Synthesis of a new iron Lewis acid catalyst possessing a chelating cyclopentadienyldiphenylphosphinite ligand, (η5:η1-C5H4CH(Ph)OPPh2)

Article abstract of DOI:10.1016/S0022-328X(02)01257-3

A new iron Lewis acid catalyst, [(η⁵:η¹-C₅H₄CH(Ph)OPPh₂)Fe(CO)(THF)]⁺[BF₄]^- featuring a cyclopentadienyl ring that is tethered to a terminal diphenylphosphinite moiety

Full text of DOI:10.1016/S0022-328X(02)01257-3

Journal of Organometallic Chemistry 654 (2002) 202Á209
/
www.elsevier.com/locate/jorganchem
Synthesis of a new iron Lewis acid catalyst possessing a chelating
cyclopentadienyldiphenylphosphinite ligand, (h5:h1-
C5H4CH(Ph)OPPh2)
Michael F. Mayer, M. Mahmun Hossain *
Department of Chemistry, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA
Received 30 January 2002; received in revised form 6 February 2002; accepted 6 February 2002
Abstract
A new iron Lewis acid catalyst, [(h5:h1-C5H4CH(Ph)OPPh2)Fe(CO)(THF)]ꢀ[BF4]ꢁ featuring a cyclopentadienyl ring that is
tethered to a terminal diphenylphosphinite moiety was created. This is the first complex to contain a chelating h5:h1-
cyclopentadienyl-phosphinite ligand. This new racemic iron Lewis acid was tested and found to be an effective catalyst in an
aziridine-forming reaction. An asymmetric alcohol that is a precursor of the chiral catalyst was synthesized with greater than 95% ee.
Upon ligation of the diphenylphosphinite to the iron center, a 3:2 mixture of diastereomers was formed. The diastereomeric mixture
of the asymmetric catalysts was utilized in the aziridine-forming reaction and produced the cis-aziridine with low enantiomeric
excess. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: h5:h1-Cyclopentadienyl-phosphinite ligand; Iron Lewis acid catalyst; Aziridines
1. Introduction
synthesized and found to catalyze a cyclopropanation
reaction, but no enantioselectivity was observed [4]. The
lack of enantioselectivity from 6 was attributed to the
free rotation of the chiral cyclopentadienyl (Cp) moiety,
with respect to the catalytic binding site. Thus, a further
criterion for our new catalyst is to link a chiral Cp
moiety to the metal center in order to prevent rotation
of the Cp moiety. A linked design of this type is a
We have embarked on the synthesis of a new
asymmetric catalyst with the following characteristics;
(1) the metal is iron because it is abundant, inexpensive,
and relatively non-toxic, (2) it is based upon an achiral
complex, [(h5-C5H5)(CO)2Fe(THF)]ꢀ[BF4]ꢁ (1), that is
known to be effective for inducing a variety of transfor-
mations, and (3) the catalyst has a chiral metal center in
order to provide the most effective transfer of chirality.
Previously, the iron Lewis acid complex (1) has been
shown to catalyze a variety of reactions [1]. A similar
complex (2), that has a chiral iron center (Fig. 1), was
challenging endeavor [5Á7]. Herein, we report the
/
synthesis of a new asymmetric catalyst that embodies
the criteria discussed above.
found to be ineffective for inducing the DielsÁAlder
/
2. Results and discussion
reaction [2] due, in part, to the reduced Lewis acidity of
the complex. However, the more electron-deficient
The new synthesis started from the known methyl
complex, (h5-C5H5)Fe(CO)2CH3 (7) [8]. The Cp ring
was deprotonated and lithiated with s-BuLi. Initially,
the lithiated Cp complex was treated with acetyl
chloride; however, the reaction did not produce any
substituted Cp complexes. We assumed that the reaction
was unsuccessful due to the competitive deprotonation
of the a-protons of acetyl chloride; therefore, we
employed an acid chloride that possessed no a-protons.
analog (3) was found to catalyze the DielsÁAlder
/
reaction [2]. Other electron-deficient variants (4 and 5)
were also shown to be very effective enantioselective
DielsÁAlder catalyst [3]. The complex 6 (Fig. 1) was
/
* Corresponding author. Fax: ꢀ
/
1-414-229-5530.
