Performance of chiral tetracarbonylmolybdenum pyrindanyl amine complexes in catalytic olefin epoxidation

Article abstract of DOI:10.1016/j.jorganchem.2018.01.003

Tetracarbonylmolybdenum(0) complexes of the type cis-[Mo(CO)₄(L)] containing chiral 7-(1-pyrindanyl) amine ligands were prepared and found to be effective precatalysts for the epoxidation of achiral (cis-cyclooctene) and prochiral (DL-limonene and trans-β-methylstyrene) olefins at 55 °C. Epoxides were the only products formed from cis-cyclooctene (100% yield) and trans-β-methylstyrene (100% selectivity at 82–85% conversion), and the main products formed from DL-limonene (80–82% 1,2-epoxide selectivity at 85% conversion). Characterization of recovered catalysts revealed that the precatalysts were transformed in situ to stable polyoxomolybdate salts containing the β-octamolybdate anion [β-Mo₈O₂₆]^4?, which was responsible for the catalytic reaction.

Full text of DOI:10.1016/j.jorganchem.2018.01.003

Accepted Manuscript
Performance of chiral tetracarbonylmolybdenum pyrindanyl amine complexes in
catalytic olefin epoxidation
Patrícia Neves, Lucie S. Nogueira, Anabela A. Valente, Martyn Pillinger, Isabel S.
Gonçalves, Ivo E. Sampaio-Dias, Carlos A.D. Sousa, Fabio Rizzo-Aguiar, José E.
Rodríguez-Borges
PII:
S0022-328X(18)30003-2
10.1016/j.jorganchem.2018.01.003
JOM 20241
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 19 December 2017
Revised Date: 3 January 2018
Accepted Date: 5 January 2018
Please cite this article as: Patrí. Neves, L.S. Nogueira, A.A. Valente, M. Pillinger, I.S. Gonçalves, I.E.
Sampaio-Dias, C.A.D. Sousa, F. Rizzo-Aguiar, José.E. Rodríguez-Borges, Performance of chiral
tetracarbonylmolybdenum pyrindanyl amine complexes in catalytic olefin epoxidation, Journal of
Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.01.003.
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ACCEPTED MANUSCRIPT
Manuscript for J. Organomet. Chem.
3 January 2018
Performance of chiral tetracarbonylmolybdenum pyrindanyl
amine complexes in catalytic olefin epoxidation
a
a
a
Patrícia Neves , Lucie S. Nogueira a, Anabela A. Valente , Martyn Pillinger ,
a
b
b
Isabel S. Gonçalves , Ivo E. Sampaio-Dias , Carlos A. D. Sousa , Fabio
Rizzo-Aguiar c, José E. Rodríguez-Borges b
* Corresponding authors.
E-mail addresses: igoncalves@ua.pt (I.S. Gonçalves), atav@ua.pt (A.A. Valente)
a
Department of Chemistry, CICECO - Aveiro Institute of Materials, University of Aveiro,
Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
b
LAQV/REQUIMTE, Department of Chemistry and Biochemistry, Faculty of Sciences,
University of Porto, 4169-007 Porto, Portugal
c
IFSP – Federal Institute of Education, Science and Technology of São Paulo, Campus São
Paulo, 01109-010, São Paulo, SP, Brazil
ABSTRACT
Tetracarbonylmolybdenum(0) complexes of the type cis-[Mo(CO)4(L)] containing chiral 7-
(1-pyrindanyl) amine ligands were prepared and found to be effective precatalysts for the
epoxidation of achiral (cis-cyclooctene) and prochiral (DL-limonene and trans-β-
methylstyrene) olefins at 55 ºC. Epoxides were the only products formed from cis-
cyclooctene (100% yield) and trans-β-methylstyrene (100% selectivity at 82-85%
conversion), and the main products formed from DL-limonene (80-82% 1,2-epoxide
selectivity at 85% conversion). Characterization of recovered catalysts revealed that the
precatalysts were transformed in situ to stable polyoxomolybdate salts containing the
octamolybdate anion [
-Mo8O26]4‒, which was responsible for the catalytic reaction.
β-
β
Keywords: Molybdenum; Tetracarbonyl complexes; Olefin epoxidation; Chiral ligands; 1-
pyrindane derivatives; Oxidative decarbonylation
1

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1. Introduction
The family of group 6 metal carbonyl complexes is versatile and effective for several
catalytic reactions such as hydrogenation, hydrosilylation, hydrogermylation, hydroxylation,
isomerization, olefin metathesis and metathesis polymerization, radical reactions, and
(ep)oxidation (e.g. of alkenes, amines, alcohols and sulfides) [1-5]. The application of these
carbonyl complexes in catalysis first arose in the late 1960’s [6,7] with the use of Mo(CO)6 as
a precatalyst for the epoxidation of olefins (monoolefins [6], diolefins and functionalized
olefins [7]), using an organic hydroperoxide as oxidant, namely tert-butylhydroperoxide
(TBHP) or cumene hydroperoxide. Halcon (later Oxirane, Arco Chemical, and Lyondell
Petrochemical [7]) claimed the application of Mo(CO)6 for the industrial catalytic production
of the bulk chemical propylene oxide, which is used in a variety of plastics, foams and paints
[3,8-10]. More recently, in 2007, Dow Global Technologies Inc. claimed the application of
various families of molybdenum compounds as homogeneous catalysts to produce epoxides
via liquid-phase reaction of olefins with organic hydroperoxides [11].
Since the turn of the century, there has been growing interest in the use of molybdenum
carbonyl complexes as precursors to MoVI catalysts for oxidation reactions, with most of the
research being focused on dicarbonyl and tricarbonyl complexes [2-5,12-14]. From 2010
onwards some of us have been investigating tetracarbonyl complexes of the type cis-
[Mo(CO)4L] (L = N,N-ligand) for the epoxidation of aliphatic and cyclic olefins (e.g.
terpenes), using TBHP as oxidant [15-19]. The Mo0 complexes can be used directly as
catalyst precursors since they undergo oxidative decarbonylation in situ (by reaction with the
oxidant) to give oxomolybdenum compounds in higher (usually +6) oxidation states. A
variety of oxidized compounds have been isolated and include the one-dimensional
molybdenum oxide/bipyridine polymer [MoO3(2,2'-bipy)] from cis-[Mo(CO)4(2,2'-bipy)]
(2,2'-bipy = 2,2'-bipyridine) [15], the octanuclear cluster [Mo8O24(di-tBu-bipy)4] from cis-
[Mo(CO)4(di-tBu-bipy)] (di-tBu-bipy = 4,4'-di-tert-butyl-2,2'-bipyridine) [15], and the
tetranuclear species [Mo4O12(pypzH)4] from cis-[Mo(CO)4(pypz)] (pypzH = 2-[3(5)-
pyrazolyl]pyridine) [16].
To the best of our knowledge, tetracarbonylmolybdenum complexes containing chiral
organic ligands have not yet been studied in catalytic olefin epoxidation. Herein we report on
the synthesis of two chiral tetracarbonylmolybdenum pyrindanyl amine complexes and their
use as precatalysts for the epoxidation of cis-cyclooctene (used as a benchmark substrate),
2

