Cyclopentadienyl molybdenum alkyl ester complexes as catalyst precursors for olefin epoxidation

Article abstract of DOI:10.1039/c4cy00738g

New molybdenum complexes of the type [CpMo(CO)₃X] containing ligands of the formula X = CHR²CO(OR¹) where R¹ = ethyl (1), menthyl (4), and bornyl (5) and R² = H; R¹ = ethyl and R² = methyl (2) and phenyl (3) have been synthesized and characterized by NMR and IR spectroscopy and X-ray crystallography. These compounds have been applied as catalyst precursors for achiral and chiral epoxidation of unfunctionalized olefins with tert-butyl hydroperoxide (TBHP) as the oxidant at 22 °C (in CH₂Cl₂) and 55°C (in CHCl₃). The substrates cis-cyclooctene, 1-octene, cis- and trans-stilbene, and trans-β-methylstyrene were selectively and quantitatively converted into their epoxides using a catalyst:substrate:oxidant ratio of 1:100:200 within 4 h at room temperature in CH₂Cl₂ and within 15 min at 55°C in CHCl₃. Complexes 1-5 are precursors of active epoxidation catalysts and turnover frequencies (TOFs) of ca. 1200 h^-1 are obtained with cis-cyclooctene as the substrate. No enantioselectivity is observed with trans-β-methylstyrene as the substrate despite the application of enantiomerically pure precatalysts. In situ monitoring of catalytic epoxidation of cis-cyclooctene with complex 5 by ¹H and ¹³C NMR spectroscopy suggests that the chiral alkyl ester side chain is retained during oxidation with TBHP. During epoxidation, the primary catalytic species is the dioxo complex [CpMoO₂X]. After near complete conversion of cis-cyclooctene to its epoxide, further oxidation of the dioxo complex to oxo-peroxo complex [CpMo(η²-O₂)(O)X] takes place. The oxo-peroxo complex is also an active epoxidation catalyst.

Full text of DOI:10.1039/c4cy00738g

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Cyclopentadienyl molybdenum alkyl ester
complexes as catalyst precursors for olefin
epoxidation†
Cite this: Catal. Sci. Technol., 2014,
4, 4219
*
Nidhi Grover, Alexander Pöthig and Fritz E. Kühn
New molybdenum complexes of the type [CpMo(CO)3X] containing ligands of the formula X = CHR2CO(OR1)
where R1 = ethyl (1), menthyl (4), and bornyl (5) and R2 = H; R1 = ethyl and R2 = methyl (2) and phenyl (3)
have been synthesized and characterized by NMR and IR spectroscopy and X-ray crystallography.
These compounds have been applied as catalyst precursors for achiral and chiral epoxidation of
unfunctionalized olefins with tert-butyl hydroperoxide (TBHP) as the oxidant at 22 °C (in CH2Cl2) and
55 °C (in CHCl3). The substrates cis-cyclooctene, 1-octene, cis- and trans-stilbene, and trans-β-
methylstyrene were selectively and quantitatively converted into their epoxides using a catalyst :
substrate : oxidant ratio of 1 : 100 : 200 within 4 h at room temperature in CH2Cl2 and within 15 min at
55 °C in CHCl3. Complexes 1–5 are precursors of active epoxidation catalysts and turnover frequencies
(TOFs) of ca. 1200 h−1 are obtained with cis-cyclooctene as the substrate. No enantioselectivity is
observed with trans-β-methylstyrene as the substrate despite the application of enantiomerically pure
precatalysts. In situ monitoring of catalytic epoxidation of cis-cyclooctene with complex 5 by 1H and
13C NMR spectroscopy suggests that the chiral alkyl ester side chain is retained during oxidation with
TBHP. During epoxidation, the primary catalytic species is the dioxo complex [CpMoO2X]. After near
complete conversion of cis-cyclooctene to its epoxide, further oxidation of the dioxo complex to
oxo–peroxo complex [CpMo(η2-O2)(O)X] takes place. The oxo–peroxo complex is also an active epoxida-
tion catalyst.
Received 5th June 2014,
Accepted 10th July 2014
DOI: 10.1039/c4cy00738g
www.rsc.org/catalysis
the well-examined methyltrioxorhenium (MTO)-H2O2 system.16–20
During the past decade, monomeric CpMo tricarbonyl com-
Introduction
Since the development of the Halcon–ARCO process for
industrial propylene oxide production using molecular Mo(VI)
catalysts and organic hydroperoxides in the homogeneous
phase, improvements in stability, selectivity and applicability
of specialized molybdenum complexes for homogeneous
and heterogeneous epoxidation catalysis have been focal
points of studies.1 Mononuclear complexes such as [MoO2X2Ln]
(X = halide, alkyl, siloxy; L = mono- or bidentate ligand),2–6
[η5-(C5R5)Mo(CO)3X] (R = H, CH3, CH2Ph; X = halide, Me, Et,
ansa-bridge, CF3)7–11 and binuclear complexes of the type
[(η5-(C5R5)2M2O5] (M = Mo, W)12,13 have been used as epoxi-
dation catalysts with tert-butyl hydroperoxide (TBHP) or H2O2
as the oxidant.13–15 Cyclopentadienyl (Cp) Mo complexes some-
times display activities similar to or even higher than that of
plexes have been established as suitable precursors for cata-
lytically active molybdenum dioxo or oxo–peroxo complexes,
which are formed in situ with organic hydroperoxides after
oxidative decarbonylation.21–23 Homogeneous epoxidation
activities of CpMo complexes as catalysts have been com-
pared in a recent review10 and other reviews have discussed
heterogenization and catalytic applications of the aforemen-
tioned category of molybdenum catalysts.24–26
Enantiopure epoxides are valuable in organic synthesis
and ubiquitous in pharmaceutical, agrochemical and other
fine chemical industrial applications.27 Numerous molybdenum-
based complexes have been utilized in enantioselective cataly-
sis.28,29 Specifically for epoxidation of unfunctionalized
prochiral alkenes, chiral dioxo-molybdenum-based complexes
have been extensively studied in both homogeneous and
heterogeneous catalysis.28,30 However, the limited enantio-
selectivity achieved with such complexes is, in general, a
consequence of either weakly coordinating chiral ligands or
transition states that are symmetrical during oxygen transfer
from the oxo–bisperoxo species.31 Although stereoselective
epoxidation in the homogeneous phase with readily available
Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center and
Department of Chemistry, Technische Universität München, D-85747 Garching bei
München, Germany. E-mail: fritz.kuehn@ch.tum.de; Fax: +49 89 289 13473;
Tel: +49 89 289 13096
† Electronic supplementary information (ESI) available. CCDC 934898–934900.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cy00738g
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Mo catalysts is a lucrative target, only very few examples are
reported in the literature and the enantiomeric excess (ee)
does not exceed ca. 20% (for trans-β-methylstyrene as the
substrate).32 Efforts towards the synthesis of chiral CpMo cat-
alysts mostly involve the introduction of chiral substituents
on the Cp ring.32,33 However, as a consequence of the fast
rotation of the Cp ring in solution, chiral information is lost,
and hence, the ee obtained is very poor. The rotation of the
chiral Cp ligand can be suppressed by an ansa-bridge from
the Cp ligand to the Mo centre that is coordinated either in
a heteroatomic fashion14 or may be σ-C bound.34 In these
cases, the chiral centres are located either at the ansa-bridge
directly or at substituents at the bridge, which is apparently
too far away from the metal to be able to effectively transfer
chiral information to the substrate. Royo et al. have investi-
gated a chiral oxazoline-substituted Cp molybdenum complex,
which forms a heteroatomic ansa-bridge in order to introduce
chiral centres in close proximity to the metal centre.14 How-
ever, the oxazoline moiety decoordinates and loss of the
Cp ligand during catalysis occurs. Hence, the efficiency of
stereoselectivity in catalytic epoxidation with [CpMo(CO)3X]
precatalysts also depends on the strength of the Mo–X bond.