E-mail address: mahmun@csd.uwm.edu (M.M. Hossain).
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 2 5 7 - 3

M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á
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203
Fig. 1. Selected iron Lewis acid catalysts.
Consequently, the lithiated Cp complex was treated with
benzoyl chloride. The reaction was successful (Eq. (1))
order to generate the alkoxide. Subsequent reaction of
the alkoxide with chlorodiphenylphosphine produced
the diphenylphosphinite complex 10 (Eq. (3)). Several
strongly basic reagents were initially employed (NaH, t-
BuLi, n-BuLi) but produced unsatisfactory results.
When the deprotonation was performed with s-butyl
lithium, most of the alcohol was converted to the desired
product. After the reaction, the solvent was removed
under reduced pressure and the resulting solids were
and a 60% yield of the benzoylated cyclopentadienylÁ
/
ironÁmethyl complex 8 was isolated after column
/
chromatography. This reaction was amenable to upscal-
ing on the order of several grams. In a gram scale
reaction, it was found to be crucial that the concentra-
tions of the reactants in solution were not increased,
otherwise, the product yield dramatically decreased. The
benzoylated product was stable to air-transfer and could
be stored under nitrogen at 0 8C for several months
without decomposition.
extracted with pentaneÁether. Evaporation of the sol-
/
vents resulted in an orange viscous liquid. Integration of
1
the H-NMR spectrum of the resulting liquid showed
that it consisted of 70% diphenylphosphinite complex
while the remainder was, in equal proportions, un-
reacted alcohol and one additional unidentified cy-
ð1Þ
clopentadienylÁ
/
ironÁmethyl by-product.
/
The ketone 8 was quantitatively reduced to alcohol 9
with five equivalents of NaBH4 (Eq. (2)). Column
chromatography was not necessary for purification of
this product. Instead, the resulting crude was dissolved
ð3Þ
into an etherÁwater mixture and the aqueous layer was
/
Photolysis of the terminally substituted diphenylpho-
sphinite complex 10 resulted in the formation of the first
complex to possess a chelating h5:h1-cyclopentadienyl-
linked diphenylphosphinite ligand (Eq. (4)), although
further extracted with ether. Upon evaporation of the
combined ether extracts under reduced pressure, the
pure alcohol 9 was obtained in a 98% yield. The alcohol
was stable and could be stored indefinitely under
nitrogen at 0 8C.
many similar h5:h1-cyclopentadienyl-linked diphenyl-
1
7]. The H-
phosphine ligands have been reported [5Á
/
NMR spectrum of the crude reaction mixture showed
that the desired product was present as a 1:1 pair of
diastereomers. Each diastereomer exhibited a signal
near 0 ppm that is characteristic of a methyl group
directly connected to iron in complexes of this type.
Each signal is split into a doublet due to the three-bond
coupling of the methyl protons to the phosphorus atom
across the iron center. The coupling constants of the two
ð2Þ
In the next step, the alcohol 9 was deprotonated in

204
M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á209
/
diastereomers are similar but not equivalent, 5.8 and 6.1
Hz, respectively.
asymmetric version of the catalyst was undertaken.
ð6Þ
ð4Þ
In order to introduce asymmetry into the new
synthetic scheme, an asymmetric reduction of the ketone
8 was chosen. The chiral oxazaborolidine developed by
E.J. Corey et al. (CBS catalyst) [10] seemed most
applicable for this particular substrate. The CBS cata-
lyst was prepared by a modified procedure reported by
Wright et al. [11].
Initially, this photolytic reaction produced a signifi-
cant amount of the by-product 6-phenylfulvene. In
order to suppress the formation of the by-product, the
volume of the reaction solution was increased and the
reaction solution was placed as close as possible to the
light source (400 W mercury vapor lamp). Under these
conditions, the reaction was over in about 20 min and a
negligible amount of the 6-phenylfulvene was formed.
In the final step of the new synthetic route, the methyl
The benzoyl complex 8 was treated with a THF
solution of the CBS catalyst in conjunction with BH3×
/
THF. The reaction was monitored by TLC and found to
take much longer (1 h) than many of the substrates
reported by Corey’s group (5 min). After the ketone was
consumed (monitored by TLC), a couple drops of water
were added and the reaction was stirred until the
solution ceased to bubble.
complex 11 was treated with tetrafluoroboric acidÁ
/
diethyletherate. The reaction mixture was quenched
with excess THF and the solvents were removed under
reduced pressure resulting in a green powder of racemic
iron Lewis acid 12 (Eq. (5)). The crude green product
was purified several times by recrystalization from
CH2Cl2ÁTHF.
