Cyclopentadienyl 1,2- and 1,3-disubstituted cobalt sandwich compounds {η5-[MeOC(O)]2C5H3} Co(η4-C4Ph4): Precursors for sterically hindered bidentate chiral and achiral ligands

Article abstract of DOI:10.1021/om300027c

Reaction of the sodium cyclopentadienyl Na{C₅H ₄[C(O)OMe]} with methyl chloroformate, ClC(O)OMe, in a 2:1 molar ratio was found to result in a mixture of sodium salts of 1,2- and 1,3-dicarbomethoxycyclopentadienyls. This mixture, on refluxing in toluene with CoCl(PPh₃)₃ and diphenylacetylene, resulted in the formation of cyclopentadienyl 1,3- and 1,2-diester derived cobalt sandwich compounds {η⁵-[MeOC(O)]₂C₅H ₃}Co(η⁴-C₄Ph₄) (1, 2) in 85% yield. 1,3- and 1,2-diesters 1 and 2 were converted to the dicarboxylic acids 3 and 4 by refluxing with aqueous KOH in ethanol. The diacyl chloride of 4 was generated in situ by the reaction of 4 with oxalyl chloride and this, on further reaction with ferrocene under Friedel-Crafts conditions, yielded the novel bis-metallocenyl acenequinone 5, having both the iron and cobalt sandwich units in the same compound. The dicarboxylic acid 3 on reaction with oxalyl chloride followed by (S)-2-amino-3-methyl-1-butanol, triethylamine, and mesyl chloride was converted to the novel 1,3-bis(oxazoline) cyclopentadienyl-derived bidentate chiral complex [η⁵-1,3-(4-iPr-2-Ox)₂C ₅H₃]Co(η⁴-C₄Ph₄) (6; Ox = oxazolinyl). Reaction of 6 with Pd(OAc)₂ in acetic acid at 95 °C resulted in the formation of the chiral palladium complex 7. The utility of this palladium complex as a chiral catalyst for the asymmetric rearrangement of trichloroacetimidates to trichloroacetamides has been evaluated.

Full text of DOI:10.1021/om300027c

Article
pubs.acs.org/Organometallics
Cyclopentadienyl 1,2- and 1,3-Disubstituted Cobalt Sandwich
Compounds {η5-[MeOC(O)]2C5H3} Co(η4-C4Ph4): Precursors for
Sterically Hindered Bidentate Chiral and Achiral Ligands
Nem Singh and Anil J. Elias*
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India
S
* Supporting Information
ABSTRACT: Reaction of the sodium cyclopentadienyl Na{C5H4[C(O)OMe]} with methyl chloroformate, ClC(O)OMe, in a
2:1 molar ratio was found to result in a mixture of sodium salts of 1,2- and 1,3-dicarbomethoxycyclopentadienyls. This mixture,
on refluxing in toluene with CoCl(PPh3)3 and diphenylacetylene, resulted in the formation of cyclopentadienyl 1,3- and 1,2-
diester derived cobalt sandwich compounds {η5-[MeOC(O)]2C5H3}Co(η4-C4Ph4) (1, 2) in 85% yield. 1,3- and 1,2-diesters 1
and 2 were converted to the dicarboxylic acids 3 and 4 by refluxing with aqueous KOH in ethanol. The diacyl chloride of 4 was
generated in situ by the reaction of 4 with oxalyl chloride and this, on further reaction with ferrocene under Friedel−Crafts
conditions, yielded the novel bis-metallocenyl acenequinone 5, having both the iron and cobalt sandwich units in the same
compound. The dicarboxylic acid 3 on reaction with oxalyl chloride followed by (S)-2-amino-3-methyl-1-butanol, triethylamine,
and mesyl chloride was converted to the novel 1,3-bis(oxazoline) cyclopentadienyl-derived bidentate chiral complex [η5-1,3-(4-
iPr-2-Ox)2C5H3]Co(η4-C4Ph4) (6; Ox = oxazolinyl). Reaction of 6 with Pd(OAc)2 in acetic acid at 95 °C resulted in the
formation of the chiral palladium complex 7. The utility of this palladium complex as a chiral catalyst for the asymmetric
rearrangement of trichloroacetimidates to trichloroacetamides has been evaluated.