In view of the strategic importance of enantiopure
epoxides in many industrial endeavors and the efficiency of
CpMo complexes in achiral epoxidation,10 complexes 1–5
have been synthesized (Scheme 1). The ligands utilized are
alkyl moieties of the type –CHR2–COOR1 (R1 = ethyl (1),
menthyl (4), and bornyl (5), R2 = H; R1 = ethyl, R2 = methyl (2)
and phenyl (3)) where the chiral information is located at the
R1 group in complexes 4 and 5. Compared to the inductive
effect of the chloro (electron-withdrawing) and the methyl
(electron-donating) group in complexes [CpMo(CO)3Cl] and
[CpMo(CO)3Me], respectively, the alkyl ester group renders
the metal centre more electron poor than the methyl substitu-
ent but less than the chloro substituent since the electron-
withdrawing ester group is not directly bound to the metal.
This is useful for verifying ligand effects on the Lewis acidity
of the metal centre in the precatalyst and establishing a cor-
relation with catalytic activity in the epoxidation reaction.
Furthermore, complexes 1–5 are designed to be less active
than the chloro and methyl analogues and thus may allow
for better thermodynamic and kinetic control during enantio-
selective epoxidation as well as in situ monitoring of the cata-
lyzed reaction. The presence of the alkyl ester moiety also
eliminates the possibility of β-hydrogen elimination decom-
position processes, which are possible for complexes where
molybdenum is attached to a large alkyl group.35
In this work, the synthesis, characterization and applications
of CpMo complexes 1–5 for epoxidation of unfunctionalized
olefins such as cis-cyclooctene, 1-octene, trans- and cis-stilbene
and trans-β-methylstyrene with TBHP as the oxidant are
reported. In addition, the progress of catalytic epoxidation of
cis-cyclooctene has been monitored by 1H and 13C NMR spec-
troscopy in order to confirm that the chiral side chain remains
coordinated during catalysis.
Experimental
Methods and materials
[Mo(CO)6], NaH (60% dispersion in mineral oil), ethyl chloro-
acetate, ethyl lactate, ethyl mandelate, (–)-borneol, (–)-menthol,
tert-butyl hydroperoxide (TBHP, ~5.5 M solution in n-decane),
and trans-β-methylstyrene were purchased from Sigma
Aldrich. All manipulations involving air-sensitive materials
were performed under an argon atmosphere using standard
Schlenk techniques and dry solvents. Menthyl and bornyl
chloroacetate ligands were synthesized by the reaction of
L-(–)-menthol and (–)-borneol with chloroacetyl chloride and
N,N-dimethylamine at 0–30 °C. Ethyl lactate mesylate and
ethyl mandelate mesylate were also obtained by reaction with
mesyl chloride in toluene.36 Ethyl chloroacetate was degassed
and dried by the freeze–pump–thaw method and ethyl lactate
mesylate and ethyl mandelate mesylate were distilled at low
pressures prior to use for the synthesis of complexes 1–5.
High-resolution NMR spectra were recorded using a Bruker
Avance DPX-400 spectrometer. 1H and 13C spectra are
referenced to solvent residual signals37 and 95Mo spectra to an
internal standard of 2 M Na2MoO4 in D2O set at 0 ppm. Solid-
state NMR spectra were recorded using a Bruker Avance 300
spectrometer at room temperature in a 4-mm ZrO2 rotor at
10 kHz or 12 kHz. Adamantane was used as an external (sec-
ondary) standard for referencing TMS (1H: 2.00 ppm, 13C:
29.472 ppm). IR spectra were recorded using a Varian ATR-
FTIR instrument. Thermogravimetric analyses were performed
with a Netzsch TG 209 system at a heating rate of 10 °C min−1
under argon. Microanalyses were performed in the Mikro-
analytisches Labor of the Technische Universität München,
Garching. Mass spectra were recorded with Finnigan MAT 311
Scheme 1 Synthesis of cyclopentadienyl molybdenum tricarbonyl alkyl ester complexes 1–5.
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A and MAT 90 spectrometers. Catalysis was performed under
an ambient atmosphere and catalytic runs were monitored
using a Varian CP-3800 instrument equipped with an FID, an
Optima 5 Amine column (cis-cyclooctene, 1-octene, trans- and
cis-stilbene) and an Optima Delta Amine column (trans-β-
methylstyrene).
The yellow-orange oily residue obtained after removing
THF in vacuo was purified by washing with cold, dry Et2O
(3 × 10 mL). 40 mL of dry THF was then added followed by
dropwise addition of a degassed solution of a chloro or a
mesylate reagent (1.05 equiv.) in 10 mL of THF under steady
argon flow in the dark at −40 °C. The reaction flask was
stirred at r.t. in the dark for a suitable time followed by evap-
oration of the solvent to dryness. The obtained residues were
extracted with dry pentane or hexane and concentrated under
vacuum. The red-yellow products were then purified by
column chromatography (SiO2). Complexes 1–5 eluted as
yellow bands after a deep red band of Cp2Mo2(CO)6 using
n-pentane : diethyl ether = 9 : 1. The fractions were concen-
trated under vacuum and 1–5 were obtained as yellow solids/
oil in yields 54–85%.
CpMo(CO)3(CH2COOC2H5) (1). Reaction time = 6 h, yield =
73%. Bright yellow solid. 1H NMR (400 MHz, C6D6) δ 4.62
(s, 4H, Cp), 4.07 (q, J = 7.1 Hz, 2H), 1.88 (s, 2H, Mo–CH2),
1.11 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (101 MHz, C6D6)
δ 240.63, 226.99 (Mo–CO), 181.07 (CO), 93.44 (Cp), 59.37
(COOCH2CH3), 14.74 (COOCH2CH3), −3.77 (Mo–CH2). 95Mo
(C6D6) = δ −1546. Elemental analysis calcd. (%): C 43.39,
H 3.64; found: C 43.56, H 3.70. IR (cm−1, C6D6) 2026 s (Mo–CO),
1926 vs. (Mo–CO), 1682 w (ester CO).