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DL-limonene and trans-β-methylstyrene. The catalytic performances of the complexes are
compared with results reported previously for related complexes. Characterization of
recovered catalysts established that the tetracarbonyl complexes are oxidized to
octamolybdate salts of the type (LH)4[Mo8O26] (rather than to hybrid complexes or materials
where the organic ligand remains coordinated to the metal center, which has been reported in
the literature for different tetracarbonylmolybdenum organo complexes).
2. Experimental
2.1. Materials and methods
Molybdenum hexacarbonyl (Fluka), cis-cyclooctene (95%, Alfa Aesar), DL-limonene
(≥95%, Merck), trans-β-methylstyrene (99%), tert-butylhydroperoxide (5.0-6.0 M in decane),
α,α,α-anhydrous trifluorotoluene (≥99%), anhydrous acetonitrile (99.8%), hexane (99%), 1,2-
dichloroethane (99%), n-pentane (>95%, Carlo Erba), diethyl ether (99.8%), undecane (99%),
(R)-(+)-α-methylbenzylamine
(98%),
(S)-(−)-α-methylbenzylamine
(98%),
37%
formaldehyde solution, and sodium triacetoxyborohydride (97%) were purchased from
Sigma-Aldrich unless otherwise indicated, and used as received.
Microanalyses for C, H and N were carried out at the Department of Chemistry,
University of Aveiro, with a Truspec Micro CHNS 630-200-200 elemental analyzer. FT-IR
spectra in the 300−4000 cm−1 range were collected using KBr (Sigma-Aldrich, 99%, FT-IR
grade) pellets and a Mattson-7000 infrared spectrophotometer. Attenuated total reflectance
(ATR) FT-IR spectra were measured using a Specac Golden Gate Mk II ATR accessory
having a diamond top plate and KRS-5 focusing lenses (256 scans, resolution 4 cm‒1).
1
Solution H and 13C NMR spectra were measured with Bruker CXP 300 (Department of
Chemistry, University of Aveiro), Avance-300 (Department of Chemistry, University of
Aveiro) or Avance III 400 (Centro de Materiais da Universidade do Porto – CEMUP)
instruments. Chemical shifts are quoted in ppm from TMS. Mass spectra were recorded on a
LTQ Orbitrap XL mass spectrometer (Thermo Fisher Scientific, Bremen, Germany)
controlled by LTQ Tune Plus 2.5.5 and Xcalibur 2.1.0 (CEMUP). Optical rotations were
measured on a thermostated Jasco P-2000 digital polarimeter using a sodium lamp. Flash
chromatography was performed on silica gel (Merck 60, 230–240 mesh), and analytical TLC
was carried out on pre-coated silica gel plates (Merck 60 F254, 0.25 mm) using UV light
3