/
ð7Þ
ð5Þ
After the reaction, the solvent was removed under
reduced pressure and the crude reaction mixture was
dissolved in diethyl ether. The solution was passed
through a plug of silica gel and dried over sodium
sulfate; the solvent was removed under reduced pressure
to provide the chiral alcohol 9 in 80% yield with 97% ee
(Eq. (7)).
The catalytic activity of the new racemic iron Lewis
acid complex (12) was tested in an aziridine forming
reaction (Eq. (6)). This transformation was chosen
because the achiral iron Lewis acid (1) is known to
catalyze the reaction of imines and diazo compounds to
form aziridines [1a]. Further, no general catalytic
asymmetric method exists to form aziridines specifically
from imines and diazo compounds [9]. Indeed, the Lewis
acid 12 was found to be an effective catalyst for the
formation of aziridines from aryl imines and ethyl
diazoacetate. The reaction of N-benzylidene aniline
with ethyl diazoacetate afforded 54% yield of cis-
aziridine. In another reaction, N-(4-nitrobenzylidene)a-
niline and ethyl diazoacetate were reacted in the
presence of the racemic Lewis acid (12) and produced
a 62% yield of cis and trans-aziridine in a 4:1 ratio.
These results were similar to those produced from the
achiral dicarbonyl iron Lewis acid catalyst (1) [1a].
Thus, knowing that the complex 12 is an effective
catalyst for aziridination reaction, the synthesis of an
The alcohol illustrated in Eq. (7) is assigned as the (R)
enantiomer. The stereochemistry of the alcohol was not
experimentally determined; rather, its stereochemistry
was based upon a transition state model [10] proposed
by Corey et al. as shown in Fig. 2.
Fig. 2. Transition state model of the enantioselective reduction.

M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á
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209
205
The asymmetric alcohol was subjected to the same
treatment as the racemic alcohol in order to convert it
into the diphenylphosphinite. The asymmetric termin-
ally functionalized diphenylphosphinite was photolyzed
as previously described in order to produce the chelated
complex (Eq. (8)). The metal center of this new complex
is additionally chiral. Upon ligation of the diphenylpho-
sphinite to the iron center, a 3:2 mixture of diastereo-
mers was formed in contrast to the 1:1 mixture that
resulted from photolysis of the racemic diphenylpho-
sphinite. Although disappointing, this result was not
unusual. In fact, a 3:2 mixture was also observed by
other groups when they synthesized analogous chelated
complexes with initial chirality in the Cp-P linker and
with secondary chirality at the metal center
[6].
ð10Þ
3. Conclusions
The new complex, [(h5:h1-C5H4CH(Ph)OPPh2)Fe-
(CO)(THF)]ꢀ[BF4]ꢁ (12), is the first to feature a
chelating h5:h1-cyclopentadienyl-linked-phosphinite li-
gand. Furthermore, while the metal was attached, the
chelating ligand was synthesized with high ee (ꢀ95%).
/
Upon ligation of the diphenylphosphinite to the iron
center, a 3:2 mixture of diastereomers was formed. The
diastereomeric mixture of the asymmetric catalysts was
utilized in the aziridine forming reaction produce the
cis-aziridine with low enantiomeric excess. The lack of
enantioselectivity may stem from the ability of the iron
Lewis acid to invert configuration at the metal center
after the labile THF ligand dissociates from the com-
plex. Thus, future work is aimed at replacing both CO
ligands of 1 with linkages to the Cp ring in order to
preclude inversion at the metal center.