X-ray crystallography
Data were collected using a single-crystal X-ray diffractometer
equipped with a CCD detector (APEX II, κ-CCD), a rotating
anode (Bruker AXS, FR591) with MoKα radiation (λ = 0.71073 Å)
and a graphite monochromator. Depending on the complex,
either a Montel-type focusing optic (compound 2) or a fine-
focus sealed tube (compounds 1 and 5) was used. Collected
data were analyzed by using the SMART software package.38
The measurements were performed on single crystals coated
with perfluorinated ether. The crystals were fixed on top of a
glass fiber and transferred to the diffractometer. The crystals
were frozen under a stream of cold nitrogen. A matrix scan
was used to determine the initial lattice parameters. Reflec-
tions were merged and corrected for Lorenz and polarization
effects, scan speed, and background using SAINT.39 Absorp-
tion corrections including odd- and even-ordered spherical
harmonics were performed using SADABS.39 Space group
assignments were based on systematic absences, E statistics,
and successful refinement of the structures. Structures
were solved by direct methods with the aid of successive dif-
ference Fourier maps and refined against all data using the
APEX 2 software38,40 in conjunction with SHELXL-9741 and
SHELXLE.42 Unless otherwise stated, methyl hydrogen atoms
were refined as a part of rigid rotating groups with a C–H dis-
tance of 0.98 Å and Uiso(H) = 1.5 × Ueq(C). Other H atoms were
placed in calculated positions and refined using a riding
model with methylene and aromatic C–H distances of
0.99 and 0.95 Å, respectively, and Uiso(H) = 1.2 × Ueq(C). If
not mentioned otherwise, non-hydrogen atoms were refined
with anisotropic displacement parameters. Full-matrix least-
CpMo(CO)3(CH(CH3)COOC2H5) (2). Reaction time = 24 h,
1
yield = 54%. Yellow solid. H NMR (400 MHz, C6D6) δ 4.63
(s, 5H, Cp), 4.05 (q, 2H), 2.90 (d, J = 7.1 Hz, 1H, Mo–CH),
1.61 (d, J = 7.1 Hz, 3H, Mo–CH–CH3), 1.11 (t, J = 7.1 Hz, 3H).
13C{1H} NMR (101 MHz, C6D6) δ 241.00, 228.28, 227.63
(Mo–CO), 182.63 (CO), 93.90 (Cp), 59.33 (COOCH2CH3),
23.61 (Mo–CH), 14.68 (COOCH2CH3), 11.32 (Mo–CH–CH3).
95Mo (C6D6) = δ −1484. Elemental analysis calcd. (%):
C 45.10, H 4.08, Mo 27.71; found: C 45.72, H 4.23, Mo 25.65.
IR (cm−1, C6D6) 2010 s (Mo–CO), 1914 vs. (Mo–CO), 1666 vs.
(ester CO).
CpMo(CO)3(CH(Ph)COOC2H5) (3). Reaction time = 48 h,
1
yield = 65%. Yellow solid. H NMR (400 MHz, C6D6) δ 7.63
squares refinements were carried out by minimizing
w(Fo − Fc
(d, J = 7.3 Hz, 2H), 7.15 (m, 2H), 6.95 (t, J = 7.3 Hz, 1H), 4.48
(s, 5H, Cp), 4.15 (s, 1H, Mo–CH), 4.04 (m, 2H), 1.08 (t, 3 H).
13C{1H} NMR (101 MHz, C6D6) δ 241.35, 228.81, 228.72
(Mo–CO), 178.80 (CO), 147.97 (MoCH(COOEt)C (phenyl)),
128.15, 124.74 (phenyl ring C, one signal not clearly observed
due to C6D6 solvent residual peaks in the same region), 94.82 (Cp),
59.53 (COOCH2CH3), 21.39 (Mo–CH), 14.53 (COOCH2CH3).
95Mo (C6D6) = δ −1515. Elemental analysis calcd. (%): C 52.96,
P
2
2 2
)
with the SHELXL-9741 weighting scheme.
Neutral atom scattering factors for all atoms and anoma-
lous dispersion corrections for non-hydrogen atoms were
taken from the International Tables for Crystallography.43
Images of the crystal structures were generated by PLATON.44
Full refinement was straightforward for complex 1, while the
ethyl moieties in 2 were refined using split-layer positions.
Due to physically meaningless ADPs, the following restraints
were applied: SIMU for C1 > C5 (Cp moiety) and ISOR for
C10 of complex 5.
H 3.95, Mo 23.50; found: C 54.10, H 4.06, Mo 22.43. IR (cm−1
,
C6D6) 2023 s (Mo–CO), 1930 vs. (Mo–CO), 1689 w (ester CO).
CpMo(CO)3(CH2COOMenthyl) (4). Reaction time = 12 h,
yield = 70%. Bright yellow oil. 1H NMR (400 MHz, C6D6)
δ 4.85–4.95 (m, 1H), 4.7 (s, 4H, Cp), 2.1–2.3 (m, 2H), 1.84–1.97
(m, 2H), 1.41–1.60 (m, 3H), 1.20–1.38 (m, 1H), 1.07–1.09
(m, 1H), 0.89–1.04 (m, 10H), 0.67–0.8 (m, 1H). 13C{1H} NMR
(101 MHz, C6D6) δ 240.72, 226.91, 226.77 (Mo–CO), 180.71
(CO), 93.44 (Cp), 73.00 (MoCH2(CO)OC*H(menthyl)), 47.78
(CH2C(iPr)C*HO–(CO)–CH2Mo), 41.89 (CH(Me)CH2C*HO–(CO)–
CH2Mo), 34.65 (C(iPr)CH2CH2CH(Me)), 31.75 (CH2–CH(Me)–CH2–),
Crystallographic data (excluding structure factors) for
the structures have been deposited in the Cambridge Crystal-
lographic Data Centre as supplementary publication no.
CCDC-934898 (1), CCDC-934899 (2) and CCDC-934900 (5).
General procedure for synthesis of 1–5
Na[CpMo(CO)3] (1 equiv.) was prepared by refluxing
Mo(CO)6 and NaCp in freshly distilled, dry THF overnight.
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26.65 (CH(CH3)2), 23.82 (C(iPr)CH2CH2CH(Me)), 22.43, 21.16,
16.76 (CH3 groups (menthyl)), −3.36 (Mo–CH2). 95Mo (C6D6) =
δ −1553. Elemental analysis calcd. (%): C 54.30, H 5.92,
Mo 21.69; found: C 55.36, H 6.28, Mo 21.85. IR (cm−1, C6D6)
2028 s (Mo–CO), 1934 vs. (Mo–CO), 1679 w (ester CO).
