HOMOGENEOUS CATALYSIS


In homogeneous processes, the design and synthesis of the appropriate ligands is crucial in order to fine tune the activity and the selectivity of the metal catalysts.

Over the years, our group has acquired a strong expertise in the synthesis of ligands for applications in transition metal-catalyzed processes and, in particular, chiral ligands for asymmetric catalysis.



CHIRAL LIGAND DESIGN

Some examples of the ligands synthesized in our group are shown below:

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CARBONYLATION PROCESSES

Several research projects were completed in our lab focusing on carbonylation processes such as asymmetric hydroformylation of alkenes, alkoxy- and hydroxycarbonylation of alkenes, co- and terpolymerization of CO/alkenes, aminocarbonylation and double carbonylation of aryl iodides.

The group has published articles and (co-)authored several reviews, books and book chapters in the area of carbonylation processes.

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Carbonylation reactions studied in our laboratory

 

 

Asymmetric carbonylation.

Rh/ asymmetric hydroformylation of alkenes

In the Rh-catalyzed asymmetric hydroformylation of alkenes, the steric and electronic properties of the Rh-coordinated ligands largely control the activity and regioselectivity of the reaction, the presence of the appropriate chiral structure of the ligands leading to potential enantioinduction.

In our group, several families of chiral diphosphite ligands were designed, synthesized and applied to the asymmetric hydroformylation of various alkene substrates such as vinyl arenes, vinyl and allyl ethers and norbornene.

These investigations led to the publication of several articles, reviews and books/book chapters in the area of asymmetric hydroformylation.

For instance, we recently reported the efficient use of C1-symmetric 1,3-diphosphite ligands derived from glucofuranose (Eur. J. Org. Chem. 2009, 1191 – 1201.) in the Rh-catalyzed hydroformylation of dihydrofurans excellent chemo- and regioselectivity together with ee’s up to 88%. (Adv. Synth. Catal. 2010, 352, 463 – 477)

 

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Our group had previously reported that when 2,5-dihydrofuran is the substrate, high degrees of isomerization to the corresponding vinyl ether are observed. This competing isomerization process has a large impact on the enantioselectivity of the hydroformylation reaction because each isomer yields the opposite enantiomer of the same aldehyde. Thus, a high degree of isomerization results in a low enantiomeric excess in the product. (Organometallics 1992, 11, 3525 – 3533; Can. J. Chem. 2001, 79, 560–565).

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In a recent study, the key step for the transfer of chirality in the hydroformylation of 2,3- or 2,5-dihydrofuran was further investigated and the results obtained are schematically represented below:

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The coordination behavior of the ligands under catalytic reaction conditions was investigated using high-pressure NMR spectroscopy:

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Selected regions of the 1H and 31P{1H} NMR spectra of complex [RhH(CO)2(P-P)] in toluene-d8 recorded at variable temperatures.

 

Pd/asymmetric hydro- and alkoxycarbonylation of alkenes

Our group has investigated several catalytic systems for the Pd-catalyzed asymmetric hydroxy- and methoxycarbonylation of alkenes such as vinyl arenes (Dalton Trans., 2008, 853–860) and norbornene (Adv. Synth. Catal. 2009, 351, 1813 – 1816; Dalton Trans., 2012, 41, 6980–6991)

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Hydro- and alkoxycarbonylation of alkenes

 

We recently reported our investigations into the use of several mono- and bidentate phosphine ligands in the Pd-catalyzed methoxycarbonylation of norbornene wherein unprecedented chemo- and stereoselectivities were achieved at room temperature.

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Pd-catalyzed methoxycarbonylation of norbornene

Ligands used in our recent study of the Pd-catalyzed methoxycarbonylation of norbornene

 

The first completely chemo- and stereoselective Pd-catalyzed methoxycarbonylation of norbornene was reported. This study showed that norbornene can be chemo- and stereoselectively functionalized by controlling the reaction conditions and using the appropriate chiral ligands. Subtle modifications to the palladium-phosphine catalyst were shown to significantly affect its activity and selectivity.

 

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Molecular structures of Pd complexes synthesized in this study

NMR investigations showed that the protonated neomethyldiphenylphosphine is formed under catalytic conditions by reaction of the free ligand and in situ-generated HCl. The addition of acid is thus not required with this system since reaction of the catalyst precursor with methanol under CO pressure produces 2 equivalents of HCl and leads to the formation of the catalytically active species.

The protonation of a diphosphine ligand under methoxycarbonylation conditions was also observed and the isolation and characterization of the diprotonated diphosphine was carried out. This diprotonated compound was tested under catalytic conditions as a ligand and acid source and with [PdCl2(COD)] as metal source wherein and excellent conversion and high selectivity to the ester were achieved. The presence of chloride ligands proved to be crucial to the attainment of high activity and selectivity. The reaction of the diprotonated ligand with [PdCl2(COD)] evidenced the formation of the Pd hydride species [PdCl(H)(P-P)].

