New indenyl phosphinooxazoline complexes of iron and their catalytic activity in the Mukaiyama aldol reaction

Article abstract of DOI:10.1016/j.tetlet.2010.03.090

New phosphinooxazoline (PHOX) η⁵-indenyl complexes of iron were synthesized and applied as catalysts in the Mukaiyama aldol reaction. Reaction of three different PHOX ligands with [Fe(η⁵-Ind)I(CO)₂] afforded the iodide sal

Full text of DOI:10.1016/j.tetlet.2010.03.090

Tetrahedron Letters 51 (2010) 2855–2858
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New indenyl phosphinooxazoline complexes of iron and their catalytic activity
in the Mukaiyama aldol reaction
Matthew Lenze a, Sergey L. Sedinkin a, Nigam P. Rath b, Eike B. Bauer a,
*
a University of Missouri-St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
b Department of Chemistry and Biochemistry and Center for Nanoscience, University of Missouri-St. Louis, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
New phosphinooxazoline (PHOX)
lysts in the Mukaiyama aldol reaction. Reaction of three different PHOX ligands with [Fe(
afforded the iodide salts of three complexes of the general formula [Fe(
g
5-indenyl complexes of iron were synthesized and applied as cata-
5-Ind)I(CO)2]
5-Ind)(CO)(PHOX)]+ in 73–81%
Received 1 March 2010
Revised 13 March 2010
Accepted 19 March 2010
Available online 25 March 2010
g
g
isolated yields. The molecular structure of one of the new complexes was determined, revealing a pseudo
octahedral coordination geometry about the iron center. The iron complexes are catalytically active in the
Mukaiyama aldol reaction between aldehydes and 1-(tert-butyldimethylsilyloxy)-1-methoxyethene to
give the corresponding aldol adducts (3 mol % catalyst, 15 min, room temperature, 48–83% isolated
Keywords:
Iron complexes
Catalysis
Phosphinooxazolines
Mukaiyama aldol reaction
Indenyl ligand
yields). A previously synthesized iron complex of the general formula [Fe(g
5-Cp)(CO)(PHOX)]+ was found
to be catalytically active in the title reaction as well, but needed three hours at room temperature to con-
vert the starting materials to the products.
Ó 2010 Elsevier Ltd. All rights reserved.
Iron catalysis is a field of intensive research in organometallic
chemistry.1 Iron based catalyst systems have a number of advanta-
ges over other transition metals typically employed in catalysis.
Iron is cheap, abundant, nontoxic and environmentally friendly.
Its non-toxicity is especially important for applications in the phar-
maceutical industry.2
oxidation of benzylic methylene groups by t-BuOOH than iron
complexes of the general formula [Fe(PHOX)2]+OTfꢀ (3,
OTfꢀ ¼ CF3SOꢀ).7 We tentatively ascribed this difference in
3
reactivity to the fact that the latter group of complexes already
has open coordination sites available, whereas an open site must
first be created for the complexes 2.7
Consequently, iron catalysts have been applied in a number of
organic transformations, such as oxidations,3a,b reductions,3c car-
bon–carbon bond forming reactions,3d–f carbon–heteroatom bond
forming reactions,3g,h polymerizations3i in addition to other reac-
tions.3k,3l Typically in situ catalyst systems are applied,3a,3d but
preformed, well defined catalyst systems allow for easier investi-
gations of the catalytically active species and for mechanistic stud-
ies.3i In order for an iron complex to be catalytically active, open
coordination sites are required. They can be provided in situ by
combining a substoichiometric amount of the ligand and a metal
salt or by preformed complexes, in which coordination sites are
only loosely occupied by solvents or donating atoms of chelating
ligands.
We were interested in alternative ways to create open coordi-
nation sites on iron. The indenyl ligand C9Hꢀ (Ind) is known to eas-
7
ily undergo an
g
5 to
g
3 haptotropic shift in metal complexes,8 and
thus opening coordination sites at the metal (Scheme 1).
Consequently, indenyl complexes tend to be more active in li-
gand exchange reactions, than their cyclopentadienyl analogs
(such as 2 in Fig. 1), sometimes referred to in the literature as
the ‘indenyl effect’.8,9 The indenyl effect has been described to par-
ticipate in catalytic reactions.10 We thus set out to synthesize iron
I-
X
+
Phosphinooxazolines (PHOX, 1, Fig. 1) which represent a biden-
tate ligand class,4 have widely been applied in transition metal cat-
alyzed organic transformations.5 We have recently synthesized
and structurally characterized the first chelating iron PHOX com-
plexes 2 (Fig. 1).6 These complexes are coordinatively saturated,
but more significantly, they showed less catalytic activity in the
R2
Fe
O
OC
R1
N
Ph2P
PPh2
N
R2
R1
O
2
1
X = H, R1 = R2 = CH3, a
X = H, R1 = H, R2 = Ph, b
X = CF3, R1 = R2 = CH3, c
R1 = R2 = CH3, a
R
R
1 = H, R2 = Ph, b
1 = R2 = H, d
* Corresponding author. Fax: +1 314 516 5342.