CpMo(CO)3(CH2COOBornyl) (5). Reaction time = 12 h,
yield = 85%. Yellow solid. 1H NMR (400 MHz, C6D6) δ 5.13–5.17
(m, 1H), 4.66 (s, 4H, Cp), 2.42–2.55 (m, 1H), 2.28–2.38
(m, 1H), 1.88–2.01 (m, 2H), 1.66–1.80 (m, 1H), 1.53–1.60
(m, 1H), 1.30–1.47 (m, 2H), 1.14–1.24 (m, 1H), 0.70–1.01
(m, 9H), 0.43–0.55 (m, 1H). 13C{1H} NMR (101 MHz, C6D6)
δ 240.58, 227.00, 226.85 (Mo–CO), 181.48 (CO), 93.53 (Cp),
79.27 (Mo–CH2(CO)OC*H(bornyl)), 48.91, 48.04, 45.40 (bridgehead
Bruker© TopSpin spectrometer, data collection could be auto-
mated. The 1H spectrum (16 scans (~1.5 min)) was first
recorded at 5 min after addition of TBHP followed by the 13C
spectrum (164 scans (~8.5 min)). Thus, in an alternating
manner, 1H and 13C spectra were collected at 10-minute
intervals for a total time of 4 h.
1
Data analysis for H NMR experiments: using NMR soft-
ware MestReNova©, the characteristic signal of the internal
standard mesitylene centred at 6.65 ppm was integrated to
3 H in all 1H NMR spectra. The concentration of oxidized
species in each spectrum was then determined by the inte-
gral of the Cp signal with correlation for 5 H.
quaternary C of the bornyl moiety), 37.54, 28.56, 27.74 Results and discussion
(–CH2– (bornyl)), 19.92, 18.99, 13.98 (CH3 groups (bornyl)),
Synthesis and characterization of complexes 1–5
−3.35 (Mo–CH2). 95Mo (C6D6) δ −1555. Elemental analysis
calcd. (%): C 54.55, H 5.49, Mo 21.79; found: C 54.81, H 5.62,
Mo 21.91. IR (cm−1, C6D6) 2023 s (Mo–CO), 1931 vs. (Mo–CO),
1669 s (ester CO).
Complexes 1–5 were synthesized in yields of 54–85% by proce-
dures analogous to previous reports for the synthesis of complex
145,46 using Na[CpMo(CO)3] as the metal precursor and either
α-chloroesters (complexes 1, 4 and 5) or α-methanesulfonoxyesters
(for 2 and 3) as alkylating agents (Scheme 1).47
Epoxidation catalysis
Complexes 1–3 and 5 are yellow solids and complex 4 is
obtained as a bright yellow oil after column purification. All
complexes are soluble in benzene, toluene, tetrahydrofuran
and dichloromethane. In the solid state, they are stable in air
and moisture for several hours, but in solution, they are sig-
nificantly more sensitive and show visible decomposition
associated with a colour change from yellow to blue-green
and formation of blue residues overnight. Compounds 1–5
can be handled briefly under ambient conditions without any
apparent decomposition, but similar complexes are known to
be susceptible to either slow photochemical transformation
in solution to μ-CO-bridged species48 or rearrangement to the
η3-coordinated side chain on the loss of a carbonyl ligand.49
Complexes similar to 2 are known to be susceptible to β-hydrogen
elimination50 but in the case described here, decomposition
of 2 to an α-alkenyl type of complex was not observed. These
complexes are stable for over a year when stored in the dark
under argon at −30 °C. While complexes 2 and 3 decompose
at 150 °C and 110 °C, respectively, Mo complexes 4 and 5
bearing menthyl and bornyl moieties decompose at 210 °C
and 205 °C, respectively, as shown by TGA-MS measure-
ments. Although elemental analyses of complexes 2, 3 and 4
All catalytic investigations were carried out under air either
in CH2Cl2, under solvent-free conditions (22 °C) or in CHCl3
(55 °C). Under standardized conditions, 1 mol% or 0.1 mol%
catalyst and the substrate were dissolved in 5 mL of solvent.
Catalysis was started with the addition of TBHP to the cata-
lyst and substrate reaction mixture. Aliquots were obtained
from the reaction flask at suitable time intervals and treated
with activated MnO2 to destroy the oxidant followed by filtra-
tion through a MgSO4 plug to remove traces of water. Appro-
priate amounts of external standard solutions (indane and
p-xylene for cis-cyclooctene, toluene and mesitylene for
1-octene, hexadecane and octadecane for cis- and trans-stilbene,
1,2,4-trimethylbenzene and tetraline for trans-β-methylstyrene
in isopropanol) were then added to the aliquot. The sample
was injected into a GC-MS instrument with a column pre-
calibrated with r2 = 0.999 to the chosen substrate. Enantio-
meric excess (ee) was calculated from the integration of the
two peaks relative to the calibration method optimized with
pure sample injections of (2S,3S)-2-methyl-3-phenyloxirane
and (2R,3R)-2-methyl-3-phenyloxirane.
1
give poor results, solution H, 13C and 95Mo NMR and mass
Typical reaction conditions and data acquisition for NMR
study of catalytic epoxidation
spectra (see the ESI†) do not indicate any impurities. For-
mation of Mo oxides is a possibility under combustion analy-
sis conditions, which is a probable cause for deviation from
calculated results.
For catalytic epoxidation of cis-cyclooctene, a mixture of
ca. 0.1 mmol of 5 and a known amount of mesitylene was
1
dissolved in 0.4 mL of CDCl3 in an NMR tube and its H and
NMR spectroscopy
13C spectra were recorded. 10 equiv. of cis-cyclooctene was
1
then added to the NMR tube and after mixing properly, H
1H NMR spectra of compounds 1–5 (in C6D6) show the C5H5
ligand in the range 4.48–4.70 ppm and 93.44–94.82 ppm in
13C NMR (Table 1). These chemical shift values for the Cp
ligand are in the range observed for other structurally similar
[CpMo(CO)3X] complexes (where X = Cl and CH3). The
deshielding effect of the ester group on Mo–CH2 protons is
and 13C NMR were measured. Subsequently, 20 equiv. of
TBHP (5.5 M n-decane solution) was added at 22 °C and
mixed with the precatalyst and substrate solution. It was nec-
essary to shim the magnet again after addition of TBHP for
field locking. Using the multizg acquisition program of a
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Table 1 Selected NMR spectroscopic data for complexes 1–5 and comparison with [CpMo(CO)3Cl] and [CpMo(CO)3CH3]
1Ha
Cp
13C{1H}a
Cp
Complex
Mo–αCH
Mo–αCH
C(O)O
Mo–CO
95Moc
1b
1
2
3
4
5
4.68
4.62
4.63
4.48
4.70
4.66
4.62
4.42
1.84 (s, 2H)
1.88 (s, 2H)
2.87–2.93 (q, 1H)
4.15 (s, 1H)
2.16 (m, 2H)
1.93 (m, 2H)
—
—
−3.94
−3.78
23.61
21.39
−3.36
−3.35
—
—
227.04, 240.63
226.99, 240.63
—
93.44
93.90
94.82
93.44
93.53
95.58
92.41
181.05
182.63
178.80
180.71
181.48
—
−1546
227.63, 228.28, 241.00
228.72, 228.81, 241.35
226.77, 226.91, 240.72
226.85, 227.00, 240.58
225.21, 242.99
−1484
−1349
−1553
−1555
−887
[CpMo(CO)3Cl]
[CpMo(CO)3CH3]
0.39
1.44
—
227.37, 240.49
−1736
a
b
c
All signals are referenced to deuterated solvent C6D6 δ 7.16 (for 1H) and δ 128.06 (for 13C{1H}). Ref. 45. All signals are referenced to 2 M
Na2MoO4 in D2O set at 0 ppm.