ð8Þ
4. Experimental
4.1. General considerations
Since the separation of the diastereomers of the
chelated diphenylphosphinite complex was not achieved,
the mixture of diastereomers was employed in the final
step of the synthesis. The chelated complex was treated
with tetraflouroboric acid to produce the iron Lewis
All operations were performed under a dry nitrogen
atmosphere using standard Schlenk techniques. All
reaction vessels were flamed under vacuum and filled
with nitrogen prior to use. HPLC grade dichloro-
methane (EM Science) was distilled under nitrogen
from phosphorus pentoxide. Reagent grade benzene
(EM Science) and toluene (Mallinckrodt) were distilled
under nitrogen from sodium. Reagent grade ether (EM
Science) and THF (EM Science) were distilled under
nitrogen from the sodium-benzophenone ketyl. Techni-
cal grade pentane (Phillips) was mixed with concen-
trated sulfuric acid, washed with sodium bicarbonate,
washed with distilled water, dried over sodium sulfate,
and distilled from sodium. HPLC grade methanol (EM
Science) was distilled under nitrogen from magnesium
iodide. HPLC grade ethyl acetate (Spectrum) was dried
over 4A molecular sieves. Untreated reagent grade
hexanes (EM Science) were used for column chromato-
graphy. All other reagents were commercially available
and used as received. Flash chromatography was
acidÁTHF adduct (Eq. (9)). This product was recrys-
/
tallized several times and resulted in green powder
similar to the racemic one.
ð9Þ
The diastereomeric mixture of the asymmetric iron
Lewis acidÁTHF complexes was employed in the
/
catalytic reaction of ethyl diazoacetate and N-
benzylidene aniline. The reaction gave, mainly, the cis-
aziridine in 5% ee (Eq. (10)).
performed using EM Science silica gel 60, 70Á230
mesh. Thin-layer chromatography was performed using
/

206
M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á
/
209
1
400 mesh). H-NMR
EM Science F254 silica gel (230Á
/
solution to control the rapid evolution of bubbles. The
reaction was stirred for 90 min and then the solvent was
removed under reduced pressure. The crude solid was
dissolved in ether and transferred to a separatory funnel.
The ether layer was extracted three times with distilled
water and dried over Na2SO4. The solvent was removed
under reduced pressure to provide relatively pure
alcohol in 98% yield. Dissolving the crude alcohol in
ether and passing the solution through a pad of silica gel
spectrometry was performed using a Bruker 250 or a 300
MHz spectrometer. 13C-NMR spectrometry was per-
formed at 62.9 MHz. 31P-NMR spectrometry was
performed at 121.5 MHz. IR spectra were obtained as
described on a PerkinÁ
/
Elmer 1600 series FTIR. GCÁ
/
MS (EI) was performed at either 70 or 15 eV. Elemental
analysis was performed using a CE Instruments EA
1110 analyzer set up for carbon, hydrogen, and nitrogen
analysis.
further purified the alcohol. 1H-NMR (CDCl3): d 7.50Á
7.30 (m, 5H), 5.46 (d, Jꢂ3.2 Hz, 1H, CHOH), 4.95 (m,
1H), 4.65 (m, 2H), 4.61 (m, 1H), 2.15 (d, Jꢂ3.2 Hz, 1H,
CHOH), 0.19 (s, 3H). 13C-NMR (CDCl3): d 217.1,
142.7, 128.6, 128.2, 126.3, 84.6, 84.2, 83.1, 70.8, ꢁ22.4.
/
/
4.2. (h5-C5H4COPh)(CO)2FeCH3 (8)
/
A 3.46 g (18.0 mmol) sample of methyl complex 7 was
dissolved into 150 ml THF. The solution was cooled to
ꢁ78 8C. Over 20 min, 1.75 equivalents of 1.3 M s-
/
IR (CH2Cl2): nmax 3590, 2960, 2890, 2004, 1946, 1050
cmꢁ1. EIMS m/z (%): 298 (5) [Mꢀ], 270 (7) (Mꢀ-CO),
/
BuLi, 23.5 ml (30 mmol), were added to the THF
solution. A separate solution was prepared by adding
5.12 ml (43 mmol) of benzoyl chloride to 75 ml THF
242 (31) (Mꢀ-2CO), 227 (100) (Mꢀ-2CO, Ã
/
CH3), 153
CPhꢀ). HRMS Calc. for C15H14O3Fe
298.0292, Anal. Found: 298.0283.
