A Cyclometalated Ruthenium-NHC Precatalyst for the Asymmetric

Article pubs.acs.org/Organometallics

A Cyclometalated Ruthenium-NHC Precatalyst for the Asymmetric Hydrogenation of (Hetero)arenes and Its Activation Pathway Daniel Paul,†,§ Bernhard Beiring,†,§,∥ Markus Plois,‡,⊥ Nuria Ortega,†,# Slawomir Kock,†,& Danny Schlüns,†,¶ Johannes Neugebauer,*,†,¶ Robert Wolf,*,‡ and Frank Glorius*,† †

Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany ‡ University of Regensburg, Institute of Inorganic Chemistry, Universitätsstrasse 31, D-93053 Regensburg, Germany ¶

S Supporting Information *

ABSTRACT: This study describes the structural investigation of a highly versatile ruthenium-NHC (N-heterocyclic carbene) catalyst complex, which has been established for the asymmetric hydrogenation of various aromatic compounds. A complex containing an unusual doubly deprotonated NHC ligand was isolated and identified as the precatalyst to this complex. When its subsequent reactivity was monitored, two additional precatalysts, featuring partially hydrogenated naphthyl substituents, were characterized spectroscopically. Ligand hydrogenation appears to be a key activation process en route to the active catalyst.

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INTRODUCTION

Scheme 1. Ru-SINpEt-Catalyzed Asymmetric Hydrogenations

For decades, enantioselective hydrogenation has been a wellestablished method to access saturated building blocks with high enantiopurity. The asymmetric hydrogenation of aromatic compounds, however, remained largely unexplored prior to 2000. The development of reactive catalysts, mainly based on iridium and ruthenium, for asymmetric arene hydrogenation has enabled access to a wide range of homochiral saturated (hetero)cycles from their aromatic precursors.1 N-heteroarenes are often ideal substrates for hydrogenation, as CN bonds can be rapidly reduced by many catalyst systems. The hydrogenated products often feature important bioactive structural motifs and are therefore attractive targets for pharmaceutical discovery.2 The derivatization of arenes followed by a late-stage enantioselective hydrogenation can serve as a complementary route to the more typical laborious methods for the synthesis of saturated (hetero)cycles. Inspired by the work of Chaudret, Borowski, Sabo-Etienne, and Leitner,3 we recently developed a highly effective and versatile chiral Ru-NHC (N-heterocyclic carbene9) catalyst for the enantioselective hydrogenation of (hetero)aromatic compounds (Scheme 1). This catalyst featuring the NHC ligand SINpEt (N,N′-bis(naphthylethyl)imidazolidinium-2-ylidene) selectively reduces the heteroaromatic ring of i.e. (benzo)furans, (benzo)thiophenes, chromones, flavones, indolizines, and the carbocyclic ring of quinoxalines.4,5 Surprisingly, SINpEt provided the best results for all substrate classes. Its framework is key to its success, as slight variations to this system (i.e., exchanging the naphthyl for phenyl or cyclohexyl) causes dramatic decreases in reactivity and selectivity.4f,6 © XXXX American Chemical Society

Despite its unique reactivity profile and overall versatility, structural and mechanistic information about this Ru-SINpEt catalyst system has been lacking. Herein, we report the characterization and structural identification of precatalyst 2-A , which features an unusual doubly deprotonated NHC ligand, Received: September 7, 2016

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DOI: 10.1021/acs.organomet.6b00702 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics as well as 2-B and 2-D. NMR, ESI-MS, and DFT measurements have provided insight into the activation pathway of the precatalyst. Structural elucidation of Ru species 2-B and 2-D shows that the reversible hydrogenation of the NHC naphthyl residues is key to the generation of catalytically active species 2-C.