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and/or an ethanolic solution of phosphomolybdic acid (followed by gentle heating) for
visualization.
2.2. Synthesis
2.2.1. General protocol for reductive amination
In a round bottomed flask, the appropriate amine was dissolved in anhydrous CH2Cl2
(20 mL) followed by addition of anhydrous Na2SO4 and the carbonyl compound (ketone 3 or
formaldehyde) (Scheme 1). The reaction was left under magnetic stirring for 4 h at room
temperature. After that period, the reducing agent, NaBH(OAc)3, was carefully added and the
reaction allowed to proceed for a further 20 h. The reaction mixture was filtered under
reduced pressure and saturated NaHCO3 (20 mL) was added to the filtrate. The phases were
transferred to a separating funnel and the aqueous phase was washed with CH2Cl2 (3 × 20
mL). The organic extracts were combined and dried over anhydrous Na2SO4, filtered, and the
solvent removed in vacuo. The crude oil was chromatographed using AcOEt as eluent,
affording the desired amine.
2.2.1.1. (R,R)-N-(1-phenylethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine (4)
Following the general protocol for reductive amination, (R)-(+)-α-methylbenzylamine
(0.73 mL, 5.6 mmol) was reacted with ketone 3 (0.500 g, 3.75 mmol) and NaBH(OAc)3
(1.193 g, 5.63 mmol). After the work-up procedure and purification, amine 4 was obtained as
a brown oil (0.715 g, 80%). 1H-NMR (CDCl3, 400 MHz): δ = 8.40 (d, J = 4.9 Hz, 1H, H-2),
7.50–7.39 (m, 3H, ArH), 7.35–7.28 (m, 2H, ArH), 7.27–7.20 (m, 1H, ArH), 7.05 (dd, J = 7.5,
5.0 Hz, 1H, H-3), 4.24 (q, J = 6.6 Hz, 1H, NHCH(CH3)Ph), 4.12 (t, J = 7.7 Hz, 1H, H-7),
2.79 (ddd, J = 16.1, 8.9, 2.9 Hz, 1H, H-5syn), 2.68–2.57 (m, 1H, H-5anti), 2.53 (br s, 1H, NH),
1.97 (dtd, J = 12.8, 7.8, 3.0 Hz, 1H, H-6syn), 1.58 (ddd, J = 17.1, 12.8, 8.9 Hz, 1H, H-6anti),
13
1.47 (d, J = 6.6 Hz, 3H, NHCH(CH3)Ph) (see Fig. 1 for atom labelling). C-NMR (CDCl3,
101 MHz): δ = 165.5 (C-7a), 147.8 (CH, C-2), 146.5 (C, Cipso), 136.5 (C, C-4a), 132.6 (CH,
C-4), 128.4 (2CH, 2 × Cmeta), 127.4 (2CH, 2 × Cortho), 127.1 (CH, Cpara), 122.2 (CH, C-3),
62.5 (CH, C-7), 58.7 (CH, NHCH(CH3)Ph), 33.8 (CH2, C-6), 28.3 (CH2, C-5), 24.4 (CH3,
NHCH(CH3)Ph). ESI-MS: calculated for [C16H19N2]+, 239.15; obtained, 239.19. ꢀꢁꢂꢃ ℃ = +
28.6 (c1, CHCl3).
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2.2.1.2. (S,S)-N-(1-phenylethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine (5)
Following the general protocol for reductive amination, (S)-(−)-α-methylbenzylamine
(0.73 mL, 5.6 mmol) was reacted with ketone 3 (0.500 g, 3.75 mmol) and NaBH(OAc)3
(1.193 g, 5.63 mmol). After the work-up procedure and purification, amine 5 was obtained as
a brown oil (0.751 g, 84%). 1H-NMR (CDCl3, 400 MHz): δ = 8.40 (d, J = 4.9 Hz, 1H, H-2),
7.50–7.41 (m, 3H, ArH), 7.36–7.28 (m, 2H, ArH), 7.27–7.20 (m, 1H, ArH), 7.05 (ddd, J =
7.5, 4.9, 0.4 Hz, 1H, H-3), 4.25 (q, J = 6.6 Hz, 1H, NHCH(CH3)Ph), 4.12 (t, J = 7.7 Hz, 1H,
H-7), 2.80 (ddd, J = 16.1, 8.8, 2.9 Hz, 1H, H-5syn), 2.73–2.45 (m, 2H, H-5anti + NH), 1.97
(dtd, J = 12.8, 7.8, 3.0 Hz, 1H, H-6syn), 1.60 (ddd, J = 17.1, 12.8, 8.8 Hz, 1H, H-6anti), 1.48 (d,
13
J = 6.6 Hz, 3H, NHCH(CH3)Ph). C-NMR (CDCl3, 101 MHz): δ = 165.2 (C, C-7a). 147.8
(CH, C-2), 146.0 (C, Cipso), 136.7 (C, C-4a), 132.7 (CH, C-4), 128.8 (2 × CH, ArC), 127.5 (2
× CH, ArC), 127.2 (CH, ArC), 122.3 (CH, C-3), 62.3 (CH, C-7), 58.6 (CH, NHCH(CH3)Ph),
33.6 (CH2, C-6), 28.2 (CH2, C-5), 24.5 (CH3, NHCH(CH3)Ph). ESI-MS: calculated for
[C16H19N2]+, 239.15; obtained, 239.39. ꢀꢁꢂꢃ ℃ = − 27.9 (c1, CHCl3).
2.2.1.3. (R,R)-N-methyl-N-(1-phenylethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine
[(R,R)-pyC5H5N(CH3)CH(CH3)Ph)=L1]
Following the general protocol for reductive amination, amine 4 (0.700 g, 2.94 mmol)
was reacted with an aqueous 37% solution of formaldehyde (0.33 mL, 4.4 mmol) and
NaBH(OAc)3 (1.869 g, 8.82 mmol). After the work-up procedure and purification, amine L1
was obtained as a brown oil (0.579 g, 78%). 1H-NMR (CDCl3, 400 MHz): δ = 8.60–8.38 (m,
1H, H-2), 7.55–7.48 (m, 2H, ArH), 7.45 (dd, J = 7.6, 1.3 Hz, 1H, ArH), 7.35–7.28 (m, 2H,
ArH), 7.26–7.20 (m, 1H, ArH), 7.05 (dd, J = 7.5, 4.9 Hz, 1H, H-3), 4.57 (t, J = 7.7 Hz, 1H,
H-7), 4.16 (d, J = 5.0 Hz, 1H, N(CH3)CH(CH3)Ph), 2.88 (ddd, J = 16.2, 8.8, 4.7 Hz, 1H, H-
5syn), 2.74 (ddd, J = 16.2, 8.4, 7.7 Hz, 1H, H-5anti), 2.14–1.93 (m, 5H, H-6syn + H-6anti
+
N(CH3)CH(CH3)Ph), 1.49 (d, J = 6.7 Hz, 3H, NHCH(CH3)Ph). 13C-NMR (CDCl3, 101
MHz): δ = 164.6 (C, C-7a), 148.2 (CH, C-2), 145.8 (C, Cipso), 137.1 (C, C-4a), 133.0 (CH, C-
4), 128.4 (2CH, 2 × Cmeta), 127.8 (2CH, 2 × Cortho), 126.9 (CH, Cpara), 121.8 (CH, C-3), 65.2
(CH, C-7), 63.0 (CH, N(CH3)CH(CH3)Ph), 33.3 (CH3, N(CH3)CH(CH3)Ph) 28.3 (CH2, C-6),
24.3 (CH2, C-5), 21.3 (CH3, N(CH3)CH(CH3)Ph). ESI-MS: calculated for [C17H21N2]+,
253.17; obtained, 253.15. ꢀꢁꢂꢃ ℃ = + 26.2 (c1, CHCl3).
5