been quite facile and excellent chiral catalysts for enantiose-
lective aza-Claisen rearrangements have resulted from such
studies.4,9,10
INTRODUCTION
■
Stable organometallic sandwich compounds having one steri-
cally bulky cyclobutadienyl or cyclopentadienyl ring (e.g.,
C4Ph4, C5Ph5) have recently attracted considerable interest
from the perspective of synthesizing novel achiral as well as
chiral ligands.1,2 Preparation of catalysts for high-yielding and
highly enantioselective reactions ranging from palladium-
catalyzed cross-coupling reactions to aza-Claisen rearrange-
ments have been reported using such ligands.3,4 Among such
sandwich compounds, (η5-Cp)Co(η4-C4Ph4) has been the
choice for many chemists, due to the relative ease of its
synthesis and excellent air and moisture stability.5,6 Sustained
interest in this compound is indicated by the fact that improved
protocols are currently being reported for the synthesis of a
range of its analogues.7 Although very similar to ferrocene in
stability, the relatively poor reactivity of the Cp ring makes
cyclopentadienyl derivatization of (η5-Cp)Co(η4-C4Ph4) quite
difficult. For example, lithiation of the Cp ring required for the
synthesis of the first planar chiral bidentate ligand based on this
sandwich compound, [η5-1,2-(PPh2)(SBut)C5H3]Co(η4-
C4Ph4), was made possible only by using the superbase t-
BuLi/KOBut, also in poor yields.8 However, the syntheses of
palladacycles by ortho palladation of the oxazoline-derived (η5-
Cp)Co(η4-C4Ph4) and its cyclobutadiene linked dimer have
Disubstitution on a Cp ring of sandwich compounds has
been an elegant route for making achiral as well as planar chiral
bidentate ligands. While many such ligands are known with
ferrocene,11 only a handful of examples having 1,2- and 1,3-
disubstitution on the Cp ring of (η5-Cp)Co(η4-C4Ph4) have
been reported so far.12 Most of these have a methyl group as
one of the Cp substituents, which limits the range of their
utility. Richards and co-workers have very recently reported a
reaction of (η5-Cp)Co(η4-C4Ph4) with mercuric acetate
followed by iodine to generate the difunctional 1,2- and 1,3-
diiodo derivatives (η5-C5H3I2)Co(η4-C4Ph4), albeit in moderate
yields, with the 1,2-isomer as the major product.13 In this paper
we report an easy and straightforward method for the synthesis
of cyclopentadienyl 1,2- and 1,3-diester derivatives of (η5-
Cp)Co(η4-C4Ph4) in good overall yields and also report the
synthesis of useful difunctional derivatives from the same. We
have attempted to maximize the yields of the cyclopentadienyl
1,3-diester derivative and report herein the synthesis of a chiral
Received: January 13, 2012
Published: February 22, 2012
© 2012 American Chemical Society
2059
dx.doi.org/10.1021/om300027c | Organometallics 2012, 31, 2059−2065

Organometallics
Article
Table 1. X-ray Crystal Structure Parameters of Compounds 1, 2, 5, and 6
param
1
2
5
6
formula
mol wt
cryst syst
space group
a (Å)
C37H29CoO4
596.53
C37H29CoO4
596.53
monoclinic
P21/c
C45H31CoFeO2
718.48
C45H43CoN2O2
702.74
orthorhombic
P212121
13.958(2)
15.306(2)
17.085(2)
90
triclinic
triclinic
P1
P1
̅
̅
10.440(2)
11.605(2)
13.574(2)
91.814(3)
102.502(3)
113.468(3)
1459.9(4)
2
8.132(1)
20.642(3)
17.443(3)
90
11.079(3)
12.484(3)
13.700(3)
116.292(4)
90.592(5)
103.413(5)
1638.5(7)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
95.169(3)
90
90
90
2915.9(8)
4
3650.1(7)
4
T/K
298
298
298
298
λ (Å)
0.710 73
1.357
0.710 73
1.359
0.710 73
1.456
0.710 73
1.279
ρcalcd (g/cm3)
μ (mm−1
)
0.628
0.629
0.989
0.511
goodness of fit
θ range (deg)
1.199
1.317
1.018
1.184
1.55−25.0
14081
1.53−25.00
27598
1.67−25.00
15768
1.79−25.50
35049
total no. of rflns
no. of unique rflns
no. of obsd data (I > 2σ(I))
Rint
5124
5136
5737
6420
4494
4914
3801
6203
0.045
0.047
0.070
0.045
a
R1 (F2 > 2σ(F2)), wR2 (F2)
0.0758, 0.1667
0.0704, 0.1454
0.0635, 0.1414
0.0435, 0.1009
0.04(1)
Flack param
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(|Fo − Fc2|)2]/∑|w(Fo )2|}1/2
.
1,3-bis(oxazoline) derivative of (η5-Cp)Co(η4-C4Ph4) which
can function as a bidentate chiral ligand. We also report the
synthesis of a novel chiral palladium complex based on this
ligand which has shown promising catalytic activity in the
asymmetric aza-Claisen rearrangement of trichloroacetimidates.
Kα (λ = 0.710 73 Å) sealed tube. All crystal structures were solved by
direct methods. The program SAINT (version 6.22) was used for
integration of the intensity of reflections and scaling. The program
SADABS was used for absorption correction. The crystal structures
were solved and refined using the SHELXTL (version 6.12) package.17
All hydrogen atoms were included in idealized positions, and a riding
model was used. Non-hydrogen atoms were refined with anisotropic
displacement parameters. Table 1 gives the data collection and
structure solution parameters for compounds 1, 2, 5, and 6. Selected
bond distances and angles for compounds 1, 2, and 4−6 are given in
the Supporting Information.