(84) (C5H4Ä
/
and, subsequently, cooled to
ꢁ
78 8C. The (h5-
/
C5H4Li)(CO)2FeCH3 solution was transferred to the
BzCl solution drop-by-drop over 30 min through a
cannula. The combined solutions were stirred for 2 more
4.4. (h5-C5H4CH(OPPh2)Ph)(CO)2FeCH3 (10)
The alcohol complex 9, 680 mg (2.3 mmol), was
dissolved into 60 ml ether and the solution was cooled to
h at ꢁ78 8C. The solution was allowed to warm to
/
room temperature (r.t.) and was passed through a pad of
silica gel (prewetted with ether) under N2. The silica was
rinsed with 50 ml ether and the combined solutions were
evaporated under reduced pressure to provide a brown
oil. The crude product was separated on a silica gel
ꢁ
/
78 8C and 1.5 equivalents of 1.3 M s-BuLi, 2.63 ml
(3.4 mmol), was added to the alcohol solution. The
reaction was stirred at ꢁ78 8C for 24 h. Then 1.5
/
equivalents of ClPPh2, 0.65 ml (3.4 mmol), was added
and the reaction mixture was stirred for another 24 h at
column, eluting with 0Á/20% ether in pentane. The first
ꢁ78 8C. The solvent was removed under reduced
/
fraction contained unreacted methyl complex (7). The
1
second and third fractions were shown by H-NMR to
pressure. The resulting crude mixture was washed three
times with a mixture of pentane and ether (1:3). The
combined extracts were reduced under vacuum to oil.
The 1H-NMR spectrum showed, by integration, the
desired diphenylphosphinite complex, which comprised
greater than 70% of the crude extracts. This crude could
not be successfully purified further without causing
serious degradation. Due to the impurities and instabil-
contain the desired product with other impurities
(mostly unreacted benzoyl chloride). These last two
fractions were combined and chromatographed again on
the silica gel. The center of a brown band (fraction 2)
that came down when eluted with 8% ether in pentane
contained pure product, 3.2 g (60% yield). 1H-NMR
1
(CDCl3): d 7.79Á
7.45 (m, 2H), 5.43Á
Jꢂ
2.3 Hz, 2H), 0.26 (s, 3H). 13C-NMR (CDCl3): d
215.1, 192.2, 138.6, 132.3, 128.5, 128.1, 92.4, 90.8, 85.9,
/
7.76 (m, 2H), 7.60Á
/
7.55 (m, 1H), 7.50Á
/
ity, this complex was not fully characterized. H-NMR
/
5.41 (t, Jꢂ2.3 Hz, 2H), 4.98Á
/
/
4.96 (t,
(CDCl3): d 7.7Á
/
7.1 (m, 15H), 5.62Á
/
5.59 (d, Jꢂ/8.5 Hz,
/
1H, CHOPPPh2), 4.83 (d, Jꢂ/1.8 Hz, 1H), 4.59 (s, 2H),
4.49 (d, Jꢂ1.9 Hz, 1H), 0.09 (s, 3H). IR (toluene): nmax
/
ꢁ
/
20.7. IR (CH2Cl2): nmax 2968, 2896, 2016, 1961, 1650,
1455, 1294, 708 cmꢁ1. EIMS (15 eV) m/z (%): 296 (3)
3000, 2918, 2005, 1948, 1607, 1489, 1000, 690 cmꢁ1
.
[Mꢀ], 268 (23) (Mꢀ-CO), 240 (51) (Mꢀ-2CO), 225 (86)
4.5. (h5:h1-C5H4CH(OPPh2)Ph)(CO)FeCH3 (11)
(Mꢀ-2CO, Ã
/
CH3), 105 (54) (PhCOꢀ), 77 (42) (Phꢀ), 56
(100) (Feꢀ). HRMS Anal. Calc. for C15H12O3Fe
296.0136, Found: 296.0131. Anal. Calc. for
C15H12O3Fe: C, 60.84; H, 4.08. Found: C, 61.17; H,
4.14%.