E-mail address: bauere@umsl.edu (E.B. Bauer).
Figure 1. Phosphinooxazoline ligands (1) and iron complexes (2) thereof.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.090

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M. Lenze et al. / Tetrahedron Letters 51 (2010) 2855–2858
two diastereomers can form during synthesis. The diastereomeric
ratio of the isolated material of 4b was determined to be 94:6, as
ring slip
assessed by NMR (1H, 31P).
Fe
Fe
L
L
L
L
To unequivocally establish the structures of the new iron com-
plexes, an X-ray structure for 4a was determined (details are given
in Supplementary data).15 The molecular structure is shown in Fig-
ure 2, along with key structural data. Bond length and angles are
similar to those of the structurally related complex 2a.6 The molec-
ular structure of 4a confirms the piano stool type coordination
geometry around the iron, in which one of the carbonyl ligands
L
L
η5 coordination
η
3 coordination
open coordination
site at the metal
Scheme 1. The indenyl effect.
and the iodo ligand in the precursor complex [Fe(g
5-Ind)I(CO)2]
3 is substituted by the PHOX ligand 1a. The bond angles around
iron range from 83.76(11)° for the N(1)–Fe–P(1) angle to
99.61(18)° for the C(10)–Fe–N(1) angle. Thus, the coordination
geometry of the complexes is best described as slightly distorted
octahedral.
PHOX indenyl complexes and anticipated that these might act as
gentle Lewis acids in solution. The Mukaiyama aldol reaction is a
Lewis acid catalyzed coupling reaction of silyl enol ethers and car-
bonyl compounds.11 Accordingly, the catalytic activity of the new
iron complexes in the Mukaiyama aldol reaction was tested.
First, synthetic access to indenyl PHOX iron complexes was tar-
We next employed the new complexes as catalysts for initial
screening in the Mukaiyama aldol reaction of aldehydes (5) and
1-(tert-butyldimethylsilyloxy)-1-methoxyethene (6), which exhib-
its enhanced reactivity, to obtain the product 7 (Table 1). The title
reaction proceeded only in acetonitrile as the solvent, and only
trace amounts of products were detected, when diethyl ether,
THF, CH2Cl2 and toluene solvents were employed. For complexes
4, after a 15 min reaction time at room temperature, only traces
of the aldehyde starting material were detected by GC/MS, and
the products were observed in about 50–90% yields. Complex 2a
(Fig. 1) was employed as a catalyst as well; most significantly, it
showed decreased catalytic activity in the title reaction (Table 1,
entries 1 and 2). The precursor complex 3 showed slightly less cat-
alytic activity than the iron PHOX complexes in the test reaction in
Table 1 (entry 3). Aliphatic aldehydes formed a number of uniden-
tified side products (Table 1, entry 6); only butyraldehyde exhib-
ited a clean reaction to the corresponding aldol adduct (Table 1,
entry 5).
geted. Accordingly, when the known complex [Fe(
g
5-Ind)I(CO)2]
(3)12 was refluxed with one equivalent of the PHOX ligand 1a13
in pentane for 3 h, the iodide salt of the complex [Fe(
g
5-In-
d)(CO)(1a)]+ (4a) was isolated in 81% yield as a light red solid after
washing with diethyl ether (Scheme 2). Applying similar condi-
tions with ligands 1b and 1c gave the iodide salts of the complexes
[Fe(g g
5-Ind)(CO)(1b)]+ (4b) and [Fe( 5-Ind)(CO)(1c)]+ (4c) as or-
ange-red solids, both in 73% isolated yield (Scheme 2).
The new iron complexes 4a, 4b and 4c were characterized by
NMR (1H, 13C, 31P), MS (including HRMS), and IR. The coordination
of the PHOX ligand was best seen by the large downfield shift of
the phosphorus signal in the 31P NMR spectrum. The free ligands
have chemical shifts of around ꢀ4.5 ppm, whereas the iron com-
plexes exhibited signals between 61.3 and 67.0 ppm. The cyclopen-
tadienyl portion of the coordinated indenyl ligand shows
a
characteristic pattern of signals in the 1H and 13C NMR spectra.14
The three protons gave distinct signals between 5.70 and
4.88 ppm in the 1H NMR spectrum and the five carbon atoms gave
distinct signals between 110.0 and 64.5 ppm in the 13C NMR spec-
trum. The IR spectra also clearly show coordination of the ligands.