Synthesis of Na{[MeOC(O)]2C5H3}. Na{[MeOC(O)]C5H4} (6.00
g, 41.00 mmol) was dissolved in 40 mL of THF and cooled to 0 °C
using an ice bath. To this cooled solution was added MeOC(O)Cl
(1.93 g, 20.50 mmol) dropwise with stirring. The ice bath was
removed, and the reaction mixture was stirred for 30 min at room
temperature followed by 30 min at 50 °C. After the reaction mixture
was cooled, ethyl ether (100 mL) was added to precipitate the sodium
salt of the diester. The dark red solvent layer was removed using a
pipet after the precipitate settled down. This process was repeated
three to four times until the solvent layer turned colorless. Afterward,
the precipitate was dried on a hot plate at 150 °C under nitrogen. The
dried precipitate was cooled and dissolved in THF (50 mL) to get a
red-brown solution. The solution was filtered under nitrogen, and
evaporation of the clear solution gave Na{[MeOC(O)]2C5H3} (yield
3.56 g, 17.45 mmol, 85%). This was freshly prepared and used for the
next step.
EXPERIMENTAL SECTION
■
General Methods. All manipulations of the complexes were
carried out using standard Schlenk techniques under a nitrogen
atmosphere. Tetrahydrofuran, xylene, and toluene were freshly
distilled from sodium benzophenone ketyl under a nitrogen
atmosphere and used. The sodium salt of carbomethoxycyclopenta-
diene,14 tris(triphenylphosphine)cobalt chloride,15 and trichloroaceti-
midates16 were prepared according to literature procedures. Methyl
chloroformate, ethyl chloroformate, triphenylphosphine (Spectro-
chem), dimethyl carbonate, L-valinol, mesyl chloride, and diphenyla-
cetylene (Aldrich) were used as received.
1
Instrumentation. H and 13C{1H} spectra were recorded on a
Bruker Spectrospin DPX-300 NMR spectrometer at 300 and 75.47
MHz, respectively. IR spectra in the range 4000−250 cm−1 were
́
recorded on a Nicolet Protege 460 FT-IR spectrometer as KBr pellets.
Elemental analyses were carried out on a Carlo Erba CHNSO 1108
elemental analyzer. Mass spectra were recorded on a Bruker Micro-
TOF QII quadrupole time-of-flight (Q-TOF) mass spectrometer.
Optical rotations of the chiral compounds were measured on an
Autopol V (Rudolph Research, Flanders, NJ) instrument. All the
rotations were measured at 589 nm (sodium D line) using chloroform
as solvent, and readings were cross-checked by taking measurements at
two different concentrations (200 and 400 mg/100 mL). The
enantiomeric excess was determined by a Shimadzu LC6AD HPLC
instrument fitted with a Daicel OD-H chiral column. All HPLC
analyses used to determine enantiomeric purity were calibrated with
samples of the racemate.
{η5-1,3-[MeOC(O)]2C5H3}Co(η4-C4Ph4) (1) and {η5-1,2-[MeOC-
(O)]2C5H3}Co(η4-C4Ph4) (2). Co(PPh3)3Cl (8.80 g, 10.00 mmol) was
added to a solution of Na{[MeOC(O)]2C5H3} (2.35 g, 11.50 mmol)
in 10 mL of THF, and the mixture was stirred for 5 min. To the
resultant solution was added diphenylacetylene (4.10 g, 23.00 mmol)
in 50 mL of toluene, and the mixture was refluxed for 5 h. The reaction
mixture was cooled, solvent was evaporated off, and the residue was
chromatographed on a neutral alumina column. Triphenylphosphine
was removed by eluting the column with hexane. When the polarity
was gradually increased (5% ethyl acetate−95% hexane), the first
fraction came out, which on evaporation of the solvent gave a yellow
X-ray Crystallography. Suitable crystals of compounds 1, 2, and
4−6 were obtained by slow evaporation of their saturated solutions in
ethyl acetate/hexane mixtures. Single-crystal diffraction studies were
carried out on a Bruker SMART APEX CCD diffractometer with a Mo
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Organometallics
Article
crystalline powder characterized as 1. Yield: 2.35 g, 4.00 mmol, 40%.
Mp: 182−184 °C. Anal. Found: C, 74.33; H, 4.96. Calcd for
C37H29O4Co: C, 74.49; H, 4.90. IR (ν, cm−1): 1729, 1705 vs (CO).
1H NMR (δ, 300 MHz, CDCl3): 3.28 (6H, s, CH3), 5.29 (2H, s,
CpH), 5.84 (1H, s, CpH), 7.21−7.33 (12H, m, m-/p-PhH), 7.40−7.42
(8H, m, o-PhH). 13C NMR (δ, 75 MHz, CDCl3): 51.38 (CH3), 77.92
(C4Ph4), 85.28, 86.91, 89.49 (CpC), 127.15, 128.14, 128.75, 133.93
(PhC), 165.45 (CO). HRMS: calcd for C37H29O4Co 596.1398, found
596.1402.