partly offset by the metal centre, which is coordinated to
three backbonding CO ligands, and therefore, these protons
appear upfield (1.88–2.16 ppm) for 1, 4 and 5. On the other
hand, the Mo–αCH signals of 2 and 3 are highly deshielded,
appearing downfield at 2.90 and 4.15 ppm, respectively. This
deshielding is also reflected in the respective 13C NMR shifts
of the α-carbon observed at 21.39 and 23.61 ppm for com-
plexes 2 and 3, respectively, in contrast to the highly upfield
Mo–αCH2 signals ranging from −3.35 to −3.78 ppm for 1, 4
and 5.
NMR spectra of complexes 4 and 5 and the X-ray structure
of 5 (Flack parameter 0.02(7), see the ESI†) confirm the
enantiopurity of the prepared complexes.
95Mo NMR chemical shifts are regarded as a suitable indi-
cator of the electronic situation or the Lewis acidity of the
metal centre.57 The 95Mo chemical shifts of complexes 1, 4
and 5 are similar, seen at ca. −1550 ppm. For complexes 2
and 3, the chemical shifts are observed at −1484 ppm and
−1349 ppm, respectively. These lie in between the 95Mo shift
of known tricarbonyl complexes [CpMo(CO)3Cl] (at −887 ppm)
and [CpMo(CO)3CH3] (at −1736 ppm). The trend in chemical
shifts (–Cl compared to the –CH3 complex) can be interpreted
to indicate that an electron-withdrawing substituent at the
Mo centre shows a downfield shift in comparison with an
electron-donating group.57 This implies that the Mo centre
is more electron-deficient in compound 3 compared to that
in complex 2, which is in agreement with the expected sub-
stituent effects (–CH3 vs. –C6H5 at α-C in conjunction with
the ester functional group).
In 13C NMR spectra, the carbonyl resonances of complexes
2–5 (in C6D6) appear as three well-resolved peaks, one each
for the two syn carbonyls and one for the third trans carbonyl
known to appear downfield to the two cis CO.51 Similar to
[CpMo(CO)3X] complexes (X = Me and Cl), compound 1 shows
no such inequivalence in the chemical shift for the terminal
carbonyl groups and only two signals are observed.
Variable temperature 13C NMR studies of 5 in C6D6 dem-
onstrate that the stereoelectronic asymmetry observed in the
form of two suitably resolved peaks for the electronically
inequivalent ‘cis-CO’ is present even until 70 °C. Coalescence
of the two carbonyl signals does not occur even at this high
temperature (Fig. S6, ESI†). Solid-state 1H MAS and 13C CPMAS
NMR spectra of 5 have been compared to those of [CpMo(CO)3Me]
(Fig. S7, ESI†). For complex 5, all three Mo-bound carbonyl
groups show chemical shift anisotropy in the solid state and
appear at 226.59, 230.16 and 242.25 ppm, in contrast to
[CpMo(CO)3Me] where the two cis-CO are equivalent at
230.3 ppm and the third appears at 242.3 ppm. This suggests
that the possible fluxional processes, namely, rotation of Cp
about the Mo-(η5-Cp) C5 axis, rapid interchangeability equiva-
lence of the square pyramidal basal cis-CO ligands, rotation
about the Mo–αC σ bond, and Berry-type pseudorotation52–56
might be slower or restricted probably due to the presence of
bulky substituents in compounds 2–5. However, for the
[CpMo(CO)3Me] complex, no such inequivalence is apparent
and only two carbonyl peaks can be observed in solution and
solid-state 13C NMR spectra. Complexes 2–5 are thus exam-
ples of monomeric cyclopentadienyl tricarbonyl molybdenum
piano-stool complexes where the barriers to various fluxional
processes involving Cp and basal ligands are significant even
at high temperatures. It is worth noting that the solution
Vibrational spectroscopy
The carbonyl ester group in 1–5 absorbs in the range of
1666–1690 cm−1, which is typical for complexes of this type.46
The absorption frequencies for the terminal carbonyl groups
differ only about 15 cm−1 in 1–5. The chemical shift anisot-
ropy, which is observed in solution and solid-state NMR spec-
tra of metal-bound carbonyl groups, is not clearly evident in
the absorption pattern and only two bands are seen in the
range of 2023–2028 cm−1 and 1926–1931 cm−1 58
This is not
.
altogether unusual as deviation from the original D4h geome-
try to the slightly perturbed C3v geometry in piano-stool com-
plexes results in experimental observation of only two bands,
A′ and A′′, out of the three possible νCO
.
Thermogravimetry and mass spectrometry
Thermogravimetric analysis combined with mass spec-
trometry data for complexes 2–5 indicate that the loss of a
Mo-bound carbonyl group (as CO+) initiates their decomposi-
tion. This is followed by a nearly simultaneous loss of
Cp, the other two carbonyls or the alkyl ester side chain.
These transformations are responsible for the complete
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decomposition of the precatalysts. The propensity of both
processes is accentuated when one CO is lost. Mass spectrome-
try and decomposition points determined from TGA-MS are
given in Table 2.
Single-crystal X-ray diffraction
Crystals of complexes 1, 2 and 5 were obtained from a
pentane–diethyl ether solvent mixture by slow vapor diffusion
and were suitable for single-crystal X-ray diffraction experi-
ments. The crystal structures of these complexes prove indis-
putably the η1-coordination of the ester side chain (Fig. 1–3).
The bond length Mo1–C9 (Mo–αC) in the crystal structures
of complexes 1, 2 and 5 differs significantly. This bond dis-
tance is the shortest for complex 1 (2.325(2) Å) due to the
least steric demand in the absence of any α-C substituent,
while in complex 2 (2.377(2) Å), the presence of the methyl
substituent exerts a higher steric demand and thus the bond
length increases. For complex 5, the bond length Mo1–C9 is
2.349(5) Å, which is in between those for 1 and 2. Here, the
steric influence is a result of the bulky bornyl ester group
even though there is no additional α-C substituent. The
Mo–αC bond length in complex [CpMo(CO)3CH3] is 2.326(3) Å,59
which is almost identical to the bond length in complex 1.
