Cooperative Catalysis at MetalâSulfur Bonds - Accounts of Chemical

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Cooperative Catalysis at Metal−Sulfur Bonds Lukas Omann,† C. David F. Königs,† Hendrik F. T. Klare,* and Martin Oestreich* Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany CONSPECTUS: Cooperative catalysis has attracted tremendous attention in recent years, emerging as a key strategy for the development of novel atom-economic and environmentally more benign catalytic processes. In particular, Noyori-type complexes with metal−nitrogen bonds have been extensively studied and evolved as privileged catalysts in hydrogenation chemistry. In contrast, catalysts containing metal−sulfur bonds as the reactive site are out of the ordinary, despite their abundance in living systems, where they are assumed to play a key role in biologically relevant processes. For instance, the heterolysis of dihydrogen catalyzed by [NiFe] hydrogenase is likely to proceed through cooperative H−H bond splitting at a polar nickel−sulfur bond. This Account provides an overview of reported metal−sulfur complexes that allow for cooperative E−H bond (E = H, Si, and B) activation and highlights the potential of this motif in catalytic applications. In recent years, our contributions to this research field have led to the development of a broad spectrum of synthetically useful transformations catalyzed by cationic ruthenium(II) thiolate complexes of type [(DmpS)Ru(PR3)]+BArF4− (DmpS = 2,6-dimesitylphenyl thiolate, ArF = 3,5-bis(trifluoromethyl)phenyl). The tethered coordination mode of the bulky 2,6dimesitylphenyl thiolate ligand is crucial, stabilizing the coordinatively unsaturated ruthenium atom and also preventing formation of binuclear sulfur-bridged complexes. The ruthenium−sulfur bond of these complexes combines Lewis acidity at the metal center and Lewis basicity at the adjacent sulfur atom. This structural motif allows for reversible heterolytic splitting of E−H bonds (E = H, Si, and B) across the polar ruthenium−sulfur bond, generating a metal hydride and a sulfur-stabilized E+ cation. Hence, this activation mode provides a new strategy to catalytically generate silicon and boron electrophiles. After transfer of the electrophile to a Lewis-basic substrate, the resulting neutral ruthenium(II) hydride can either act as a hydride donor (reductant) or as a proton acceptor (Brønsted base); the latter scenario is followed by dihydrogen release. On the basis of this concept, the tethered ruthenium(II) thiolate complexes emerged as widely applicable catalysts for various transformations, which can be categorized into (i) dehydrogenative couplings [Si−C(sp2), Si−O, Si−N, and B−C(sp2)], (ii) chemoselective reductions (hydrogenation and hydrosilylation), and (iii) hydrodefluorination reactions. All reactions are promoted by a single catalyst motif through synergistic metal−sulfur interplay. The most prominent examples of these transformations are the first catalytic protocols for the regioselective C−H silylation and borylation of electron-rich heterocycles following a Friedel−Crafts mechanism.

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the ordinary.4 This situation is somewhat contradictory since polar metal−sulfur bonds are often found in living systems, where they are assumed to play a key role in important biological processes such as the heterolysis of dihydrogen catalyzed by hydrogenases.5 Therefore, extensive work has been carried out to investigate the mechanism of this activation mode. The active site of these metalloenzymes consists of a bimetallic core coordinated by terminal and bridging thiolate ligands, usually derived from cysteine (Scheme 1, top).6 In case of the prevalent [NiFe] hydrogenases, a plausible way of dihydrogen activation relies on the precoordination of H2 at the nickel center, followed by cooperative H−H bond splitting at the polar metal−sulfur bond. The overall bond activation event results in the formation of a metal hydride and a protonated

INTRODUCTION The synergistic interplay of a metal and a coordinating ligand, referred to as metal−ligand cooperation (MLC), has emerged as a powerful tool for the catalytic activation of small molecules. 1 In these reactions, the cooperating ligand participates directly in the bond activation step along with the metal center, and both undergo a reversible chemical transformation. The ligand is therefore not a pure spectator or ancillary ligand but is part of the reactive site. Among the various strategies that are known for MLC,1 the bond activation can occur across the M−L bond of a Lewis-acidic metal center and an adjacent Lewis-basic ligand. This structural motif can be considered as a transition metal frustrated Lewis pair (FLP).2 Complexes with polar metal−nitrogen bonds are the most prominent representatives and have evolved as well-established catalysts in hydrogenation chemistry.3 In contrast, catalysts incorporating metal−sulfur bonds as the reactive site are out of © 2017 American Chemical Society