On further increasing the polarity using 7% ethyl acetate−93%
hexane, a second fraction was collected, which on evaporation gave a
reddish yellow crystalline solid characterized as 2. Yield: 2.68 g, 4.50
mmol, 45%. Mp: 175−177 °C. Anal. Found: C, 74.30; H, 4.80. Calcd
for C37H29O4Co: C, 74.49; H, 4.90. IR (ν, cm−1): 1708 vs (CO). 1H
NMR (δ, 300 MHz, CDCl3): 3.26 (6H, s, CH3), 4.86 (1H, s, CpH),
5.25 (2H, s, CpH), 7.23−7.30 (12H, m, m-/p-PhH), 7.45−7.47 (8H,
m, o-PhH). 13C NMR (δ, 75 MHz, CDCl3): 51.41 (CH3), 77.76
(C4Ph4), 86.39, 87.44, 89.64 (CpC), 127.04, 128.12, 128.87, 134.13
(PhC), 165.13 (CO). HRMS: calcd for C37H29O4Co 596.1398, found
596.1399.
72.65, 77.21, 79.25, 79.58, 84.47, 92.16 (CpC), 75.07 (C4Ph4), 127.26,
128.24, 128.46, 133.62 (PhC), 189.69 (CO). HRMS: calcd for
C45H31O2CoFe 718.1005, found 718.1008.
[η5-1,3-(4-iPr-2-Ox)2C5H3]Co(η4-C4Ph4) (6). The crude acid 3
(0.55 g, 0.96 mmol) was dissolved in CH2Cl2 (15 mL). Oxalyl chloride
(0.15 g, 1.17 mmol) and DMF (1 drop) were added sequentially.
Upon addition of the latter, gas evolution was observed. The resulting
solution was stirred at room temperature. After 30 min, the solution
was concentrated using a rotary evaporator. Byproducts and excess
oxalyl chloride were removed by repeated extraction of the residue
with CH2Cl2 (3 × 20 mL) to yield the acid chloride as a red-brown
solid, which was used directly in the next step.
(S)-2-amino-3-methyl-1-butanol (L-valinol; 0.23 g, 2.23 mmol) was
taken up in a mixture of triethylamine (4 mL) and CH2Cl2 (15 mL). A
solution of the crude acid chloride in 20 mL of CH2Cl2 was also
transferred to this flask. The resulting solution was stirred at room
temperature and, after 2 h, was cooled to 0 °C using an ice bath. Mesyl
chloride (0.35 g, 3.05 mmol) was added, and the resulting solution was
warmed to room temperature. After it was stirred for 16 h, the solution
was washed with 30 mL of saturated aqueous sodium bicarbonate and
30 mL of brine using a separating funnel. The organic layer was dried
over Na2SO4, filtered, and concentrated using a rotary evaporator. The
residue was purified using a silica gel column with a 7/1 hexane/ethyl
acetate mixture as eluent. Evaporation of the solvent gave 5 as a yellow
crystalline solid. Yield: 0.51 g, 0.73 mmol, 73%. Mp: 101−103 °C.
[α]D35 = −35° (c 0.20 in CHCl3). Anal. Found: C, 76.79; H, 6.08; N,
4.14. Calcd for C45H43O2N2Co: C, 76.91; H, 6.17; N, 3.99. IR (ν,
[η5-1,3-(COOH)2C5H3]Co(η4-C4Ph4) (3). Potassium hydroxide
(0.56 g, 10.00 mmol) dissolved in 5 mL of water was mixed with 1
(0.60 g, 1.00 mmol) in 75 mL of ethyl alcohol, and the mixture was
refluxed for 30 h. The reaction was quenched with 2 M HCl (25 mL).
After extraction with CH2Cl2 (80 mL), the organic phase was washed
with 100 mL of 2 M aqueous HCl, dried over anhydrous sodium
sulfate, filtered, and concentrated under reduced pressure to give the
orange solid 3. Yield: 0.55 g, 0.96 mmol, 96%. Mp: >330 °C. Anal.
Found: C, 74.02; H, 4.47. Calcd for C35H25O4Co: C, 73.94; H, 4.43.
1
cm−1): 1655 vs (CN). H NMR (δ, 300 MHz, CDCl3): 0.73−0.76
(3H, m, CHCH3), 0.83−0.90 (3H, m, CHCH3), 1.48−1.53 [2H, m,
CH(CH3)2], 3.36−3.42 (2H, m, CHCH2), 3.45−3.49 (1H, m,
CHCH2), 3.56−3.65 (2H, m, CHCH2), 3.73−3.82 (1H, m,
CHCH2), 5.16 (1H, s, CpH), 5.25 (1H, s, CpH), 5.79 (1H, s,
CpH), 7.17−7.30 (12H, m, m-/p-PhH), 7.42−7.45 (8H, m, o-PhH).