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2.2.1.4. (S,S)-N-methyl-N-(1-phenylethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine
[(S,S)-pyC5H5N(CH3)CH(CH3)Ph)=L2]
Following the general protocol for reductive amination, amine 5 (0.700 g, 2.94 mmol)
was reacted with an aqueous 37% solution of formaldehyde (0.33 g, 4.4 mmol) and
NaBH(OAc)3 (1.869 g, 8.82 mmol). After the work-up procedure and purification, amine L2
was obtained as a brown oil (0.549 g, 74%). 1H-NMR (CDCl3, 400 MHz): δ = 8.52–8.46 (m,
1H, H-2), 7.55–7.48 (m, 2H, ArH), 7.45 (dd, J = 7.6, 1.2 Hz, 1H, ArH), 7.35–7.28 (m, 2H,
ArH), 7.26–7.20 (m, 1H, ArH), 7.05 (dd, J = 7.5, 4.9 Hz, 1H, H-3), 4.57 (t, J = 7.7 Hz, 1H,
H-7), 4.16 (br s, 1H, N(CH3)CH(CH3)Ph), 2.88 (ddd, J = 16.1, 8.8, 4.7 Hz, 1H, H-5syn), 2.74
(ddd, J = 16.3, 8.1, 7.7 Hz, 1H, H-5anti), 2.15–1.97 (m, 5H, H-6syn + H-6anti
+
N(CH3)CH(CH3)Ph), 1.49 (d, J = 6.7 Hz, 3H, NHCH(CH3)Ph). 13C-NMR (CDCl3, 101
MHz): δ = 164.2 (C, C-7a), 148.2 (CH, C-2), 145.9 (C, Cipso), 137.1 (C, C-4a), 132.5 (CH, C-
4), 128.4 (2CH, 2 × Cmeta), 127.9 (2CH, 2 × Cortho), 126.9 (CH, Cpara), 121.9 (CH, C-3), 65.2
(CH, C-7), 63.0 (CH, N(CH3)CH(CH3)Ph), 33.4 (CH3, N(CH3)CH(CH3)Ph), 28.3 (CH2, C-6),
24.3 (CH2, C-5), 21.3 (CH3, N(CH3)CH(CH3)Ph). ESI-MS: calculated for [C17H21N2]+,
253.17; obtained, 253.19. ꢀꢁꢂꢃ ℃ = − 27.3 (c1, CHCl3).
2.2.2. General procedure for synthesis of tetracarbonyl complexes
In a Schlenk tube, Mo(CO)6 and organic ligand L1 or L2 were reacted in refluxing
anhydrous CH3CN (40 mL) for 30 min under nitrogen. Solvent was removed from the
resultant brown mixture by evaporation under reduced pressure, giving a brown solid that
was washed with hexane (2 × 40 mL) and pentane (20 mL), and finally vacuum-dried.
2.2.2.1. cis-[Mo(CO)4(L1)] (1)
Mo(CO)6 (0.31 g, 1.17 mmol) was reacted with L1 (0.30 g, 1.19 mmol), giving 1 in
54% yield (0.29 g). Anal. Calcd for C21H20N2MoO4 (460.33): C, 54.79; H, 4.38; N, 6.08.
Found: C, 54.40; H, 4.45; N, 6.10%. Selected FT-IR (KBr, cm−1): 359 (m), 705 (m), 796 (m),
1
944 (m), 1455 (m), 1822 (vs), 1868 (vs), 1892 (s), 2013 (s), 2854 (w), 2927 (m). H NMR
(CDCl3, 300 MHz): δ = 8.58 (d, 1H, H-2), 7.6–7.2 (overlapping signals and multiplets, 6H,
aryl-H and H-4), 7.14 (dd, 1H, H-3), 4.71 (t, 1H, H-7), 4.48 (q, 1H, N(CH3)CH(CH3)Ph),
2.93-2.63 (overlapping multiplets, 2H, H-5syn and H-5anti), 2.6-2.0 (m, 5H, H-6syn + H-6anti
N(CH3)CH(CH3)Ph), 1.95 (d, 3H, N(CH3)CH(CH3)Ph).
+
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2.2.2.2. cis-[Mo(CO)4(L2)] (2)
Mo(CO)6 (0.27 g, 1.02 mmol) was reacted with L2 (0.26 g, 1.03 mmol), giving 2 in
64% yield (0.29 g). Anal. Calcd for C21H20N2MoO4 (460.33): C, 54.79; H, 4.38; N, 6.08.
Found: C, 54.57; H, 4.79; N, 6.13%. Selected FT-IR (KBr, cm−1): 363 (m), 705 (m), 795 (m),
1
944 (m), 1454 (m), 1822 (vs), 1868 (vs), 1893 (s), 2013 (s), 2857 (w), 2927 (m). H NMR
(CDCl3, 300 MHz): δ = 8.58 (d, 1H, H-2), 7.6–7.2 (overlapping signals and multiplets, 6H,
aryl-H and H-4), 7.14 (dd, 1H, H-3), 4.71 (t, 1H, H-7), 4.48 (q, 1H, N(CH3)CH(CH3)Ph),
2.93-2.63 (overlapping multiplets, 2H, H-5syn and H-5anti), 2.6-2.0 (m, 5H, H-6syn + H-6anti
N(CH3)CH(CH3)Ph), 1.95 (d, 3H, N(CH3)CH(CH3)Ph).
+
2.3. Catalytic tests
The catalytic reactions were carried out at 55 ºC under autogenous pressure in 10 mL
borosilicate reactors possessing a Teflon valve for sampling, under magnetic stirring (1000
rpm). The reactor containing the catalyst (18 µmol), substrate (1.8 mmol) and 2 mL of co-
solvent was immersed in a thermostated oil bath (under stirring) and, after 10 min, 2.75 mmol
of oxidant (also pre-heated) was added (corresponding to the initial instant of the reaction).
cis-Cyclooctene (Cy), DL-limonene (DL-Lim), trans-β-methylstyrene (tbms) were the
substrates; 1,2-dichloroethane (DCE) and α,α,α-trifluorotoluene (TFT) were used as co-
solvents (other than the decane present in the oxidant solution) for reactions of Cy and DL
-
Lim/tbms, respectively.
The reactions were monitored by analysis of samples taken at different reaction times
using a Varian 3900 GC equipped with a flame ionization detector and a capillary column
(J&W Scientific DB-5, 30 m × 0.25 mm × 0.25 µm). Undecane was used as internal standard
(experimental range of error of less than 5%). The reaction products were identified by GC-
MS (Trace GC 2000 Series Thermo Quest CE Instruments GC; Thermo Scientific DSQ II)
using He as the carrier gas.
A biphasic solid/liquid reaction mixture was obtained for all systems after 24 h and,
with the objective of identifying the active species, the liquid phase was separated from the
solid phase by centrifugation (3000 rpm). The solids were thoroughly washed with diethyl
ether (3 × 10 mL) and pentane (2 × 10 mL), dried at ambient temperature overnight, and
finally vacuum-dried (ca. 0.1 bar) at 60 ºC for 1 h. These recovered solids are denoted as i-
sub-run1, where i = 1 or 2, and sub = Cy, DL-Lim or tbms.
7

