E. P.; Semikolenova,
N. V.; Zakharov, V. A. Organometallics 2009, 28, 3225.
(h) Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.;
Tellmann, K. P.; White, A. J. P.; Williams, D. J. J. Chem.
Soc., Dalton Trans. 2002, 1159.
Dalton Trans. 2006, 2427.
(48) (a) Stynes, D. V.; Noglik, H.; Thompson, D. W. Inorg.
Chem. 1991, 30, 4567. (b) Weiss, H.; Strähle, J. Z.
Naturforsch., B 1984, 39, 1453.
(49) (a) Bhadbhade, M. M.; Srinivas, D. Polyhedron 1998, 17,
2699. (b) Chen, K.; Costas, M.; Que, L. Jr. J. Chem. Soc.,
Dalton Trans. 2002, 672.
(50) (a) Haselhorst, G.; Wieghardt, K.; Keller, S.; Schrader, B.
Inorg. Chem. 1993, 32, 520. (b) James, M.; Kawaguchi, H.;
Tatsumi, K. Polyhedron 1997, 16, 1873. (c) Cámpora, J.;
Naz, A. M.; Palma, P.; Rodríguez-Delgado, A.; Álvarez, E.;
Tritto, I.; Boggioni, L. Eur. J. Inorg. Chem. 2008, 1871.
(51) (a) Bart, S. C.; Chłopek, K.; Bill, E.; Bouwkamp, M. W.;
Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am.
Chem. Soc. 2006, 128, 13901. (b) Lu, C. C.; Bill, E.;
Weyhermüller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem.
Soc. 2008, 130, 3181.
(40) (a) Hoyt, J. M.; Sylvester, K. T.; Semproni, S. P.; Chirik, P.
J. J. Am. Chem. Soc. 2013, 135, 4862. (b) D’Souza, B. R.;
Lane, T. K.; Louie, J. Org. Lett. 2011, 13, 2936.
(c) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009,
131, 8772.
(41) (a) Eames, E. V.; Sánchez, R. H.; Betley, T. A. Inorg. Chem.
2013, 52, 5006. (b) Yang, M.; Kim, E.; Jeong, J. H.; Min, K.
S.; Lee, H. Inorg. Chim. Acta 2013, 394, 501. (c) Duncan, J.
S.; Zdilla, M. J.; Lee, S. C. Inorg. Chem. 2007, 46, 1071.
(d) Nienkemper, K.; Kotov, V. V.; Kehr, G.; Erker, G.;
Fröhlich, R. Eur. J. Inorg. Chem. 2006, 366. (e) Kreischer,
K.; Kipke, J.; Bauerfeind, M.; Sundermeyer, J. Z. Anorg.
Allg. Chem. 2001, 627, 1023.
(42) Schmidt, R.; Welch, M. B.; Knudsen, R. D.; Gottfried, S.;
Alt, H. G. J. Mol. Catal. A: Chem. 2004, 222, 9.
(43) Esteruelas, M. A.; López, A. M.; Méndez, L.; Oliván, M.;
Oñate, E. Organometallics 2003, 22, 395.
(44) Lenze, M.; Bauer, E. B. J. Mol. Catal. A: Chem. 2009, 309,
117.
(52) Some of the isolated products contained small amounts (~5
mol%) of silyl impurities, as assessed by 1H NMR
spectroscopy. All 13C{1H} NMR spectra showed baseline
purity.
(53) The syn/anti ratios were determined based on the 3JHH
coupling constants of the H–C–OSiR3 proton (syn: <1 Hz,
anti: ~4.5 Hz); see: Bifulco, G.; Dambruoso, P.; Gomez-
Paloma, L.; Riccio, R. Chem. Rev. 2007, 107, 3744.
(54) Lin, J.-M.; Liu, B.-S. Synth. Commun. 1997, 27, 739.
(55) de Nanteuil, G.; Waser, J. Angew. Chem. Int. Ed. 2011, 50,
12075.
(45) Görl, C.; Alt, H. G. J. Mol. Catal. A: Chem. 2007, 273, 118.
(46) (a) Shejwalkar, P.; Rath, N. P.; Bauer, E. B. Dalton Trans.
2011, 40, 7617. (b) Souza, V. R.; Rechenberg, H. R.;
Bonacin, J. A.; Toma, H. E. Spectrochim. Acta, Part A 2008,
71, 1296.
(56) Bruker Analytical X-ray, Madison, Wisconsin (USA), 2010.
(57) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
(47) (a) England, J.; Gondhia, R.; Bigorra-Lopez, L.; Petersen, A.
R.; White, A. J. P.; Britovsek, G. J. P. Dalton Trans. 2009,
Synthesis 2014, 46, 57–66
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