The diphenylphosphinite complex 10 (400 mg) was
dissolved into 60 ml toluene in a pyrex photolytic
chamber surrounding a quartz, water-jacketed, 400 W
mercury vapor lamp. The apparatus was submerged into
a bucket of ice water. The solution was continually
bubbled with N2 gas throughout the reaction. The
solution (1 ml) was periodically removed and an IR
spectrum was obtained in order to monitor the extent of
the reaction. The solution was irradiated for a total of
10 h in order for the IR spectrum to show that the
4.3. (h5-C5H4CH(OH)Ph)(CO)2FeCH3 (9)
A 1.91 g (6.4 mmol) sample of benzoyl complex 8 was
dissolved into 50 ml of MeOH and cooled to 0 8C. The
NaBH4, 1.24 g (32 mmol), was slowly added to the

M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á
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209
207
reaction had gone at least halfway. The solvent was
removed under vacuum. An 1H-NMR spectrum
(CDCl3) was obtained of the crude product and showed
the two product diastereomers in addition to the by-
product, 6-phenylfulvene, but no starting material
remained. The crude product was separated on a silica
gel column with pentane. 6-Phenylfulvene: 1H-NMR
ether in order to remove the catalyst. Silica gel column
chromatography of the crude (2Á8% EtOAc in pentane)
provided 154 mg of the cis-aziridine (54%). No trans-
/
aziridine was isolated from the reaction.
4.8. Test for catalysis using [(h5:h1-
C5H4CH(OPPh2)Ph)(CO)Fe(THF)]ꢀ[BF4]ꢁ (12)
in the reaction of (N-4-nitrobenzylidene)aniline and ethyl
diazoacetate
(CDCl3): d 7.64Á
/
7.60 (m, 2H), 7.45Á/7.39 (m, 3H), 7.25
(s, 1H), 6.72Á6.68 (m, 2H), 6.54Á
/
/
6.52 (m, 1H), 6.37Á
/
6.33 (m, 1H). The two diastereomers were obtained and
characterized as a 1:1 mixture. Diastereomer 1: 1H-
A 31 mg (0.052 mmol, 5 mol%) sample of racemic
complex 12 was dissolved in 4 ml CH2Cl2. To the
catalyst solution, a 237 mg (1.03 mmol) sample of N-(4-
nitrobenzylidene)aniline was added. A separate solution
was created by diluting 0.150 ml at 90% (1.29 mmol)
EDA to 5 ml with CH2Cl2. The EDA solution was
NMR (CDCl3): d 7.85Á
Jꢂ7.8 Hz, 1H, CHOP), 4.74Á
(m, 1H), 4.20Á
/
7.10 (m, 15H), 5.89Á
/
5.86 (d,
4.50
0.19) (d, Jꢂ5.8
/
/
4.72 (m, 2H), 4.52Á
/
/
4.19 (m, 1H), ꢁ
/
0.17Á
/
(ꢁ
/
/
Hz, 3H, CH3). 31P-NMR (CDCl3, referenced to 80%
H3PO4): d 196.6. Diastereomer 2: 1H-NMR (CDCl3): d
7.85Á
CHOP), 4.83Á
4.50 (m, 2H), ꢁ
/
7.10 (m, 15H), 5.51Á
4.82 (m, 1H), 4.74Á
0.21Á(ꢁ0.23) (d, Jꢂ
/
5.50 (d, Jꢂ
4.72, (m, 1H), 4.52Á
6.1 Hz, 3H, CH3).
/
2.5 Hz, 1H,
added by syringe pump over 5 h to the imineÁcatalyst
/
/
/
/
solution and stirred for a total of 18 h at r.t. Then, 5 ml
THF was added to the reaction and the solution was
further stirred for 1 h. After the reaction, the crude was
passed through a silica gel plug and eluted with ether in
order to remove the catalyst. Silica gel column chroma-
/
/
/
/
31P-NMR (CDCl3, referenced to 80% H3PO4): d 195.0.
IR of 1:1 mixture (toluene): nmax 1918, 1083, 1000 cmꢁ1
.
4.6. [(h5:h1-C5H4CH(OPPh2)Ph)(CO)Fe-
(THF)]ꢀ[BF4]ꢁ (12)
tography (2Á8% EtOAc in pentane) of the crude mixture
/
provided 200 mg of the cis and trans-aziridine (62%
yield) in 4:1 ratio.