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The recovered solids 1-Cy-run1 and 2-Cy-run1 were tested for Cy reaction using the
same mass ratio of molybdenum compound:olefin:oxidant as that used for the typical reaction
conditions; after a 24 h batch run, the corresponding solids were recovered (denoted 1-Cy-
run2 and 2-Cy-run2) using the same procedure described above for the original compounds.
3. Results and discussion
3.1. Catalyst preparation
Ligands L1 and L2 were prepared from ketone 3 (Scheme 1), which was obtained from
2,3-cyclopentenepyridine using an optimized protocol previously developed by some of us
[20-22]. The reductive amination of ketone 3 with (R)-(+)-α-methylbenzylamine [(R)-PEA]
and (S)-(−)-α-methylbenzylamine [(S)-PEA] using NaBH(OAc)3 (STAB) as reducing agent
proceeded diastereoselectively, affording 4-(R,R) and 5-(S,S), respectively, as major
diastereopure amines in good yields (80-84%). The subsequent N-methylation of the
secondary amines 4 and 5 under the same protocol was accomplished using an aqueous
solution of formaldehyde, yielding the title tertiary amines, L1 and L2 (74-78%).
The tetracarbonyl complexes cis-[Mo(CO)4(L1)] (1) and cis-[Mo(CO)4(L2)] (2) were
obtained as brown solids in fair yields by reacting Mo(CO)6 directly with the respective
organic ligand L1 or L2 in refluxing CH3CN. Complexes 1 and 2 display long-term stability
in the solid-state if stored under inert atmosphere. However, in solution (the complexes are
soluble in ethanol, acetone, diethyl ether, acetonitrile, toluene, ethyl acetate, chloroform and
dichloromethane), the complexes present limited stability, being susceptible to
decarbonylation reactions. Both complexes are insoluble in hexane, pentane and water. IR
spectra of 1 and 2 in the solid-state (KBr pellets) display four CO stretching bands in the
region 1810-2020 cm−1 in a pattern consistent with a cis-tetracarbonyl geometry. The solution
1H NMR spectra confirm the presence of the organic ligands L1 and L2 and furthermore
display peak shifts that can be attributed to coordination of the ligands to the metal center.
For example, the resonances for the pyridyl ring protons H-2 and H-3 shift downfield from
8.5 and 7.05 ppm for the free ligands to 8.58 and 7.14 ppm for complexes 1 and 2 (CDCl3
solvent). Similarly, the resonances for the methyne protons at 4.58 (H-7) and 4.16 ppm
(N(CH3)CH(CH3)Ph) for the free ligands shift to 4.71 and 4.48 ppm for the complexes.
3.2. Catalysis
8

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3.2.1. General considerations using cis-cyclooctene as model substrate
Complexes 1 and 2 promoted the epoxidation of cis-cyclooctene (Cy) with TBHP at 55
ºC (DCE as cosolvent), and 1,2-epoxycyclooctane (CyO) was the only product formed (100%
selectivity) (Fig. 2). Without molybdenum species and/or TBHP, olefin conversion was
negligible, and thus metal species and the oxidant are required for the formation of active
species. There is consensus across previously reported mechanistic studies that the active
oxidizing species involves the coordination of the oxidant to the metal center [23-29]. The
kinetic curves for 1 and 2 are roughly coincident, suggesting that the two compounds possess
similar catalytic activity and stability, despite the chiral differences. High conversions were
reached at 6 h (92-94%), and the reaction was complete within 24 h.
A comparison of the catalytic results for 1 and 2 with literature data (based on epoxide
yields at 6 h, where CyO selectivity was always 100%) for neutral or ionic molybdenum
tetracarbonyl compounds bearing pyridine-containing ligands (L), under somewhat
comparable reaction conditions, indicates that the performances of 1 and 2 are far superior to
the complexes with L = 2,2'-bipy (71% CyO yield) [15], di-tBu-bipy (84%) [15], pypzH
(51%) [16], or ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (70%) [16] (Table 1). N-heterocyclic
carbene (NHC) complexes of the type [Mo(CO)4(NHC)n] (NHC = 1,3-dibenzylimidazol-2-
ylidene or 1,3-dipropylimidazol-2-ylidene (n = 2), or 3-methyl-1-picolylimidazol-2-ylidene
(n = 1)) led to poorer catalytic results (ca. 9-35% CyO yield at 24 h, 55 ºC; Table 1) [30]. At
the higher reaction temperature of 70 ºC, the complex [Mo(CO)4L] with L = N-[3-
(trimethoxysilyl)propyl]ethylenediamine led to 47% CyO yield at 50% conversion, at 24 h
(catalyst:olefin:oxidant = 1:159:317, mesitylene as cosolvent), which is much poorer than
that for 1 and 2 [31].
The catalytic results for 1 and 2 are on a par with some of the best results reported in
the literature for complexes of the type [Mo(CO)4L] tested as precatalysts for Cy reaction, at
55 ºC, specifically with L = 2-(1-pentyl-3-pyrazolyl)pyridine (93% CyO yield at 6 h) [19]
and N-(n-propyl)-2-pyridylmethanimine (100% CyO yield at 5 h) [17].
3.2.2. Epoxidation of prochiral olefins
Compounds 1 and 2 were further explored for the epoxidation of prochiral olefins,
namely trans-β-methylstyrene (tbms) and DL-limonene (DL-Lim) with TBHP at 55 ºC (TFT as
cosolvent) (Fig. 2 and 3). With tbms as substrate, complexes 1 and 2 led to similar catalytic
results (62%/85% and 59%/82% conversion at 6 h/24 h, for 1 and 2, respectively), which
9