13C NMR (δ, 75 MHz, CDCl3): 18.08, 18.24, 19.13, 19.77
1
IR (ν, cm−1): 3359 vs (−OH). H NMR (δ, 300 MHz, d6-DMSO):
4.82 (2H, s, CpH), 5.26 (1H, s, CpH), 7.18 (12H, br s, m-/p-PhH),
7.36 (8H, bs, o-PhH). 13C NMR (δ, 75 MHz, CD3OD): 75.67
(C4Ph4), 85.08, 95.76 (CpC), 126.42, 128.50, 128.85, 135.74 (PhC),
171.98 (CO). HRMS: calcd for C35H25O4CoK 607.0722, found
607.0724.
(CH(CH3)2), 32.73, 32.94 (CH(CH3)), 69.34, 69.63 (CHCH2),
72.53, 72.93 (CHCH2), 77.05 (C4Ph4), 82.02, 84.41, 86.13, 86.91,
87.47 (CpC), 126.60, 127.87, 128.85, 134.51 (PhC), 159.73, 160.12
(CN). HRMS: calcd for C45H43O2N2CoH 703.2735, found
703.2723.
{η5-1,2-[COOH]2C5H3}Co(η4-C4Ph4) (4). Compound 2 (0.60 g,
1.00 mmol) was taken in place of 1 in an identical reaction by which 3
was synthesized to give 4. Yield: 0.54 g, 0.95 mmol, 95%. Mp: 265−
268 °C. Anal. Found: C, 73.86; H, 4.46. Calcd for C35H25O4Co: C,
73.94; H, 4.43. IR (ν, cm−1): 3359 vs (−OH). 1H NMR (δ, 300 MHz,
CDCl3): 4.92 (1H, s, CpH), 5.52 (2H, s, CpH), 7.21−7.23 (12H, br s,
m-/p-PhH), 7.35−7.37 (8H, bs, o-PhH). 13C NMR (δ, 75 MHz, d6-
DMSO): 77.45 (C4Ph4), 89.47, 91.47 (CpC), 127.70, 128.49, 128.61,
133.40 (PhC), 166.68 (CO). HRMS: calcd for C35H25O4CoK
607.0722, found 607.0676.
Pd(OAc)2{[η5-1,3-(4-iPr-2-Ox)2C5H3]Co(η4-C4Ph4)} (7). Palladi-
um acetate (0.11 g, 0.50 mmol) was added to a solution of 6 (0.35 g,
0.50 mmol) in acetic acid (1.5 mL), and the mixture was stirred at 95
°C for 20 min. The yellow precipitate formed was filtered, washed with
acetic acid, and dried under vacuum to give a yellow crystalline powder
which was characterized as 7. Yield: 0.29 g, 0.31 mmol, 62% Mp: 206−
208 °C dec. [α]D35 = −95° (c 0.20 in CHCl3). Anal. Found: C, 63.29;
H, 5.38; N, 3.05. Calcd for C49H49O6N2CoPd: C, 63.47; H, 5.33; N,
3.02. IR (ν, cm−1): 1628. 1H NMR (δ, 300 MHz, CDCl3): 0.58−0.60
(3H, d, 3J = 6.0 Hz, CHCH3), 0.77−0.79 (3H, d, 3J = 6.0 Hz,
CHCH3), 1.13 (3H, s, COCH3), 1.17−1.19 (3H, d, 3J = 6.0 Hz,
CHCH3), 1.39 (3H, s, COCH3), 1.74−1.76 (1H, m, CH(CH3)2),
2.70−2.76 (1H, m, CHCH2), 2.90−2.98 (1H, m, CH(CH3)2), 3.43−
3.57 (2H, m, CHCH2), 3.68−3.74 (1H, m, CHCHH), 3.92−4.04 (1H,
m, CHCH2), 4.13−4.18 (1H, m, CHCHH), 5.55 (1H, s, CpH), 6.85
(1H, s, CpH), 7.15−7.26 (20H, PhH), 7.38 (1H, s, CpH). 13C NMR
(δ, 75 MHz, CDCl3): 15.93, 18.56, 21.28, 21.55 (CH(CH3)2), 22.47,
22.96 (COCH3), 29.22, 31.07 (CH(CH3)2), 67.32, 69.90 (CHCH2),
72.57 (CHCH2), 78.12 (C4Ph4), 84.46, 85.90, 87.04, 87.92, 91.54
(CpC), 126.87, 128.72, 128.80, 134.18 (PhC), 163.97, 167.97 (CN),
177.37, 177.44 (CO). HRMS: calcd for C49H49CoN2O6PdNa
949.1855, found 949.1839.