 

Pd/co- and terpolymerisation of CO and alkenes

Our group also has experience in the study of co- and terpolymerisation of CO and alkene(s).

In these studies, several series of bidentate nitrogen and phosphorus donor ligands derived from carbohydrates, and their corresponding Pd complexes, were synthesized and characterized.

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Ligands synthesized in our lab for application in co- and terpolymerisation of CO and alkene(s).

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Examples of ORTEP drawings of the molecular structure of [PdCl(Me)(N-N´)] complexes.

 

These species were subsequently used in CO/styrene copolymerization reactions (see for instance, Chem. Eur. J. 2004, 10, 3747-3760). Examination of the carbonylation step emmploying several isomeric cationic Pd complexes [PdMe(NCMe)(N-N´)][BAr’4] led to the proposal of the mechanism summarized below:

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Mechanism of carbonylation of the cationic palladium compounds. M: major isomer; m: minor isomer

 

 

Other carbonylation reactions.

Recently, our group realized several studies on carbonylation reactions that did not involve the use of chiral ligands, such as the Pd-catalyzed methoxycarbonylation of ethene, mono- and double aminocarobonylation of aryliodides.

 

Pd/ methoxycarbonylation of ethene

A family of new bulky diphosphine ligands were recently synthesized in our lab and successfully applied in the Pd-catalyzed methoxycarbonylation of ethene, reaching the highest activity reported. (Chem. Eur. J. 2010, 16, 6919)

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Ligands used in the Pd-catalyzed methoxycarbonylation of ethene

 

X-ray diffraction-elucidated structures of all the corresponding Pd complexes were obtained, detailed NMR studies were performed, and most intermediates in the catalytic cycle were detected and characterized.

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X-ray structures of Pd complexes used in this study


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Recorded and simulated NMR spectra

 

Pd/double carbonylation of aryl iodides using phosphine-free catalyst

We also recently reported the first phosphine-free Pd-catalyzed double carbonylation of aryl iodides, giving excellent conversions and selectivities for a wide range of aryl iodides and amine nucleophiles under atmospheric CO pressure. (Chem. Commun., 2012, 48, 1695–1697).

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Schematic representation of the Pd-catalyzed double carbonylation of aryl iodides and ORTEP drawing of the molecular structure of PdCl2(DBU)2

 

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Examples of conversions and selectivities for the ketoamide products reported in this study

 

 

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Proposed mechanism for the phosphine-free Pd-catalyzed double carbonylation of aryl iodides


 

Asymmetric hydrogenation processes.

Our group has experience in the study of asymmetric hydrogenation of substrates such as imines and functionalized alkenes. Examples of such investigations using carbohydrate-derived diphosphite and diphosphinite ligands are summarized below.

 

Ir/ hydrogenation of imines (diphosphinite or diphosphonite ligands)

Using the ligands derived from xylose shown below, enantiomeric excesses of up to 57% were achieved in the asymmetric hydrogenation of imines.

 

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Xylose-derived ligands used in this reaction

 

 

Rh/ hydrogenation of C=C bonds (diphosphonite /diphosphite)

Using the diphosphites derived from carbohydrates shown below, enantiomeric excesses of up to 93% were achieved in the asymmetric hydrogenation of C=C bonds.

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Summary of results obtained for the Rh-catalyzed hydrogenation of C=C bonds

 

 

C-C BOND FORMATION REACTIONS

 

Asymmetric reactions.

Pd/asymmetric allylic substitutions

In collaboration with the group of Prof. Montserrat Gomez (Toulouse, France), our group recently investigated the use of C2-symmetric diphosphite ligands derived from carbohydrates in asymmetric Pd-catalyzed allylic substitution processes (Chem. Commun., 2008, 6197–6199; Chem. Commun., 2011, 47, 7869–7871).

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Conversions and ee's from recycling experiments

These systems were extremely active and selective, providing 99% ee in both allylic alkylation and amination processes together with the highest turn over frequencies reported to date for these reactions.

These catalytic systems were later transfered to ionic liquid media and could be recycled several times without loss of activity or selectivity.