ACCEPTED MANUSCRIPT
parallels that observed with Cy as substrate. The catalytic reaction of tbms was 100%
selective towards the epoxide isomers (2S,3S)-2-methyl-3-phenyloxirane and (2R,3R)-2-
methyl-3-phenyloxirane, which were formed in approximately equimolar amounts
(enantiomeric excess (ee) was always ≤ 1%). Hence, asymmetric induction was negligible.
Poor asymmetric induction ability was reported for several molybdenum carbonyl
complexes (with tbms as substrate), excluding chiral molybdenum tetracarbonyl complexes,
which, to the best of our knowledge, have not been previously explored in catalytic olefin
epoxidation [32-36]. This feature may be partly due to partial ligand dissociation and/or the
pronounced distance between the olefin molecule and the chiral centers of the active
oxidizing species (negligible stereochemical constraints for chiral induction ability), and/or
involvement of radicals in the epoxidation reactions [33-36]. The catalytic results for 1 and 2
with tbms as substrate are comparable with results published for the chiral complex [(―)-
menthylCp]Mo(CO)3Cl (84% epoxide yield; 24 h, 55 ºC, CHCl3) [32], molybdenum carbonyl
complexes with ansa-bridged η5-cyclopentadienyl ligands where the stereogenic centers are
located in side chains (50-66% yield; 4 h, 55 ºC, toluene) [33], and the complex [(N-
benzyloxycarbonylprolyl)cyclopentadienyl]Mo(CO)3Me (84% yield; 24 h, 57 ºC, CHCl3)
[34] (in all these studies, total epoxide selectivity = 100% and Mo:olefin:oxidant =
1:100:200). Among these studies the maximum ee reported was 19-20% for the ansa-bridged
complexes (the enantiomer in excess was not specified) [33] and the methyl-derived complex,
which favored formation of the (2R,3R)-2-methyl-3-phenyloxirane enantiomer [32]. Further
comparisons of the tbms epoxidation results for 1 and 2 with literature data for other
molybdenum carbonyl complexes are difficult due to the broad range of reaction conditions
used. The best result reported for the complexes (R)-CpoxM(η3-C3H5)(CO)2 (M = Mo, W) and
[(R)-CpoxMo(CO)2(NCMe)]BF4 (Cpox = chiral oxazoline pendant group) was 58% conversion
(100% epoxide selectivity) after 6 h reaction at ambient temperature (Mo:olefin:oxidant =
1:100:200, CHCl3) [36]. The complexes CpMo(CO)3X with X = CHR2CO(OR1) (R1 = ethyl,
menthyl or bornyl, R2 = H; R1 = ethyl, R2 = methyl or phenyl) led to 62-70% epoxide yield
(100% selectivity) after 8.3 h reaction at ambient temperature (Mo:olefin:oxidant =
1:100:200, CH2Cl2) [35]. The two latter works reported negligible ee’s.
Complexes 1 and 2 were further tested for the epoxidation of the bio-derived olefin DL
-
limonene (DL-Lim). Limonene is produced via distillation of crude turpentine, and is a
valuable by-product of pine oil and camphor processes. Limonene epoxidation leads to
valuable products, such as limonene-1,2-monoepoxide (limonene oxide, abbreviated as LO)
and 1,2:8,9-diepoxy-p-menthane (limonene dioxide, abbreviated as LDO). The relative
10

ACCEPTED MANUSCRIPT
abundance, low cost, and structural similarity to cyclohexene oxide, make LO an attractive
choice as a biorenewable epoxide monomer for copolymerization with CO2, giving a
biodegradable polycarbonate (LDO may also be used) and, consequently, providing an
attractive green route to non-isocyanate polyurethanes without requiring the use of either
toxic isocyanate monomers or phosgene [37-39]. In the same way, LO can give polyesters by
reaction with succinic anhydride [40]. LO/LDO can be used as intermediates for the
manufacture of perfumes, flavors and fragrances, as green solvents, and as reactive diluents
in cationic and UV cure applications [39,41-43]. Geoghegan and Evans employed LO as a
starting material for the synthesis of (+)-perillyl alcohol [44], which may be useful as a
“chiral pool” starting material and as an anti-cancer agent [45].
The reaction of (racemic) DL-Lim with TBHP, in the presence of 1 or 2 at 55 ºC, led to
similar catalytic results, in parallel to that observed for the two complexes with Cy or tbms as
substrates. The DL-Lim conversions were 85% and 96-97% at 6 h and 24 h, respectively (Fig.
3). The main reaction products were LO isomers, namely (1S,4S)-(−)-cis-limonene-1,2-
epoxide and (1R,4S)-( −)-trans-limonene-1,2-epoxide (diastereomers formed from (S)-( −)-
limonene (S-Lim)), and (1S,4R)-(+)-cis-limonene-1,2-epoxide and (1R,4R)-(+)-trans-
limonene-1,2-epoxide (diastereomers formed from (R)-(+)-limonene (R-Lim)), which were
formed with total (LO) selectivity of 80-82% and 69-70%, at 6 h and 24 h, respectively
(Scheme 2).
Slight enantiomeric excesses (ee’s) of the (+) LO isomers were obtained at 24 h (ca. 4%
of isomer II relative to III and 14-16% of isomer I relative to III). On the other hand, slight
diastereomeric excess (de) of trans limonene-1,2-epoxide isomers was obtained at 24 h (3-
4% of isomer I relative to II and 6-9% of isomer III relative to IV).
Other products of the DL-Lim reaction included LDO (selectivities of 12-13% at 6 h and
19-20% at 24 h) and 1-methyl-4-(1-methylethenyl)-cyclohexane-1,2-diol (LDOH;
selectivities of 0-2% at 6 h and 8-9% at 24 h). The ratio LO/(LO+LDO) was in the range 0.8-
1, indicating high chemoselectivity in favor of the epoxidation of the endocyclic double bond.
LO selectivity decreased with time, and LDO selectivity increased (Fig. 3). On the other
hand, 8,9-epoxy-p-menth-1-ene was not formed, suggesting that LDO is formed via
consecutive epoxidation of LO.
To the best of our knowledge, there are no reports on the use of chiral molybdenum
carbonyl complexes as precatalysts for the reaction of racemic DL-Lim, although it was
reported that they may promote the reaction of the pure isomers of limonene with TBHP.
Specifically, the complex [Mo(CO)3I2(4,4'-dimethyl-2,2'-bipyridine)] led to 56% conversion
11