All terminal Mo–CO bonds and C–O bond lengths for 1, 2
and 5 are equal (within statistical error) and lie in the
expected range. Furthermore, the bond angle Cp–Mo1–C9 is
smaller for complex 1 (109.01(6)°) when compared to those
for complexes 2 and 5 (110.71(6)° and 111.0(2)°, respectively).
The torsion angle Cp–Mo1–C9–C10 is −177.8(2)° in 1, −59.8(2)°
in 2 and −51.2(4)° in 5, which indicates that the alkyl ester
moiety can rotate freely in 2 and 5 but only a staggered con-
formation of Cp–Mo1–C9–C10 is possible in complex 1. In
addition, the torsion angle C7–Mo1–C9 differs in the three
complexes significantly. It has the highest value for 1 (136.2(1)°),
which is possibly a consequence of conformation or packing
effects, while in complexes 2 (134.4(1)°) and 5 (132.7(3)°), the
higher steric demand and the gauche conformation to Mo1–Cp
make a close proximity between C9 and CO ligands possible,
making the torsion angle smaller than that for 1.
Fig. 1 ORTEP view of the single-crystal X-ray structure of compound
1. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond distances [Å], angles [°]
and torsion angles [°]: Mo1–C9 2.325(2), Mo1⋯Cp 1.9956(2), Mo1–C6
2.002(3), Mo1–C7 1.990(3), Mo1–C8 2.000(3), C6–O1 1.148(4), C7–O2
1.143(3), C8–O3 1.138(4), Cp–Mo1–C9 109.01(6), Mo1–C9–C10 116.0(2),
C6–Mo1–C9 77.7(2), C7–Mo1–C9 136.2(1), C8–Mo1–C9 78.4 (2),
Mo1–C9–C10–O4 88.1(3), Cp–Mo1–C9–C10 −177.8(2).
olefins trans- and cis-stilbene and trans-β-methylstyrene with
TBHP (5.5 M in n-decane) as the oxidant. Different molar
ratios of the catalysts, 1 mol% and 0.1 mol%, were investi-
gated and the ratio of substrate : TBHP = 1 : 2 was utilized in
all reactions, which were carried out at 22 and 55 °C. Catalytic
reactions were investigated under air in 5 mL of solvent
dichloromethane, chloroform or in the absence of a solvent.
For all catalytic reactions, <1% conversion of all substrates to
their epoxides was observed in the absence of the molybde-
num precatalyst, and similarly, the oxidant alone without the
catalyst was ineffective in any appreciable epoxidation of the
chosen substrates. Yield and TOF for each catalytic experi-
ment are given in Tables S1 and S2 in the ESI.†
During catalytic epoxidation of cis-cyclooctene with 1–5
and TBHP, an induction period is observed that lasts for
30 min to 2 h depending on the catalyst. This initial time
period is attributed to oxidative decarbonylation of the Mo(II)
Epoxidation catalysis
All complexes were applied as catalyst precursors for the epox-
idation of cis-cyclooctene and 1-octene as well as the prochiral
Table 2 Mass spectrometry data and decomposition points for complexes 1–5
Complex
MIa,b
Base peak
Methodd
Decomposition point (°C)e
1c
2
3
4
5
333
89
CI
32.5–33.5c
150
348.6 (14.8)
410.5 (13.7)
444.6 (4.6)
442.5 (19.9)
182.8
164.9
146.9
246.6
CI (+)
CI (+)
FAB (+)
FAB (+)
110 f
210–220 f
205
a
b
c
d
% Relative abundance in parenthesis. CI-MS (+) m/z are M + 1 peaks. Ref. 46. CI refers to chemical ionization; FAB refers to fast atom
bombardment method. Determined by TGA-MS under an inert argon atmosphere with Al2O3 correction; temperature gradient 10 K min−1
.
e
f
Gradual decomposition, triggered by the loss of one CO+.
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of cis-cyclooctene was accompanied by evolution of heat after
the addition of TBHP, indicating that oxidative decarbonylation
is exothermic. This is, at least in part, responsible for faster
conversions to cyclooctene oxide along with the smaller dilu-
tion factor and thus increased TOFs (210–500 h−1) (Fig. 4(c)).
At a higher reaction temperature of 55 °C, conversion of
cis-cyclooctene to its epoxide is very fast and quantitative
yields are obtained within 10 min after addition of the oxi-
dant (Fig. 4(d)). There is no clearly discernible induction
period for these catalysis experiments and TOFs are 780 h−1
(1, 4, 5), 1020 h−1 (2) and 1190 h−1 (3). TOFs for catalytic
epoxidation of cis-cyclooctene in the case of complexes
[CpMo(CO)3Cl]21 and [CpMo(CO)3CH3]11 are 1300 h−1 and
820 h−1, respectively, using a catalyst : substrate : TBHP ratio
of 1 : 100 : 200 at 55 °C. For catalysis with complexes 2 and 3,
within 5 min of addition of TBHP, rapid evolution of gases is
observed with a simultaneous increase in temperature over
55 °C. This temperature increase is a result of oxidative
decarbonylation of the tricarbonyl complexes and is respon-
sible for the high activity (as indicated by TOFs) of these
complexes. During catalysis with complexes 1, 4 and 5, such
violent exothermic reactions are not observed; however, due
to the high reaction temperature, the conversion of cis-cyclooctene
is very fast, and for all complexes, quantitative yield of the
epoxide is obtained within 15 min.
Fig. 2 ORTEP view of the single-crystal X-ray structure of compound 2.
Thermal ellipsoids are drawn at a 50% probability level. The hydrogen
atoms and the disorder in the ester moiety are omitted for clarity.
Selected bond distances [Å], angles [°] and torsion angles [°]: Mo1–C9
2.377(2), Mo1⋯Cp 2.0118(3), Mo1–C6 1.997(2), Mo1–C7 1.997(3),
Mo1–C8 2.004(2), C6–O1 1.145(3), C7–O2 1.145(3), C8–O3 1.141(3),
Cp–Mo1–C9 110.71(6), Mo1–C9–C10 108.3(2), C6–Mo1–C9 78.9(1),
C7–Mo1–C9 134.4(1), C8–Mo1–C9 73.6(1), Mo1–C9–C10–O4 96.3(3),
Cp–Mo1–C9–C10 −59.8(2).
The stilbene substrates are selectively transformed to their
respective epoxides in yields of up to 50% within 4 h and
these yields only marginally increase up to 65% after 24 h
(Fig. 4(e) and (f)). A more challenging substrate, 1-octene, is
converted to its epoxide slowly, and yields of about 40%
are obtained after 24 h with 1 mol% of the catalysts. The
conversion of the terminal alkene (1-octene) and aromatic
substrates (stilbene, methylstyrene) is both poor and slow rel-
ative to cis-cyclooctene. This can be due to deactivation of the
primary catalyst before complete epoxidation of these sub-
strates. There is little influence of the increasing steric bulk
of the ester alkyl group from ethyl (1, 2 and 3) to menthyl (4)
or bornyl (5) on catalytic activity, which is not surprising as
the electronic situation at the metal centre is similar for the
three α-carbon-unsubstituted precatalysts. In addition, the
reaction site is farther from the ethyl group or the sterically
encumbered menthyl or bornyl groups located at the end of
the oxoalkyl side chain.