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RESULTS AND DISCUSSION Exposure of [Ru(cod)(2-methylallyl)2] (cod = cycloocta-1,5diene), featuring labile ligands, to free NHC, generated in situ via deprotonation of the imidazolidinium salt, gives rise to the ruthenium-NHC catalyst. The catalyst is formed in situ by mixing the components in the presence of the substrate directly. In this case, elevated temperature (60 °C) needs to be applied during the hydrogenation reaction. The reaction temperature can be lowered to −10 °C in the case of pyridone hydrogenation, when ruthenium and ligand precursor are mixed with KOtBu and prestirred in hexane at 70 °C for 12 h before substrate and hydrogen gas are added.4f,h In order to better understand the cause of the distinct reactivity of the chiral Ru-SINpEt catalyst, the corresponding achiral system Ru-ICy was first examined, as it displays the same reactivity and selectivity as Ru-SINpEt in hydrogenation and has been used for the synthesis of the corresponding racemic products. Colorless crystals of the bis(carbene) complex [Ru(ICy)2(2-methylallyl)2] (1) were obtained in 53% yield from the reaction of [Ru(cod)(2-methylallyl)2] with 2 equiv of ICy generated in situ by addition of equimolar amounts of ICy·HCl and KOtBu (Scheme 2).7,8 X-ray

Figure 1. Molecular structure of [Ru(ICy)2(2-methylallyl)2] (1) (left) and [Ru(SINpEt)(SINpEt′′)] (2-A) (right) showing 40% probability ellipsoids. Hydrogen atoms (other than H5a and H5b) and a THF molecule present in the crystal lattice of 2-A have been omitted for clarity. Selected bond lengths (Å) and angles (deg) of 1: Ru1−C1 2.0942(12), Ru1−C16 2.2506(13), Ru1−C17 2.1698(13), Ru1−C18 2.2334(14), C16−C17 1.4157(19), C17−C18 1.4197(19), C17−C19 1.5109(19), C1′−Ru1−C1 86.38(7), C17−Ru1−C17 99.23(7), C1−Ru1−C17′ 103.79(5); C1−Ru1−C17 134.95(5). Selected bond lengths (Å) and angles (deg) of 2-A: Ru1−C1 2.015(3), Ru1−C5 2.232(3), Ru1−C19 2.175(3), Ru1−C28 2.008(3), Ru1−C35 2.181(3), Ru1−C36 2.141(3), Ru1−C37 2.180(3), Ru1− C38 2.327(4), C1−Ru1−C5 75.98(13), C1−Ru1−C28 107.35(14), C5−Ru1−C19 82.69(12), dihedral angle between the planes C35,36,37,38/C35,43,44,38 39.16.

in n-hexane. 1H NMR analysis of the reaction mixtures, however, showed that 2-A is, in fact, the major product in this reaction. The SINpEt′′ ligand coordinates to ruthenium via the carbon of the central imidazolylidin-2-ylidene ring (Ru1−C1 2.015(3) Å), a CH2 group (formed by deprotonation of a methyl substituent) (Ru1−C5 2.232(3) Å), and a deprotonated naphthyl group (Ru1−C19 2.175(3) Å). The second, intact SINpEt ligand binds to ruthenium through its central carbene carbon atom (C28) and four carbon atoms (C35−C38) of the neighboring naphthyl substituent. This η4-coordination mode of the naphthyl ring to ruthenium is unusual for a ruthenium(II) complex. An η6-mode of coordination is much more common.11 η4-Coordinated polyarene ligands have been observed in the structures of highly reduced polyarene metalates, which display a high degree of metal to ligand back-bonding.12 Although numerous monocyclometalated NHC complexes containing deprotonated alkyl, vinyl, and aryl groups have been characterized,13−15 examples in which two substituents on the same NHC ligand are metalated are rare.16 To the best of our knowledge, 2-A represents the first complex containing a doubly cyclometalated carbene ligand featuring sp2- and sp3hybridized carbon atoms bound to the metal center. The 1H and 13C{1H} NMR spectra, in both deuterated THF and toluene, revealed the presence of two diastereomers of [Ru(SINpEt)(SINpEt′′)] (2-A). The diastereomers are present in a 70:30 ratio and interconvert slowly on the NMR time scale at room temperature.17 The isolated complex 2-A showed the same reactivity and enantioselectivity in asymmetric hydrogenation as the precatalyst formed using the general procedure described previously. The hydrogenation of 2-methylbenzofuran is fast and proceeds at relatively low catalyst loadings. Using 0.5 mol % of catalyst and 10 bar of hydrogen pressure (R)-2-