The free ligands 1a, 1b and 1c show characteristic tC=N stretching
frequencies between 1630 and 1655 cmꢀ1, respectively,6 which
shift significantly to lower wavenumbers (1607 to 1617 cmꢀ1),
when coordinated to the iron center. The IR spectra of each of
the complexes also showed a single tCꢁO stretch between 1941
and 1959 cmꢀ1, as expected for an iron complex with a single car-
bonyl ligand. The carbonyl ligand was also observed in the 13C
NMR spectra of the complexes and gives a distinct doublet be-
tween 217.6 and 221.2 ppm (2JCP = 29.4 to 28.0 Hz) as previously
observed in structurally related iron complexes.6 The FAB MS spec-
tra showed molecular ion peaks for the new complexes as well as
ions for CO loss.
Acetophenone reacted much slower under the conditions in Ta-
ble 1 and gave only an inseparable mixture of products (Table 1,
entries 7 and 8).
Next, the catalyst 4a was applied in a number of reactions of
aromatic aldehydes 5 with 1-(tert-butyldimethylsilyloxy)-1-meth-
oxyethene 6 and the corresponding aldol products 7 were isolated
The resulting complexes 4 are stereogenic at the metal. Conse-
quently, for the achiral ligands 1a and 1c, racemic mixtures of the
complexes 4a and 4c were isolated. Ligand 1b is chiral, and thus,
-
+
I
ligand 1
R2
X
Fe
Fe
I
OC
R1
pentane
OC
N
Ph2P
reflux
CO
O
5 - 12 h
4
3
Figure 2. Molecular structure of 4a (depicted with 65% probability ellipsoids; H
atoms and the acetone solvate are omitted for clarity). Key bond lengths (Å) and
bond angles (°): Fe–C10, 1.746(5); Fe–N1, 1.999(4); Fe–P1, 2.2091(12); N1–C13,
1.285(6); Fe–C2, 2.100(4); Fe–C3, 2.080(4); Fe–C4, 2.100(4); Fe–C1, 2.241(4); Fe–
C5, 2.228(4); C10–O2, 1.148(6); C10–Fe–N1, 99.61(18); N1–Fe–P1, 83.76(11); C10–
Fe–P1, 95.18(14); O2–C10–Fe, 174.1(4).
a,
X = H, R1 = R2 = CH3, 86%
X = H, R1 = H, R2 = Ph, b, 73%
X = CF3, R1 = R2 = CH3, c, 73%
Scheme 2. Iron indenyl PHOX syntheses.

M. Lenze et al. / Tetrahedron Letters 51 (2010) 2855–2858
2857
in 48–83% yields (Table 2). The catalytic performance of complexes
4a and 4c were comparable, but complex 4a was used for further
studies, as its ligand 1a is synthetically more easily accessed than
the other ligands.
A variety of aromatic substitution patterns in the aldehyde is
compatible with the catalyst. The catalysts loads were 3% and the
turnover frequencies ranged from 64 to 106 hꢀ1. The new iron
indenyl complexes are thus highly active in the title reaction.
Nishiyama recently revealed that 8 mol % of the rhenium complex
Re(CO)5Br catalyzes the conversion of the silyl enol ether 6 and
aromatic aldehydes to the corresponding aldol products (1 h,
80 °C, 50–80% isolated yields).16 Mukaiyama reported that
10 mol % of an InCl3ꢂAgClO4 mixture is catalytically active in the
reaction between 6 and benzaldehyde (2 h, ꢀ78 °C, 96% isolated
yield).17 Reetz described the cationic complex [Fe(Cp)(dppe)(CO)]+
as a catalyst in the reaction between 6 and benzaldehyde after
photolytic activation (2.5 mol % catalyst, 20 h, ꢀ20 °C, 77%,
dppe = diphenylphosphinoethane).18 The catalytic activity of the
new indenyl complexes 4 described herein is, thus, comparable
to other transition metal complexes that catalyze the reactions
shown in Table 2.
The complex 4b was isolated in high diastereomeric purity (1H,
31P NMR). The Mukaiyama aldol reaction in Table 2 produces chiral
aldol products 7. We anticipated that the chiral complex 4b could
be catalytically active in enantioselective formation of the prod-
ucts. However, essentially no enantiomeric excess was observed
for the reaction of benzaldehyde with 1-(tert-butyldimethylsilyl-
oxy)-1-methoxyethene 6 (Table 2, entry 1).