ACCEPTED MANUSCRIPT
of R-(+)-Lim and 91% selectivity to the monoepoxides (1S,4R)-(+)-cis- (II) and (1R,4R)-(+)-
trans-limonene-1,2-epoxide (I), and de was less than 5% (24 h, 55 ºC, Mo:olefin:oxidant =
1:100:200, CH2Cl2) [46]. The complexes (R)-CpoxM(η3-C3H5)(CO)2 (M = Mo, W) and [(R)-
CpoxMo(CO)2(NCMe)]BF4 led to quantitative conversion of R-(+)-Lim to LO after 1 h
reaction at 55 ºC [36]; an approximately equimolar mixture of LO isomers was obtained for
the neutral complex, while 20% de of the (+)-cis isomer was obtained for the cationic
complex (Mo:olefin:oxidant = 1:100:200, CHCl3).
The few published studies that report the Mo-catalyzed reaction of DL-Lim with TBHP
use chiral oxomolybdenum compounds as (pre)catalysts (Table 2). In general, compounds 1
and 2 studied in this work led to comparable results (in terms of LO yields at 24 h) to those
obtained with the polymeric compound [((Me)2(menthyl)Sn)2MoO4(H2O)3.5] [47], and
slightly better results than those for the complex [MoO2{(−)-2,2-bis[(4S,5S)-4-
hydroxymethyl-5-phenyl-1,3-oxazolin-2-yl]propane}]
[48].
The
hybrid
polymer
[Mo2O6(trethbz)2]·H2O (trethbz = (S)-4-(1-phenylpropyl)-1,2,4-triazole) led to higher yields
(82%) [49]. The (+)-cis and (+)-trans LO isomers were predominantly formed relative to (−)-
cis and (−)-trans, respectively, using all Mo compounds, with the best ee’s observed for 1
and 2 and also [Mo2O6(trethbz)2]·H2O as (pre)catalysts. The dioxomolybdenum(VI) complex
containing the chiral oxazoline ligand led to the highest de (18-20% of trans isomers) [48].
3.2.3 Nature of active species
Complexes 1 and 2 are converted into active oxidizing species (discussed above) in situ
during the catalytic process. Structural modification of the starting complexes is somewhat
supported by the fact that the color of the Cy reaction mixtures containing (1 or
2)/olefin/TBHP changed from greenish to light brown upon addition of the oxidant to the
reactor. The metal species were isolated after a 24 h batch run, giving the solids 1-Cy-run1
and 2-Cy-run1, and characterized by ATR FT-IR spectroscopy (Fig. 4). Indicators for
oxidative decarbonylation of the precatalysts are the disappearance of the CO stretching
bands in the region 1810-2020 cm−1 and the appearance of several new bands in the Mo‒O
stretching region of 650-950 cm−1. Similar spectra were obtained for the solids isolated after
catalytic reactions with the other substrates (Fig. 4). The pattern of bands in the Mo‒O
stretching region is highly characteristic of the
β-octamolybdate anion, β
-[Mo8O26]4-, as
supported by the literature data given in Table 3 for comparison. This polyoxomolybdate had
previously been identified as the product of oxidative decarbonylation of the complexes
12

ACCEPTED MANUSCRIPT
[Mo(CO)3I2L2] (L = pyridine or 4-tert-butylpyridine), which gave the salts (LH)4[Mo8O26]
[50]. The Cy/TBHP reaction in the presence of 1-Cy-run1 or 2-Cy-run1 led to similar
catalytic results to the respective parent compounds 1 and 2 (Fig. 2). Compounds 1-Cy-run2
and 2-Cy-run2 exhibited similar ATR FT-IR spectra to 1-Cy-run1 and 2-Cy-run1, suggesting
that the polynuclear species formed is chemically stable during the catalytic reaction (Fig. 4).
On the other hand, these results suggest that the active species formed from the precursors 1
and 2 are responsible for the catalytic epoxidation process in the first and second batch runs.
4. Conclusions
The efficacy of molybdenum carbonyl complexes as precatalysts for olefin epoxidation
reactions has motivated some efforts to prepare chiral catalyst precursors in the hope that
these may promote asymmetric induction. In the present work we have described the first
report of the application of chiral molybdenum tetracarbonyl complexes in catalytic olefin
epoxidation. Although epoxides were either the main or only products formed from cis-
cyclooctene, trans-β-methylstyrene and DL-limonene, stereoselectivities were low with the
prochiral olefins. The lack of success in asymmetric epoxidation may be partly due to the
labile behavior of the 7-(1-pyrindanyl) amine ligands, which decoordinate upon oxidative
decarbonylation of the precursors to give β-octamolybdate salts in which the protonated
organic ligand is present as charge-balancing cation. Our previous work with cis-
[Mo(CO)4(L)] catalyst precursors has indicated that chelating ligands such as bipyridines and
pyrazolylpyridines tend to remain coordinated to the metal center upon oxidative
decarbonylation to give either discrete polynuclear oxo-complexes or hybrid molybdenum
oxide/organic polymers, and therefore better success in asymmetric epoxidation may be
achieved by employing chiral derivatives of these types of ligands.
Acknowledgements
This work was developed in the scope of the project (Associate Laboratory) CICECO-Aveiro
Institute of Materials-POCI-01-0145-FEDER-007679 [FCT (Fundação para a Ciência e a Tecnologia)
ref. UID/CTM/50011/2013], financed by national funds through the FCT/MEC and when applicable
co-financed by FEDER (Fundo Europeu de Desenvolvimento Regional) under the PT2020
Partnership Agreement. The FCT and the European Union are acknowledged for Ph.D. grants to L.N.
(PD/BD/109666/2015) and I.E.S-D. (SFRH/BD/93632/2013), and post-doctoral grants to P.N.
13

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(SFRH/BPD/110530/2015) and C.A.D.S. (SFRH/BPD/80100/2011) co-funded by MCTES and the
European Social Fund through the program POPH of QREN.
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Figure captions
Scheme 1. Preparation of the ligands (L1 and L2) and corresponding tetracarbonyl
complexes (1 and 2).
Scheme 2. Products formed from DL-limonene epoxidation.
Fig. 1. Atom labelling used in the NMR peak assignments.
Fig. 2. Kinetic profiles for the catalytic reactions 1/Cy (∆), 2/Cy (○), 1-Cy-run1/Cy (+), 2-
Cy-run1/Cy (-), 1/tbms (◊) and 2/tbms (□), using TBHP as oxidant at 55 ºC.
Fig. 3. Kinetic profile of the reaction of DL-Lim with TBHP, in the presence of 1 (+) or 2 (×),
at 55 ºC, and selectivity to the products LO (▲,∆), LDO (●,○) and LDOL (■,□ ) for reactions
of 1 (closed symbols) or 2 (open symbols).
Fig. 4. FT-IR spectra in the range 500-2100 cm−1 of L1 and complex 1, and ATR FT-IR
spectra of solids recovered after catalytic reactions.
17