Ferrocene Fused Acenequinone (η4-C4Ph4)Co[μ2-η5:η5-1,2-
C5H3(CO)2-(1,2-C5H3)]Fe(η5-Cp) (5). The 1,2-dicarboxylic acid 4
(0.15 g, 0.26 mmol) was dissolved in dry CH2Cl2 (10 mL). Oxalyl
chloride (0.25 g, 2.00 mmol) and DMF (1 drop) were also added, and
the mixture was stirred for 30 min at room temperature. The resultant
reddish yellow solution was evaporated completely to dryness under
high vacuum at 40 °C to give the 1,2-diacyl chloride. Without further
purification, the diacyl chloride was dissolved in dry CH2Cl2 (5 mL).
To this solution were added ferrocene (0.04 g, 0.28 mmol) and
anhydrous aluminum chloride (0.15 g, 1.13 mmol) together. The
reaction mixture was stirred for 30 min at room temperature and then
quenched with 2 M aqueous NaOH (5 mL), and the product was
extracted with ethyl ether (2 × 25 mL). Combined ether extracts were
dried with MgSO4 and filtered, and the volatiles were removed in
vacuo. The crude product was chromatographed through alumina
using an ethyl acetate/hexane (1/5) mixture as the eluent. A dark red
band was eluted out that upon evaporation of the solvent gave a dark
red crystalline solid, which was characterized as 5. Yield: 0.07 g, 0.10
mmol, 38%. Mp: 235−238 °C dec. Anal. Found: C, 75.40; H, 4.29.
Calcd for C45H31O2CoFe: C, 75.22; H, 4.35. IR (ν, cm−1): 1644 vs
(CO). 1H NMR (δ, 300 MHz, CDCl3): 4.12 (5H, s, Cp
General Procedure for Rearrangement of Allylic Trichlor-
oacetimidates Catalyzed by Palladium Complex 7. A solution of
the catalyst 15 (0.02 mmol) in CH2Cl2/CH3CN (0.50 mL) was added
to trichloroacetimidates 8a−c (1.00 mmol) in an oven-dried 2 mL
reaction vial having a stirring bar. The vial was capped, and the
solution was stirred at 37 °C/65 °C. After 10 h, the solution was
concentrated to give a semisolid. Purification by silica gel column
chromatography using 2% ethyl acetate/98% hexane provided the pure
allylic trichloroacetamides 9a−c. Chiral HPLC analysis (Shimadzu,
3
unsubstituted), 4.76−4.77 (1H, t, J = 2.7 Hz, FeCHCHCH), 4.81−
3
3
4.83 (1H, t, J = 2.7 Hz, CoCHCHCH), 4.93−4.94 (2H, d, J = 2.7
3
Hz, FeCHCHCH), 5.64−5.65 (2H, d, J = 2.8 Hz, CoCHCHCH),
7.13−7.31 (20H, m, PhH). 13C NMR (δ, 75 MHz, CDCl3): 71.38,
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Scheme 1. Synthesis of the 1,2- and 1,3-{[MeOC(O)]2C5H3}Co(C4Ph4) Derivatives
Scheme 2
Scheme 3
1,3-bis(oxazoline)-derived bidentate chiral ligand {(η5-1,3-[(4-
iPr-2-Ox)]2C5H3}Co(η4-C4Ph4) (6; Ox = oxazolinyl) (Scheme
3). Interestingly, an attempt to prepare an analogous 1,2-
bis(oxazoline) derivative was unsuccessful. Reaction of 6 with
Pd(OAc)2 in acetic acid resulted in the formation of the chiral
palladium complex 7 (Scheme 4). While previous reactions of
Daicel OD-H column, 0.5−1.0% IPA/n-hexane, 0.50 mL/min) was
carried out to determine the enantiomeric excess of 9a−c, details of
which are provided in the Supporting Information.
RESULTS AND DISCUSSION
■
A reaction of the sodium salt of the monoester MeOC(O)-
CpNa with methyl chloroformate in THF at 50 °C resulted in
the formation of a mixture of the sodium salts of 1,2- and 1,3-
diester-derived sodium cyclopentadienyl along with the
disubstituted cyclopentadienyl dimer as a side product (Scheme
1).18 The latter was removed by repeated extraction of the
reaction mixture with ethyl ether and the sodium salt was
reacted directly with Co(PPh3)3Cl and diphenylacetylene in
refluxing toluene to yield the 1,3- and 1,2-diesters {η5-
[MeOC(O)]2C5H3}Co(η5-C4Ph4) (1 and 2) in 83−85%
overall isolated yield (Scheme 1). The mixture was easily
separated and purified by column chromatography. For the
sake of comparison, we have also carried out a reaction of the
carboethoxy sodium cyclopentadienyl Na{C5H4[C(O)OEt]}
with ethyl chloroformate, ClC(O)OEt, but observed that the
reaction results in lower yield (28%) of the 1,3-diester.