 

Other reactions

Pd/ Suzuki-Miyaura reaction

In 2007, we reported two new diaryl alkyl phosphine sulfonate ligands synthesized as zwitterions. The corresponding Pd complexes were prepared and successfully used in the palladium-catalyzed Suzuki–Miyaura cross-coupling reaction of aryl bromides as well as aryl chlorides in neat water and at low Pd loading. The X-ray diffraction-determined structure of one of the complexes confirmed the coordination mode of the phosphine sulfonate ligand in a bidentate fashion. (Dalton Trans., 2007, 2859–2861)

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Ligands used in this study

 


OXIDATION PROCESSES

 

ChemCatChem 2013, 5, 1092 – 1095

Fe-catalysed olefin epoxidation with tridentate non-heme ligands and hydrogen peroxide as the oxidant.

Bernabé F. Perandones, Enrique del Río Nieto, Cyril Godard, Sergio Castillón, Pilar De Frutos, and Carmen Claver

Our group has recently developed Fe based catalysts bearing tridentate nitrogen donor ligands 1-4 in the selective epoxidation of olefins using hydrogen peroxide as oxidant. The optimum L/Fe ratio revealed to be 1/2, suggesting the formation of dimeric iron species bearing one tridentate ligand. However, further work is required to determine the role of this species in catalysis and is currently on going in our laboratory. The presence of substituents at the central N-atom of the ligands drastically lower the activity and selectivity of the system. The C2-symmetryc ligands 1 and 4 form efficient catalytic systems for the epoxidation of aromatic olefins. Lower conversions and selectivities were obtained for aliphatic olefin substrates.

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Optimisation of the L/Fe ration in the Fe-catalysed epoxidation of styrene using H2O2 as oxidant.

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Fe-catalysed epoxidation of styrene using ligands 1-4[a]

 

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Entry

Ligand

Conv. (%)[b]

Sel. (%)[b]

1

1

82

96

2

2a

53

94

3

2b

55

95

4

2c

5

80

5

2d

-

-

6

2e

-

-

7

2f

-

-

8

3

69

93

9

4

86

98

[a] General conditions: FeCl3·6H2O (0.01 mmol), ligand (0.005 mmol), acetonitrile (3 mL), styrene (0.2 mmol); H2O2 (0.6 mmol) (added dropwise during the reaction); t= 30 min. [b] Determined by HPLC using m-xylene as internal standard.

 

Fe-catalysed epoxidation of various alkenes using ligands 1 and 4.

 

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Entry

Substrate

L

Conv.(%) [b]

Sel.(%) [b][c]

Yield (%)

1

4-Me-styrene

1

93

85

70

2

4-Me-styrene

4

94

90

-

3

4-MeO-styrene

1

95

77

65

4

4-MeO-styrene

4

75

80

-

5

4-tBu-styrene

1

93

87

76

6

4-tBu-styrene

4

93

87

-

7

4-Cl-styrene

1

76

87

60

8

4-Cl-styrene

4

81

81

-

9

4-F-styrene

1

55

89

46

10

4-F-styrene

4

70

71

-

11

3-NO2-styrene

1

10

57

-

12

3-NO2-styrene

4

13

78

-

13

trans- stilbene

1

88

98

72

14

trans- stilbene

4

100

97

-

15

cis- stilbene

1

39

30[d]

-

16

cis- stilbene

4

43

35[d]

-

17

cis-cyclooctene

1

68

71

40

18

cis-cyclooctene

4

65

68

-

19

1-octene

1

7

75

-

20

1-octene

4

6

66

-

[a] General conditions: FeCl3·6H2O (0.01 mmol), ligand (0.005 mmol), acetonitrile (3 mL), substrate (0.2 mmol), H2O2 (0.6 mmol) (added dropwise during the reaction); t= 30 min. [b] Determined by HPLC or GC (for non aromatic olefin) by comparing with real samples and using m-xylene as internal standard. [c] The only by-product detected was the corresponding aldehyde, except for the disubstituted olefins cis- and trans-stilbene and cis-cyclooctene where the corresponding ketone was formed. [d] The by-products were the corresponding ketone (ca. 40%) and the trans-stilbene oxide (ca. 30%)

 

 


CO2 TRANSFORMATIONS

The Eco2CO2 project aims at exploiting a photo-electro-chemical (PEC) CO2 conversion route for the synthesis of methanol as a key intermediate for the production of fine chemicals (fragrances, flavourings, adhesives, monomers ,.. ) in a lignocellulosic biorefinery. The most crucial development in the project will be the development of a PEC reactor capable of converting CO2 into methanol by exploiting water and sun light without using expensive noble metals or precious materials.

Our group is involved in the synthesis of organic photosensitizers, based on naphthalenediimides (NDIs) structures, capable of absorbing light below 600 nm, which would be interfaced with the water-splitting catalysts.

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Nitrile-based monomers for the synthesis of 2D or 3D polymeric Covalent-Triazine Frameworks (CTF), which will be employed for the assembly of the photocatalysts for the CO2 reduction to methanol are also being synthesized.

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