Scheme 2. Formation of Precatalysts 1 and 2-A

crystallography confirmed the presence of two ICy ligands that coordinate to ruthenium in a monodentate fashion with a typical Ru−C distance of 2.0942(12) Å in both cases.9,10 The distorted-tetrahedral coordination environment of ruthenium is completed with two η3-coordinated 2-methylallyl ligands (Ru− C 2.1698(13)−2.2506(13) Å). The subsequent hydrogenation reactions catalyzed by the preformed complex 1 require a high reaction temperature of 60 °C, presumably owing to the required dissociation of the methylallyl ligands, which is necessary in order to form the active catalyst species. In contrast, a novel and structurally distinct complex is formed in the analogous reaction of 2 equiv of the homochiral carbene precursor SINpEt·HBF4 with KOtBu and [Ru(cod)(2methylallyl)2] (Scheme 2). The molecular structure of [Ru(SINpEt)(SINpEt′′)] (2-A, Figure 1) shows that one of the usually monodentate carbenes has been transformed into the unique tridentate ligand SINpEt′′ by deprotonation of the methyl and naphthyl groups. After crystallization, 2-A was isolated in a moderate yield of 30% owing to its high solubility B


Article

Organometallics methyldihydrobenzofuran was isolated in 98% yield and 96:4 er after 30 min. The hydrogen pressure could also be reduced to 1 bar, which allows for the reaction to be simply conducted in a standard flask rather than in an autoclave. Monitoring the hydrogenation of 2-methylbenzofuran at this pressure revealed an induction period of ca. 3 h (Figure 2). However, when a

Scheme 3. Formation of Complex 2-B and Excerpt from the gHMBC Spectrum of 2-B in THF-d8 at −30 °C Showing Interactions between the Carbene Carbons and Protons of the Coordinated Naphthyl (Np) Moieties

Figure 2. Hydrogenation of 2-methylbenzofuran at 1 bar of H2 pressure.

solution of the preformed catalyst was stirred under an atmosphere of H2 for 18 h prior to the addition of 2methylbenzofuran, no induction period was observed after substrate addition and direct and rapid conversion to the product occurred. These findings show that precatalyst 2-A itself is first activated by H2 before catalyzing heteroarene reduction. To gain further insight into the structure of the active catalyst, the reaction of 2-A with dihydrogen was investigated by NMR spectroscopy in the absence of substrate. Upon application of 1 bar of hydrogen pressure for 1−5 min to a solution of 2-A in THF-d8 or toluene-d8, a color change from yellow to orange-brown was observed, indicating the formation of a new species (2-B). The resulting complex 2-B was characterized by 2D NMR at −30 °C. Just one set of signals was observed, implying that, in contrast to 2-A, a single diastereomer was present. The 1H NMR spectrum, featuring four doublets between 1.7 and 0.1 ppm, corresponding to the methyl groups, indicates an unsymmetrical complex structure. The signals for one of the naphthyl moieties from each NHC ligand are shifted to high field into the olefinic region, similar to those of the η4-Rucoordinated naphthyl ring in complex 2-A. Correlations of the shifted naphthalene resonances in the 2D NMR with the carbene carbons support a π-coordination of the naphthalenes (Scheme 3). Other than the Ru-bound carbene carbons, no further Ru−C σ-bonds were detected. Together, the absence of hydride signals and the observation of four distinct methyl environments indicate that 2-B likely corresponds to a cis bisNHC-Ru(0) complex stabilized by coordinative interactions between ruthenium and one of the naphthyl moieties from each NHC (Scheme 3). A signal at m/z 877 is visible in the electrospray ionization (ESI) mass spectrum, which was assigned to the water adduct of 2-B. DFT calculations show that the structure of the ruthenium(0) complex 2-B, as represented in Scheme 3, is the lowest-energy conformer of several possible conformers investigated.7

Next, the hydrogenation reaction was monitored by 1H NMR. 2-Methylbenzofuran remained untouched, while 2-A was transformed to 2-B. Coordination of the substrate to the catalyst was not observed (Figure 3b). As soon as the signals of