A 75 mg (0.165 mmol) sample of the complex 11 was
dissolved into 8 ml CH2Cl2 and cooled to ꢁ78 8C. A
/
4.9. (h5-C5H4CH(OH)Ph)(CO)2FeCH3 ((R)-9)
0.8 equivalent of HBF4, 0.02 ml at 54 wt.% in Et2O
(0.132 mmol), was added, and the CH2Cl2 solution
turned from orange to brown. The solution was stirred
The oxazaborolidine complex, 40 mg (0.145 mmol, 25
mol%), was dissolved in 2 ml THF and cooled to 0 8C.
for 30 min at ꢁ
/
78 8C; then 5 ml THF was added. The
A 0.145 ml (0.145 mmol, 1 M) sample of BH3×
/THF was
solution was stirred for an additional 30 min, the solvent
was removed under reduced pressure, leaving a dark
green solid. The crude product was purified three times
added to the oxazaborolidine solution and stirred for 5
min. A 172 mg (0.58 mmol) sample of 8 was dissolved in
3 ml THF. The solution of 8 and 0.29 ml (0.29 mmol)
by dissolving the solid in 3 ml THF and cooling to ꢁ
/
BH3×
/
THF were added together over 8 min to the
78 8C, then reprecipitating the complex by addition of 5
ml pentane. Satisfactory NMR spectra were not ob-
tained. IR (CH2Cl2): nmax 1991 (CO), 1695, 1424, 1272,
oxazaborolidine solution and stirred at 0 8C. The
reaction was monitored by TLC (silica gel plate, 20%
etherÁ
/
80% pentane); ketone Rfꢂ
/
0.34, alcohol Rfꢂ
/
1033, 712 cmꢁ1
.
0.25. After 105 min, the reaction appeared to be
complete. The solvent was removed under reduced
pressure and the crude was redissolved in 4 ml of moist
ether and dried over anhydrous Na2SO4. The super-
natant was filtered through glass wool and the solvent
removed under reduced pressure. Silica gel chromato-
4.7. Test for catalysis using [(h5:h1-
C5H4CH(OPPh2)Ph)(CO)Fe(THF)]ꢀ[BF4]ꢁ (12)
in the reaction of N-benzylidene aniline and ethyl
diazoacetate
graphy of the crude with 4Á14% ether in pentane
/
A 64 mg (0.106 mmol, 10 mol%) sample of racemic
complex 12 was dissolved in 4 ml CH2Cl2. To the
catalyst solution, a 197 mg (1.06 mmol) sample of N-
benzylidene aniline was added. A separate solution was
created by diluting 0.155 ml at 90% (1.33 mmol) ethyl
diazoacetate (EDA) to 5 ml with CH2Cl2. The EDA
solution was added by syringe pump over 5 h to the
provided 136 mg (80% yield) of chiral alcohol.
4.10. Chiral HPLC of the alcohol complex (h5-
C5H4CH(OH)Ph)(CO)2FeCH3 ((R)-9)
A chiral Regis (S,S)-Whelk-O-1 column was em-
ployed. Using a 1% isopropyl alcoholÁ99% hexanes
/
imineÁ
/
catalyst solution and stirred for a total of 19 h at
solvent system at a flow rate of 2 ml minꢁ1 and a
pressure of 1080 psi, the racemic alcohol complex was
resolved. The peaks for the (R) and (S)-enantiomers
(racemic) were baseline resolved with column retention
r.t. Then, 5 ml THF was added to the reaction and the
solution was further stirred for 1 h. The crude mixture
was passed through a silica gel plug and eluted with

208
M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á209
/
times of 7.52 and 8.49 min, respectively, and peak area
ratio of 49.876:48.838. The peaks for the asymmetric
alcohol sample, with the measured specific rotation of
19.58, came through at 7.59 and 8.65 min and had a
peak area ratio of 98.312:1.688, respectively, which
corresponds to an enantiomeric excess of 96.6%. From
most of the asymmetric reductions, only the one
enantiomer was detected using this technique. However,
occasional reductions provided the alcohol product with
a lower enantiomeric excess (as low as 85% ee).