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Table 1
cis-Cyclooctene epoxidation at 55 ºC in the presence of compounds 1 and 2, and comparison
with literature data for molybdenum tetracarbonyl complexes.
Complexa
Reaction conditions
Conv.c (%) Select. (%)d Ref.
Mo:Cy:TBHP Cosolventb t (h)
1:100:153
1:100:153
1:100:153
DCE
DCE
DCE
DCE
None
None
DCE
None
None
None
None
6/24
6/24
6
92/100
94/100
71
100/100
100/100
100
This work
This work
[15]
1
2
[Mo(CO)4(2,2'-bipy)]
[Mo(CO)4(di-tBu-bipy)] 1:100:153
6
84
100
[15]
[Mo(CO)4(pzpyH)]
[Mo(CO)4(pzpyE)]
[Mo(CO)4(pzpyP)]
[Mo(CO)4(pyim)]
[Mo(CO)4(IBz)2]
[Mo(CO)4(IPr)2]
1:100:152
1:100:152
1:113:172
1:100:153
1:100:200
1:100:200
1:100:200
6/24
6/24
6/24
5
51/78
70/92
93/100
100
100/100
100/100
100/100
100
[16]
[16]
[19]
[17]
4/24 h ~7/~35
4/24 h ~10/~32
4/24 h ~2/~9
100/100
100/100
100/100
[30]
[30]
[Mo(CO)4(PyNHC)]
[30]
a 2,2'-bipy = 2,2'-bipyridine, di-tBu-bipy = 4,4'-di-tert-butyl-2,2'-bipyridine, pzpyH = 2-[3(5)-
pyrazolyl]pyridine, pzpyE = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate, pzpyP = 2-(1-pentyl-3-
pyrazolyl)pyridine, pyim = N-(n-propyl)-2-pyridylmethanimine, IBz = 1,3-dibenzylimidazol-2-
ylidene, IPr = 1,3-dipropylimidazol-2-ylidene, PyNHC = 3-methyl-1-picolylimidazol-2-ylidene.
b Other than decane from oxidant solution.
c cis-Cyclooctene conversion.
d Selectivity to the epoxide (CyO).
18

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Table 2
Limonene epoxidation with TBHP and chiral Mo compounds at 55 ºC.
Compounda
Sub.b
Mo:Sub:TBHP Solv.c C (%)d Y (%)e ee (%)f de (%)g Ref.
DL-Lim 1:100:153
TFT
96
67
4 (II)
14 (I)
6 (III)
3 (I)
This work
1
DL-Lim 1:100:153
TFT
97
68
4 (II)
9 (III)
4 (I)
-
5 h
5 h
This work
[47]
2
16 (I)
[(Me2RSn)2MoO4(H2O)3.5] DL-Lim 1:100:150
CH2Cl2 90
65-67
-
-
-
R-Lim
S-Lim
85
97
[Mo2O6(trethbz)2]·H2O
DL-Lim 1:100:153
TFT
90
82
59
12 (II) 3 (IV) [49]
18 (I) 8 (II)
13 (III) 18 (I)
[MoO2(oxazol)]
DL-Lim 1:138:211
DCE
90
25 (I)
20 (III)
22 (I)
R-Lim
S-Lim
87
88
68
60
-
-
[48]
19(III)
a R = menthyl; trethbz = (S)-4-(1-phenylpropyl)-1,2,4-triazole; oxazol = (−)-2,2-bis[(4S,5S)-4-
hydroxymethyl-5-phenyl-1,3-oxazolin-2-yl]propane.
b Substrates: DL-Lim = DL-limonene; R-Lim = R-(+)-limonene; S-Lim = S-(−)-limonene.
c Solv. = cosolvent added (other than decane from oxidant solution).
d Conversion of the substrate after 24 h.
e Total yield of 1,2-epoxides after 24 h.
f Enantiomeric excess at 24 h; the isomer in excess is indicated within brackets (pairs of enantiomers
I/III and II/IV).
g Diastereomeric excess at 24 h; the isomer in excess is indicated within brackets (pairs of
diastereomers I/II and III/IV).
h The isomer in excess was not specified.
19

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Table 3
Comparison of infrared bands observed for the recovered compounds 1-Cy-run1 and 2-Cy-
run1 in the range 650-950 cm‒1 with bands exhibited by β-octamolybdate salts.
Compounda
IR bands (cm‒1)
Ref.
1-Cy-run1, 2-Cy-run1b
(pyH)4[Mo8O26]
942
946
949
944
939
946
956
947
939
944
935
903
912
914
919
910
910
911
911
913
902
910
844
839
837
838
842
841
837
836
842
841
836
701
709
708
686
720
710
714
717
714
711
699,
659
675
659
670
669
665
650
632
661
651
657
This work
[50]
(tBupyH)4[Mo8O26]
[50]
(H3biim)4[Mo8O26]
[51]
(HDBU)3(NH4)[Mo8O26]·H2O
(Hmim)4[Mo8O26]
[52]
[53]
(Dhmim)4[Mo8O26]
[53]
(Hpy)4[Mo8O26]·H2O
(Bmim)4[Mo8O26]
[53]
[54]
(dmimX)3[Mo8O26]2(H3O)2·H2O
(4,4'-Hybipy)4/y[Mo8O26]b
[55]
[56]
a pyH = pyridinium; tBupyH = 4-tert-butylpyridinium; H2biim = 2,2'-biimidazole; DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene; Hmim = 1-hexyl-3-methylimidazolium; Dhmim = 1,2-dimethyl-3-
hexylimidazolium; Hpy = 1-hexylpyridinium; Bmim = 1-butyl-3-methylimidazolium; dmimX =
bis(1,2-dimethylimidazolium) p-xylenedichloride; 4,4'-bipy = 4,4'-bipyridine.
b ATR FT-IR bands.
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1) Two chiral tetracarbonylmolybdenum pyrindanyl amine complexes have been synthesized.
2) The tetracarbonyls can be used catalyst precursors for olefin epoxidation with TBHP.
3) Epoxides were obtained as the main/only products from achiral/prochiral olefins.
4) Oxidative decarbonylation of the precursors with TBHP gives β-octamolybdate salts.
1