Figure 3. 1H NMR spectra in toluene-d8 at −10 °C: (a) 2methylbenzofuran; (b) 2-B and 2-methylbenzofuran under an H2 atmosphere of 1 bar; (c) formation of hydrogenation product and of a new species after 90 min.

species 2-B decreased in intensity, the hydrogenation of 2methylbenzofuran commenced and new resonances appeared (Figure 3c). These new signals were broad and complex and appear to arise from a mixture rather than a single species. It seems as though 2-B itself is not active in hydrogenation, but that the active species is slowly formed at low H2 pressure from 2-B. Analysis of this reaction mixture by ESI mass spectrometry indicated that a ruthenium species containing 18 more hydrogen atoms in comparison to 2-A is formed under a C


Article

Organometallics dihydrogen atmosphere. This new complex (2-C) presumably arises from the partial hydrogenation of the naphthyl substituents and the hydrogenolysis of the C−H activated Ru−C bonds to the carbene ligand. The same mass of m/z 875 was observed as the only ruthenium peak at the end of the hydrogenation reaction with every reported substrate (standard conditions). The hydrogenation of 2-methylbenzofuran at 10 bar of H2 was stopped after 2.5 h and the aliquot analyzed. A second batch of substrate was added to continue the reaction (Scheme 4). The catalyst was still active and hydrogenated the second

Scheme 5. Reversible Formation of 2-D and Proposed Structure of 2-C

Scheme 4. Formation and Reactivity of Complex 2-C

batch of benzofuran with full conversion and the same enantioselectivity. Complex 2-C was the only species detected by ESI-MS after both the first and the second reaction. The 1H NMR spectrum after hydrogenation of 2-A at 100 bar of H2 pressure for 2 h shows rather broad peaks even at low temperature (−60 °C), indicating a dynamic behavior. The intensities of the signals in the aliphatic region were much larger than those observed for 2-A and 2-B. Two broad hydride peaks around −6 ppm appeared when the sample was stored under an atmosphere of hydrogen. The longitudinal relaxation of the larger peak at −6.2 ppm was small (90(±10) ms), hinting at the presence of a dihydrogen ligand. A T1 time of 167(±20) ms for the second hydride signal at −6.5 ppm is in the typical range for anionic hydride ligands. The presence of both dihydrogen and hydride ligands in the same molecule has been reported by Chaudret for the related complex [RuH2(H2)2(PCy3)2], which is active in the nonstereoselective hydrogenation of arenes (Scheme 6a).3a,c,18 Evaporation of the solvent caused the conversion of 2-C to the new Ru species 2-D, which was obtained as a brown solid. A detailed NMR analysis of 2-D revealed three partially hydrogenated naphthyl residues, a hydride signal at −9.74 ppm, a methylene group derived from methyl deprotonation at 2.16 and 0.96 ppm, and one high-field-shifted, η4-coordinated naphthyl substituent. Mass spectrometry together with the IR hydride band at 1868 cm−1 confirmed the formation of a bis(NHC)-ruthenium hydride species. DFT calculations additionally support the structure of 2-D shown in Scheme 5. When the MS peak of 2-D was fragmented within the Orbitrap, two different NHC signals with a mass difference of 4 were observed in a 1:1 ratio (Figure 4). Both NHCs act as bidentate ligands. One of the NHCs forms a ruthenacycle, while for the second NHC, one of the naphthyl moieties coordinates to ruthenium in an η4-fashion. The three noncoordinating naphthalene substituents are partially hydrogenated to 1,2,3,4tetrahydronaphthalenes. As only one species was observed in the 1H NMR spectrum, the reduction of the naphthalene substituents seems to be stereoselective. The ability of the Ru hydride complex 2-D to catalyze hydrogenation reactions was investigated by 1H NMR spectroscopy. No reaction took place when 2-methylbenzofuran was added to 2-D. After hydrogen gas (2 bar) was applied to the NMR tube, however, rapid and complete conversion of the 2methylbenzofuran to 2-methyldihydrobenzofuran was ob-

Figure 4. High-resolution mass spectrum and daughter ion spectrum of 2-D, showing the presence of two different NHC ligands.