4.15. Test for asymmetric catalysis using [(h5:h1-
C5H4CH(OPPh2)Ph)(CO)Fe(THF)]ꢀ[BF4]ꢁ
((RR),(SR)-12) in the reaction of N-benzylidene aniline
and ethyl diazoacetate
The asymmetric iron Lewis acidÁTHF adduct (3:2
/
mixture of diastereomers), 44 mg (0.073 mmol, 5 mol%),
was dissolved in 5 ml CH2Cl2. A 270 mg (1.46 mmol)
sample of N-benzylidene aniline was added to the
catalyst solution. An EDA solution (0.231 ml at 90%,
1.83 mmol in 5 ml CH2Cl2) was added by syringe pump
over 5 h to the imineÁcatalyst solution and stirred for a
/
4.11. Determination of the optical rotation of the
asymmetric alcohol complex ((R)-9)
total of 18 h at r.t. In order to remove the catalyst, the
crude was passed through a silica gel plug and eluted
with ether. The solvent was evaporated under reduced
pressure and the crude was separated by silica gel
In order to obtain stable optical rotation measure-
ments, a freshly purified sample was required. The
column chromatography (2Á8% EtOAc in pentane)
resulting in 166 mg of the cis-aziridine (42%).
/
sample was purified on a silica gel plug with pentaneÁ
/
ether (1:1) and the solvent was removed under reduced
pressure. A portion of the oil (20 mg) was dissolved into
freshly distilled methanol (1 ml) that was bubbled with
N2 gas for 20 min prior. The average rotation of ten
4.16. Chiral HPLC of cis-2-ethoxycarbonyl-1,3-
diphenylaziridine
measurements provided [a]2D5ꢂ
19.58.
/
4.12. (h5-C5H4CH(OPPh2)Ph)(CO)2FeCH3 ((R)-
10)
A chiral Regis (S,S)-Whelk-O-1 column was em-
ployed. Using a 1% isopropyl alcoholÁ99% hexanes
/
solvent system at a flow rate of 1 ml minꢁ1 and a
pressure of 460 psi, the racemic cis-2-ethoxycarbonyl-
1,3-diphenylaziridine was resolved. The peaks for the
two enantiomers of the racemate were baseline resolved
with column retention times of 16.89 and 19.00 min and
with a peak area ratio of 46.219:53.751, respectively.
The inequality in the peak areas most likely stems from
the fact that the peak widths are not equal. Utilizing
various conditions did not improve the balance in the
peak area ratio. Rather, in all cases the peak areas were
similar to this ratio and the second peak was consis-
tently shorter and wider than the peak with a shorter
retention time. In the determination of the enantiomeric
excess of the cis-aziridine derived from the asymmetric
iron Lewis acid catalyzed reaction, the above conditions
were utilized and the above peak ratio was used as the
standard for a racemic mixture. The cis-aziridine
product of the catalytic asymmetric reaction was run
and resulted in peaks at 16.71 and 18.80 min. The
respective peak areas were 36.16 and 46.05, which
translates to a 5% ee.
The reaction was carried out according to the same
procedure described for the racemic complex 10, using
0.894 g (3.0 mmol) of the complex R-9, 3.46 ml (4.5
mmol) of 1.3 M s-BuLi and 0.85 ml at 95% (4.5 mmol)
of chlorodiphenylphosphine at ꢁ
lowed by separation of the crude mixture by extraction
with pentaneÁether (1/3) resulted in 1.658 g (80%) of
complex (R)-10 and some unreacted alcohol (Â15%).
/
78 8C. Workup fol-
/
/
4.13. (h5:h1-C5H4CH(OPPh2)Ph)(CO)FeCH3
((RR),(SR)-11)
The reaction was carried out according to the same
procedure described for the racemic complex 11, using
1.658 (3.0 mmol) of the complex (R)-10. Workup
followed by separation of the crude mixture by column
chromatography at ꢁ78 8C resulted in 0.408 g (80%)
/
orange oil of diastereomers (RR)-11 and (SR)-11 in a
3:2 ratio.
4.14. [(h5:h1-C5H4CH(OPPh2)Ph)(CO)Fe-
(THF)]ꢀ[BF4]ꢁ ((RR),(SR)-12)
The reaction was carried out according to the same
procedure described for complex 12, using 0.286 g (0.63
Acknowledgements
mmol) and 0.07 ml at 54 wt.% in ether (0.50 mmol) at ꢁ
/
We gratefully thank the NIH for partial financial
support of this research project.
78 8C. Workup followed by recrystallization resulted in
a dry green powder of complexes (RR),(SR)-12.

M.F. Mayer, M.M. Hossain / Journal of Organometallic Chemistry 654 (2002) 202Á
/
209
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