Scheme 4
chiral monodentate ligands derived from (η5-Cp)Co(η4-C4Ph4)
with palladium acetate resulted exclusively in the formation of
palladacycles,1,9 compound 6 was found to be simply acting as a
chelating bidentate ligand in the synthesis of complex 7. The
identity of the chiral complex 7 was conclusively proved by
HRMS, CHN analysis, optical rotation, and NMR (1H and
13C) studies. Two peaks of COCH3 at 1.13 and 1.39 ppm in 1H
NMR and two peaks of COCH3 at 177.26 and 177.50 in 13C
NMR provided clear evidence for the two acetate groups, which
was further supported by HRMS. Three peaks at 5.55, 6.85, and
7.38 ppm for the Cp protons confirmed that compound 7 is
not a palladacycle.
The diesters 1 and 2 were hydrolyzed to the dicarboxylic
acids 3 and 4 by refluxing with aqueous KOH in ethanol. The
1,2-diacyl chloride of the 1,2-dicarboxylic acid 4 was generated
in situ by a reaction with oxalyl chloride, and it was further
reacted with ferrocene under Friedel−Crafts conditions to yield
the novel bis-metallocenyl acenequinone 5, having both iron
and cobalt sandwich units in the same compound (Scheme 2).
A similar reaction attempted using 3 did not result in any
isolable products. The 1,3-dicarboxylic acid 3 after reaction with
oxalyl chloride, followed by (S)-2-amino-3-methyl-1-butanol,
triethylamine, and mesyl chloride, was converted to the novel
A range of 1,2- and 1,3-phenyl-substituted ligands, many of
them chiral, such as phenyl bis(oxazoline) (Phebox) have
shown excellent applications in homogeneous catalysis,
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especially enantioselective synthesis.19 However, a search in the
literature of examples of 1,2 and 1,3 symmetrically Cp
disubstituted metal sandwich compounds, analogous to the
(η5-R2C5H3)Co(η4-C4Ph4) derivatives reported in the present
study, indicated only a handful of examples, mostly ferrocene
derivatives. These include the 1,2-methyl diester,20 1,2-ethyl
diester,21 1,2- and 1,3-dialdehydes,20 1,3-hydroxymethyl,20 and
1,2-diacetylene derived ferrocenes.22 In addition, synthesis of
the 1,2-phenyl diester of CpRuCp*23 and 1,3-dialkyne of
22b
CpMn(CO)3 has also been reported.
In the 1,3-bis(oxazoline) derivative 6, the hydrogen flanked
by the two oxazolinyl groups was found to resonate at 5.79
ppm, which is slightly deshielded in comparison to the reported
monooxazolinyl analogue.6a On formation of the palladium
complex 7, this hydrogen was found to be significantly
deshielded (7.38 ppm), suggesting a possible C−H···Pd
anagostic interaction similar to such anagostic interactions
reported for cyclobutadiene linked dimeric palladacycles.10a
The activity of 7 as chiral catalyst was evaluated in the
asymmetric rearrangement of trichloroacetimidates, which is
also an elegant method for the catalytic synthesis of chiral allylic
amines.9c
Figure 3. ORTEP diagram of 5 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Structural Studies on the New Compounds. Single-
crystal structures of diesters 1 and 2, diacid 4, acenequinone 5,
and 1,3-bis(oxazoline) 6 were determined. Figures 1−4 show
Figure 4. ORTEP diagram of 6 with thermal ellipsoids at the 30%
probability level. Some hydrogen atoms have been omitted for clarity.
Information. Structural analysis of the 1,2-dicarboxylic acid 4
showed both intramolecular and intermolecular OH···O
hydrogen bonding between the two carboxylic acid groups.
The intermolecular hydrogen bonding results in a linear chain
where successive cobalt sandwich units alternate on both sides
of the chain (see the Supporting Information). Crystal
structures of two acenequinones having two metallocenyl
units similar to acenequinone 5 have been reported.23 These
are (A) having both the metallocenyl units as CpRuCp* and
(B) having one CpRuCp* and other ferrocene (Figure 5). The
structures of these compounds showed some interesting
variations. In the case of compound 5 and the bis-CpRuCp*
acenequinone, the two Cp rings on both sides of the quinone
unit are planar (angles between the mean planes of the Cp rings
being 0.7(2) and 0°, respectively), while for the compound
having CpRuCp* and ferrocene, the corresponding angle is
13.8(6)°, indicating that the ruthenocene unit is placed toward
the carbonyl and the ferrocenyl unit is placed away from the
carbonyl group (Figure 5).
Figure 1. ORTEP diagram of 1 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Figure 2. ORTEP diagram of 2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
the ORTEP diagrams of compounds 1, 2, 5, and 6. ORTEP and
packing diagrams of compound 4 are given in the Supporting
However, the carbonyl stretching band of the quinone
groups of 5 was observed at 1643 cm−1, which is the same as
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Figure 5. Comparison of ring planarity of acenequinone 5 with analogous complexes.