Although epoxidation of trans-β-methylstyrene is selective
towards the epoxide product, there is negligible (within the
experimental error) stereo-differentiation during catalysis and
only equimolar amounts of (2S,3S)-2-methyl-3-phenyloxirane
and (2R,3R)-2-methyl-3-phenyloxirane are obtained. Poor ee
obtained with these complexes can be reasoned to be due to
the location of chiral information being still too far away
from the reactive metal centre.
Fig. 3 ORTEP view of the single-crystal X-ray structure of compound 5.
Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond distances [Å], angles [°] and torsion
angles [°]: Mo1–C9 2.349(5), Mo1⋯Cp 1.9917(5), Mo1–C6 2.008(5),
Mo1–C7 1.983(7), Mo1–C8 1.978(7), C6–O1 1.141(6), C7–O2 1.170(8),
C8–O3 1.161(7), Cp–Mo1–C9 111.0(2), Mo1–C9–C10 111.0(4), C6–Mo1–C9
77.6(2), C7–Mo1–C9 132.7(3), C8–Mo1–C9 73.5(2), Mo1–C9–C10–O4
91.4(6), Cp–Mo1–C9–C10 −51.2(4).
precatalyst to give the catalytically active oxomolybdenum(VI)
species (see Fig. 4 and Scheme 2).21,22,60 The concentration of
active species present in the reaction mixture is very small in
the beginning of the reaction, and therefore, conversion of
the substrate to its epoxide is originally also small. Once a
critical amount of oxidized species is formed, epoxidation of
the substrate becomes quite fast, as indicated by the steep
part of the plots in Fig. 4.
1–5 are active catalysts for the epoxidation of
cis-cyclooctene, forming cyclooctene oxide selectively and
quantitatively within 2–4 h (1 mol% catalyst) in CH2Cl2
(Fig. 4(a)). Activities in the range of 120–190 cycles per hour
are observed for cyclooctene oxide formation, which increase
to 230–360 h−1 when 0.1 mol% of the precatalysts is used
(Fig. 4(b)). In the absence of a cosolvent, catalytic epoxidation
Complexes 2 and 3 with a methyl and a phenyl substituent
on Mo–αC, respectively, are generally more active than
complexes 1, 4, and 5, which are unsubstituted at this posi-
tion. Furthermore, catalysis with 3 gives a slightly higher
yield of epoxides for nearly all substrates tested compared to
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Fig. 4 Conversion vs. time plot of different substrates with 1 mol% complexes 1–5 and TBHP oxidant at room temperature in dichloromethane
unless otherwise stated: (a) cis-cyclooctene, (b) cis-cyclooctene + 0.1 mol% catalyst, (c) cis-cyclooctene, no co-solvent, (d) cis-cyclooctene, 55 °C,
CHCl3 solvent, (e) cis-stilbene, (f) trans-stilbene, (g) 1-octene, (h) trans-β-methylstyrene. Ratio of catalyst : substrate : oxidant = 1 : 100 : 200.
Scheme 2 The oxidation of tricarbonyl precatalyst 5 with 10 equiv. of TBHP (in n-decane) results in the formation of both dioxo (I) and
oxo–peroxo (II) species at room temperature in CDCl3.
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that with complex 2. This trend may be accounted for by the
observation that the molybdenum centre appears slightly
more electron-deficient in 2 and 3 compared to those in
precatalysts 1, 4 and 5.
that although only a part of the precatalyst is converted to
the active species, the rate of epoxidation is quite high.
The signal for Cp of precatalyst 5 at 5.22 ppm and a new signal
for the oxidized complex at 6.28 ppm can both be observed
after 4 h of monitoring the catalysis reaction, confirming that
the Cp ligand is retained after the oxidative transformation.
In 13C NMR spectra, a prominent signal from the Cp ligand
of the oxidized complex (Cp(ox)) is observed at 111.3 ppm,
evolving from the Cp signal at 93.4 ppm of the tricarbonyl
precatalyst 5 and has been assigned to the dioxo complex I.
The proposal outlined in Scheme 2 asserts that in situ oxida-
tion of 5 with TBHP forms complexes I (dioxo) and (later) II
(oxo–peroxo) and both are catalytically active for olefin epoxi-
dation. The work presented here supports the possibility of
the oxidation shown in Scheme 2 but does not unambigu-
ously support the structures of I and II. A comparison with
NMR data of similar complexes21 suggests, however, that this
assignment is most likely correct. A small signal at 111.7 ppm
can be seen (Fig. 6(d)) when the amount of cyclooctene
decreases appreciably at later stages of the reaction and has
been assigned to the Cp ligand of the oxo–peroxo complex II
based on the 95Mo NMR chemical shift (see discussion
below, Fig. S11 in the ESI†) and crystallographic evidence
(Table S3 in the ESI†). Since cyclooctene is converted to its
epoxide before the amount of the oxo–peroxo complex is
significant, it is evident that the rate of oxidation of olefin
with the dioxo complex is quite high. Alternatively, this
observation suggests that the presence of the olefin (in its
NMR study of catalytic epoxidation of cis-cyclooctene
In order to confirm the stability of the Mo–R bond in the
chiral catalyst during epoxidation, we followed the progress
of catalytic epoxidation of cis-cyclooctene with TBHP using
1H and 13C NMR. Complex 5 was chosen as the precatalyst
since its oxidative transformation is slow enough to be studied
on a suitable time scale in NMR at room temperature espe-
cially in comparison with precatalysts 2, 3, [CpMo(CO)3Cl]
and [CpMo(CO)3CH3] for which the reaction was observed to
be exothermic. In analogy with previous reports,9,23 it is
proposed that the reaction of 5 with TBHP proceeds as illus-
trated in Scheme 2, and the oxidized complexes I and II are
the catalytically active species.
A mixture of ca. 0.1 mmol of 5 in CDCl3 and 10 equiv. of
cis-cyclooctene was treated with 20 equiv. of TBHP at 22 °C
1
and the reaction progress was monitored by H (Fig. 5) and
13C NMR (Fig. 6). Quantitative epoxidation of cis-cyclooctene
to its epoxide takes place within 3.5 h, as indicated by the dis-
appearance of the cis-cyclooctene multiplet at 5.68–5.85 ppm.
However, complex 5 does not undergo complete oxidative
decarbonylation and all three terminal CO signals can be
observed even after 4 h in 13C NMR (Fig. 6(a)). This indicates
Fig. 5 1H NMR profile for the reaction of 5 with 10 equiv. of cis-cyclooctene and 20 equiv. of TBHP (in decane) in CDCl3 at 22 °C (with mesitylene
as the internal standard).