served. In addition, the distinct signals of complex 2-D were no longer present. Instead, 2-C was observed by NMR as well as ESI-MS. Reversible dehydrogenation and C−H activation of ligands is not uncommon for ruthenium complexes,3b but the involvement of a naphthalene group in such processes is unexpected (Scheme 6). The main structural difference between the unreactive species 2-A and 2-B and the rapidly activated 2-D is the hydrogenated ligand. While the additional Ru−C bonds of 2-A are hydrogenolyzed quickly to form 2-B under an H2 atmosphere (vide supra), hydrogenation of the naphthyl substituents of 2-B appears to be the key activation step of the Ru-SINpEt catalyst.19 In hopes of obtaining a more stable complex, we added carbon monoxide and isonitriles to 2-D in toluene. An instant color change from dark brown to yellow was observed in both D


Organometallics

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Scheme 6. (a) Reversible Dehydrogenation and Deprotonation of [RuH2(H2)2(PCy3)2], (b) Influence of a Second Coordination Site on the Enantioselectivity, and (c) Structural Comparison of Successful Hydrogenation Substrates

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00702. Experimental procedures and spectroscopic data (PDF) Cartesian coordinates for the calculated species (XYZ) Crystallographic data (CIF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.N.: [email protected]. *E-mail for R.W.: [email protected]. *E-mail for F.G.: [email protected]. Present Addresses ∥

Axalta Coating Systems, Wuppertal, Germany. Baxter Oncology GmbH, Halle/Westfalen, Germany. # Bayer Healthcare, Wuppertal, Germany. & Crespel & Deiters, Ibbenbühren, Germany. ⊥

Author Contributions 1

13

§

These authors contributed equally.

1

cases. The H and C{ H} NMR spectra showed similar but distinctively shifted resonances. For example, the hydride signal at −9.74 ppm instead appeared at −4.58 ppm after addition of CO and at −5.04 ppm after tert-butyl isonitrile was added. Characterization of the adducts using 2D NMR techniques support the previously proposed structure of 2-D.7 Although 2D·CNtBu was isolated in small amounts, single crystals suitable for X-ray analysis have not yet been obtained. We presume that the mechanism of substrate hydrogenation follows a pathway similar to that for the hydrogenation of the ligands. The aromaticity of the substrate may be weakened by π-back-bonding of the electron-rich ruthenium center into the CC bond, and thus unproductive σ-coordination to heteroatoms may be prevented.4e All of the successful substrates possess either a butadiene moiety or a donating heteroatom adjacent to the reduced double bond. Hence, a coordination analogous to naphthalene seems possible. The importance of a heteroatom adjacent to the CC bond for achieving a high enantioselectivity has been shown by comparison of 3-methylbenzofuran with 3-methylindene reduction.4b Exchanging the oxygen for a methylene group leads to a drop in enantioselectivity from 97:3 to 58:42 er.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Klaus Bergander, M.Sc. Christoph Schlepphorst, and Dr. Kathryn Chepiga (all at WWU) for helpful discussions, M.Sc. Dirk Herrmann (University of Regensburg) for experimental assistance, Dr. Laurent Lefort (DSM) for experimental support with specialized equipment, and Dr. Klaus Ditrich (BASF) for donation of enantiomerically pure chiral amines (ChiPros). Generous financial support by the DFG (SFB 858, Leibniz award) is gratefully acknowledged.

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REFERENCES

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CONCLUSIONS

We were able to isolate and characterize the new ruthenium(II)-NHC complex 2-A. This complex was identified as the precatalyst to the extremely active and versatile Ru-SINpEt catalyst, which catalyzes the highly enantioselective hydrogenation of a wide range of heteroarenes. Complex 2-A bears a rare doubly C−H activated NHC ligand as well as an uncommon η4-naphthyl-coordinated NHC ligand. Monitoring this complex throughout the course of the catalytic reaction revealed the reversible hydrogenation of the Ru−C bonds and the naphthyl moieties. Both effects are likely crucial for generating the catalytically active species. The ability of naphthalene to π-coordinate to electron-rich metal fragments seems to be the basis for stabilizing the precursor complex. Reversal of this π-interaction under catalytic conditions presumably enables substrate coordination and subsequent reduction. E


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