Table 1
Initial Screening of Iron catalyzed Mukaiyama aldol reactions
catalyst
In conclusion, the present study shows for the first time the cat-
alytic activity of new iron indenyl PHOX complexes of the general
t-BuMe2SiO
O
OMe
O
OMe
conditions
rt
OSiMe2t-Bu
formula [Fe(g
5-Ind)(CO)(PHOX)]+ in the Mukaiyama aldol reaction
+
R
H
R
of 1-(tert-butyldimethylsilyloxy)-1-methoxyethene and a variety
of aromatic aldehydes to give the corresponding aldol products
in 48–83% isolated yields. The new complexes show high activity,
as the reaction proceeds at room temperature within 15 min. The
substitution of a Cp ligand by an indenyl ligand enhanced the cat-
alytic activity in the [Fe(L)(CO)(PHOX)]+ system. Ligand and metal
complex modifications to improve catalytic activity for other silane
substrates and to obtain enantiomeric excesses are currently
underway.
7
5
6
Entry
Aldehyde
Catalyst
Conditionsa
Yieldb (%)
1
2
3
4
5
6
7
8
R = Ph
R = Ph
R = Ph
R = Ph
R = CH3(CH2)4
2-Ethylbutanal
Acetophenone
Acetophenone
2a
2a
3
CH3CN, 25 min
CH3CN, 3 h
CH3CN, 15 min
CH3CN, 15 min
CH3CN, 5 h
CH3CN, 5 h
CH3CN, 20 min
CH3CN, 5 h
3
61
89
90
48
0d
5
4ac
4a
4a
4a
4a
50
Acknowledgments
a
Conditions: The aldehyde (0.377 mmol) was dissolved in the solvent (1.0 mL).
The catalyst (0.011 mmol) was added, followed by 1-(tert-butyldimethylsilyloxy)-
1-methoxyethene (0.377 mmol). After the time indicated, the reaction mixture was
filtered through a short pad of silica gel and analyzed by GC/MS. All reactions were
carried out at room temperature.
We thank the University of Missouri-St. Louis for support. Fund-
ing from the National Science Foundation for the purchase of the
NMR spectrometer (CHE-9974801), for the ApexII diffractometer
(MRI, CHE-0420497), and the purchase of the mass spectrometer
(CHE-9708640) is acknowledged.
b
Determined by GC/MS.
Complexes 4b and 4c showed comparable activity.
c
d
No aldol product was obtained, but unidentified side products were observed,
presumably due to elimination reactions.
Supplementary data
Table 2
Crystallographic data (excluding structure factors) for the struc-
ture in this paper has been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication CCDC 766377.
Copies of the data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk). Experimental details,
characterization data and 1H and 13C NMR spectra for ligand 1c,
all new complexes and for the catalysis products in Table 2; exper-
imental details and crystallographic data for the X-ray determina-
tion of 4a are available. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.03.090.
Iron catalyzed Mukaiyama aldol reaction
cat. 4aa
R1
O
t-BuMe2SiO
OMe
O
R1
R2
R3
OMe
CH3CN
R2
R3
H
+
rt, 15 min
R4
OSiMe2t-Bu
R4
5
6
7
c
Entry
Aldehyde
R1 = R2 = R3 = R4 = H, a
Yieldb (%)
TOF/hꢀ1
1
2
3
4
5
6
7
8
9
74d
59
62
78
70
83
76
64
70
79
48
54
98
78
80
101
91
104
101
86
93
106
64
R
R
R
R
1 = R2 = R4 = H, R3 = NO2, b
1 = OMe, R2 = R3 = R4 = H, c
2 = OMe, R1 = R3 = R4 = H, d
3 = OMe, R1 = R2 = R4 = H, e
R1 = Cl, R2 = R3 = R4 = h, f
References and notes
R
R
R
R
R
3 = CH3, R1 = R2 = R4 = H, g
1 = R3 = R4 = CH3, R2 = H, h
1 = CH3, R2 = R3 = R4 = H, i
2 = CH3, R1 = R3 = R4 = H, k
1 = R2 = R4 = H, R3 = Et, I
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11
12
Butyraldehyde, m
71
a
Conditions: The aldehyde 5 (0.377 mmol) was dissolved in CH3CN (1.0 mL). The
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methoxyethene (6, 0.377 mmol). After 15 min at rt, the solvent was removed and
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R
indices R1
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