Table 2. Aza-Claisen Rearrangement Carried Out on Trichloroacetimidates using Bis(oxazoline) Pd Complex 7
a
b
entry
R
substrate
product
solvent
reacn temp (°C)
yield (%)
ee (%) (confign)
1
2
3
4
5
Me
8a
8a
8b
8b
8c
9a
9a
9b
9b
9c
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
37
65
37
65
37
82
90
85
93
81
95 (S)
83 (S)
96 (S)
90 (S)
94 (S)
Me
n-Pr
n-Pr
PhCH2CH2
a
b
Isolated yield. Determined by HPLC analysis using chiral ODH column.
that observed for the acenequnone having CpRuCp* and
ferrocene units, possibly suggesting that the electronic effects of
CoC4Ph4 and RuCp* units on the carbonyl groups of the
acenequnone are comparable. These νCO values are lower than
the 1660 cm−1 reported for 9,10-anthraquinone and, as
suggested by Selegue and co-workers, can be rationalized by
resonance-stabilized structures.23a The crystal structure of
compound 6 (Figure 4) is the first example of the structure
of a bis-oxazoline derived metal sandwich compound. The
mean planes of the two oxazoline rings at the 1- and 3-positions
are almost in the same plane as that of the cyclopentadiene ring
with dihedral angles of 5.2(1) and 7.5(1)°.
Catalytic Studies Using the Palladium Complex 7. The
catalytic asymmetric rearrangement of trichloro- or trifluor-
oacetimidates to the corresponding chiral allylic trichloro- or
trifluoroacetamides is a well-known method utilized to evaluate
the enantioselective efficiency of many chiral catalysts.4 To
explore the catalytic activity of the palladium complex 7, we
have carried out the aza-Claisen rearrangement of three
selected trichloroacetimidates, RCHCHCH2OC(NH)CCl3
(R  Me, n-Pr, CH2CH2Ph), in the presence of 7 as catalyst.
Reactions were carried out in dichloromethane and acetonitrile
over a period of 10 h with a catalyst loading of 2 mol %, and the
results obtained are given in Table 2.
CONCLUSION
■
The first synthesis of cyclopentadienyl 1,2- and 1,3-diester
derivatives of (η5-Cp)Co(η4-C4Ph4) has been described. These
diesters are potential precursors for the synthesis of a range of
basal bulky bidentate chiral and achiral ligands with a scope of
preparing a range of catalysts. A novel ferrocene fused
acenequinone of (η5-Cp)Co(η4-C4Ph4) was synthesized using
the 1,2-dicarboxylic acid derivative of (η5-Cp)Co(η4-C4Ph4).
Reaction of the 1,3-dicarboxylic acid derivative of (η5-
Cp)Co(η4-C4Ph4) with oxalyl chloride followed by reacting
with (S)-2-amino-3-methyl-1-butanol and ring closing by mesyl
chloride resulted in the first example of a chiral bidentate 1,3-
bis(oxazoline) ligand. Reaction of this chiral bidentate ligand
with Pd(OAc)2 yielded the chiral chelated Pd complex 7. The
utility of the palladium complex as a catalyst in the asymmetric
aza-Claisen rearrangement of three different trichloroacetimi-
dates was explored. The reaction when carried out at 37 °C in
CH2Cl2 gave yields in the range of 81−85% and good ee’s
ranging from 94 to 96%. While all previous studies on aza-
Claisen rearrangements of trichloro- and trifluoroacetimidates
which resulted in good enantiomeric excess was reported with
palladacycles, the present study has shown that chiral palladium
complexes with bulky basal ligands which are not necessarily
palladacycles can also bring about the rearrangement with good
enantiomeric excess. This study further substantiates the
finding by Overman and others that the crucial factor which
enhances enantioselectivty in this rearrangement is the basal
bulkiness of the chiral ligand utilized.9a
The chiral complex 7 was found to be quite efficient in
bringing about the aza-Claisen rearrangement at ambient
temperature with good enantioselectivity. The yields of the
trichloroacetamides after the aza-Claisen rearrangement are in
the range of 81−85% and the ee’s obtained ranged from 94 to
96% when the reaction was carried out at 37 °C. An attempt to
increase the yield further by increasing the reaction temperature
to 65 °C resulted in improved yields (90−93%) but a decrease
in ee (83−90%).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for compounds 1, 2, and
4−6 and figures giving H and 13C NMR spectra of 1,3-
1
oxazoline 6, palladium complex 7, trichloroacetimidates 8a−c,
their rearranged products 9a−c, and HPLC plots of 9a−c. This
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(8) Arrayas, R. G.; Mancheno, O. G.; Carretero, J. C. Chem. Commun.
2004, 1654−1655.
material is available free of charge via the Internet at http://
pubs.acs.org.
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L. E.; Sneddon, H. F.; Ziller, J. W. Adv. Synth. Catal. 2009, 351, 3186−
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Stepnicka, P., Ed.; Wiley: Chichester, England, 2008. (c) Gupta, B. D.;
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: eliasanil@gmail.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Science and Technology (DST)
of India for financial assistance in the form of research grant to
A.J.E. (No. SR/SI/IC-43/2010). N.S. thanks the UGC India for
a research fellowship. We thank the DST-FIST and IITD for
funding of the single-crystal X-ray diffraction facility at IIT
Delhi.
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