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Fig. 6 13C NMR profile for the reaction of 5 with 10 equiv. of cis-cyclooctene and 20 equiv. of TBHP (in n-decane) in CDCl3 at 22 °C showing
spectral regions for (a) terminal CO, (b) alkylester CO, (c) mesitylene (internal standard), (d) Cp ligand, and (e) solvent CDCl3, tert-butanol,
epoxycyclooctane, and alkyl moieties CH2 and CH3 of R1 group in complex 5.
role as a reductant) affects the oxidation of the precatalyst, i.
e. by suppressing the conversion of I to II.
the dioxo complex I) and a signal at 170.8 ppm (for the ester
carbonyl) in the 13C NMR spectrum. tert-Butanol is evolved as
a side product, appearing as a broad signal in 1H spectra
from 2.9 to 3.3 ppm. The complete assignment of observed
1H and 13C NMR chemical shifts is given in Table 3.
The concentration vs. time plots illustrated in Fig. 7 follow
the progress of catalytic epoxidation and the changes in the
concentrations of the substrate, precatalyst 5 and dioxo com-
plex I. From a starting concentration of [5] = 0.249 M, the
amount of 5 present in solution after 3.5 h left unreacted is
0.079 M (31.7%) and the concentration of the catalytically
In Fig. 6(b), the signals of the ester carbonyl carbon
are not clearly observed in the period following oxidative
decarbonylation (between 15 and 45 min) but they reappear
at about 55 min. It is unclear why this might occur since the
signals of other quaternary carbon (CO group) can still be
observed. The chiral side chain does not dissociate during
the catalysis reaction, as evident from a persistent multiplet
1
in the H NMR spectrum from 4.6 to 4.76 ppm after 4 h (cor-
responding to the hydrogen at the bornyl chiral centre from
Table 3 Assignment of 1H and 13C NMR chemical shifts (in ppm) observed during catalytic epoxidation of cis-cyclooctene with 5 and TBHP oxidant in
CDCl3 at 22 °C (5: CyOc:TBHP = 1:10:20). R1 = Bornyl, CyOc = cis-cyclooctene, EpCy = epoxycyclooctane
Complex
1H NMR, δ (ppm)
13C NMR, δ (ppm)
Time
5
Cp
5.22
Cp
93.4
0–4 h
C(O)OCH
4.93–4.99
Mo–CO
(OC)OR1
Cp
226.14, 226.36, 239.67
182.3 → 184.0b
111.3
I
Cp
C(O)OCH
Cp
C(O)OCH
–HCCH–
–HC–(O)–CH–
6.28
35 min–4 h
4.6–4.76a
(OC)OR1
Cp
176.16
II
6.27
111.7c
175 min–4 h
a
(OC)OR1
–HCCH–
–HC–(O)–CH–
171.4c
CyOc
EpCy
5.68–5.85
2.64–2.75
130.0
55.5
0–165 min
15 min–4 h
a
b
See ‘Bornyl-HC–O–C( O)’ in Fig. 5 for changes in the observed multiplet of this proton. Chemical shift changes due to the change in
polarity. c Observed after 3 h when epoxidation of cyclooctene is complete.
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active dioxo species is 0.083 M (~33.3%). This suggests that
in situ generation of catalytically active complexes is not very
efficient. Alternatively, since about one-third of the precatalyst
is left unreacted during catalytic epoxidation, these results
suggest that the conversion of the cyclooctene substrate
to its epoxide takes precedence over a complete oxidative
decarbonylation of the precatalyst.
gradually (Fig. S12 in the ESI†). This may be attributed to the
auto-retardation effect of tert-butanol, which is present in the
reaction mixture after in situ oxidation of the precatalyst
and/or lower activity of the oxo–peroxo species relative to the
dioxo complex.
Accordingly, it can be confirmed that complex II (oxo–peroxo)
is also an active catalyst for olefin epoxidation.
In an attempt to evaluate how such homogeneous cata-
lysts perform in subsequent catalytic runs without isolating
the active oxo complex, we added the substrate after treating
the precatalyst with TBHP. The 95Mo NMR spectrum of
the reaction mixture on treating 5 with 50 equiv. of TBHP
in CDCl3 shows a broad signal at −628 ppm (Fig. S11 in the
ESI†). This chemical shift is similar to 95Mo signals of other
[CpMo(O)(O2)R] complexes (R = CH3, −609 ppm; R = CF3,
−709 ppm).9,23 For this reason, the persistent Cp signal
Conclusion
Cyclopentadienyl molybdenum complexes with different
oxoalkyl side chains have been synthesized and investigated
for achiral and chiral epoxidation catalysis. The chiral ligands
for the synthesized complexes are derived from cheap and
readily available chiral pool compounds. A comparison between
substituted and unsubstituted complexes at the α-carbon to
molybdenum is presented. The former are more active for
the epoxidation of olefins on account of a more electron-
deficient metal centre and 3 is a better catalyst for nearly all
substrates. 1–5 display moderate activities in epoxidation
catalysis at room temperature compared to many other half-
sandwich tricarbonyl Mo(II) complexes previously reported,
and no stereoselectivity is achieved with prochiral substrates,
probably due to the pronounced distance between the chiral
centres and the catalytic sites. Nevertheless, due to the some-
what reduced catalytic activity, insight into the progress of
the precatalyst oxidation and the epoxide formation is
gained. Cyclooctene epoxidation progress with precatalyst 5
and TBHP monitored by NMR indicates that in situ oxidation
of 5 with TBHP forms the highly active dioxo complex, and
thus, both the Cp ligand and the chiral side chain are
retained. However, ca. 32% of precatalyst 5 remain unreacted
and catalytic epoxidation takes precedence over complete oxi-
dation of the tricarbonyl precursor. The oxo–peroxo complex
is also formed on oxidation of the dioxo complex, albeit only
near-complete conversion of the substrate in the epoxidation
1
at 6.27 ppm in the H NMR spectrum and 111.7 ppm in the
13C NMR spectrum is assigned to the oxidized species (II).
After 48 h, cis-cyclooctene (10 equiv.) was added into the
NMR tube containing the pre-oxidized complex (II). The con-
centration of the oxidized complex available for epoxidation
of the substrate was determined to be ca. 0.054 M. This
amount is ~50% less than that present after 4 h of oxidation
of the precatalyst (ca. 0.11 M), indicating that either II is
slowly transformed into another species or it undergoes
decomposition. Complex II also catalyses the transformation
of cis-cyclooctene to its epoxide, although conversion occurs
1
reaction. The quantitative H NMR study also confirms that
the oxo–peroxo complex [CpMo(O)(O2)R] obtained through
oxidation of the precursor with TBHP is catalytically active
for olefin epoxidation.
Acknowledgements
N.G. is grateful to the TUM Graduate School of Chemistry for
financial support.
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