Received: February 15, 2017 Published: April 13, 2017 1258

DOI: 10.1021/acs.accounts.7b00089 Acc. Chem. Res. 2017, 50, 1258−1269

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Accounts of Chemical Research

dinuclear rhodium(III) complex 1 facilitates reversible H−H bond splitting.8 In another pioneering study, Sellmann described cooperative dihydrogen activation with cationic mononuclear rhodium(III) complex 2 bearing a tetradentate ethyl-linked benzenedithiolate ligand; a similar diazene-bridged dinuclear rhodium complex was also shown to be active in reversible dihydrogen activation (not shown).9 A few years later, Sellmann successfully extended this concept to nickel(II) thiodibenzenethiolate complex 3.10 Another remarkable example was reported by Rauchfuss, where activation of two molecules of dihydrogen with disulfide-bridged dinuclear iridium(II) complex 4 occurs homolytically for the first and then heterolytically for the second one.11 Similar heterobimetallic disulfide-bridged complexes 5 and 6 were later introduced by Mizobe and Hidai. These authors demonstrated for the first time that such complexes are potential catalysts for hydrogenation (vide inf ra).12 In another contribution by Sellmann and Prakash, dimeric thiolate-bridged ruthenium(II) complex 7 was shown to reversibly dissociate into two monomers upon activation of dihydrogen.13 A few years later, Ohki and Tatsumi introduced cationic rhodium(III) and iridium(III) complexes 8 and 9, bearing only a single monodentate sulfur-containing ligand.14 The bulky 2,6dimesitylphenyl thiolate (DmpS) ligand developed by Power15 stabilizes the coordinatively unsaturated metal center

Scheme 1. Active Site of [NiFe] Hydrogenase (X = OH or O, Cys = Cysteine), Proposed Dihydrogen Heterolysis (Top), and Cooperative Activation of E−H Bonds (E = H, Si, and B) at Transition Metal−Sulfur Bonds (Bottom)

sulfur ligand. Inspired by this unique operating mode, we envisioned to expand this strategy to the heterolytic activation of Si−H and B−H bonds, thereby providing access to silicon and boron electrophiles (Scheme 1, bottom). Cooperative E−H Bond Activation at M−S Bonds

Outside biological systems, only a few transition metal−sulfur complexes that allow for cooperative activation of small molecules have been reported (Chart 1).7 In a seminal work, Bianchini and Mealli demonstrated that disulfide-bridged

Chart 1. Metal−Sulfur Complexes for Cooperative E−H Bond Activationa

a

E = H, Si, and B. ArF = 3,5-bis(trifluoromethyl)phenyl. 1259


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Accounts of Chemical Research and prevents the formation of dinuclear sulfur-bridged dimers. These complexes proved to be exceptionally active in dihydrogen activation, as the H−H bond activation already occurs at cryostatic temperatures (−50 °C to −20 °C) under 1 atm of dihydrogen (vide inf ra). Seino and Mizobe reported that rhodium(III) complex 10 with a bidentate benzenedithiolate ligand reacts reversibly with dihydrogen. The two-point binding prevents ligand dissociation after H−H bond activation, thereby gaining increased stability and catalytic activity (vide inf ra).16 Another example recently shown by Yan included ruthenium(III) complex 11 with bulky carborane-based thiolate ligands.17 Andersen and Bergman showed for the first time that M−S bonds are also applicable to the activation of Si−H bonds; the monomeric titanocene sulfido complex 12 with a terminal MS double bond facilitates the reversible splitting of both H−H and Si−H bonds.18 This concept was then successfully applied to catalytic hydrosilylation by Stradiotto using cationic rhodium(III) and iridium(III) complexes 13 and 14 with an indene-bridged bidentate P,S-ligand.19 Both complexes activate the Si−H bond in PhSiH3 and Ph2SiH2, but unlike 14, complex 13 proved to be an effective catalyst for ketone hydrosilylation (vide inf ra). Ohki, Tatsumi, and our group introduced tethered ruthenium(II) thiolate complexes 15a−c as highly active catalysts for the heterolysis not only of H−H and Si−H but also B−H bonds. This Account summarizes our recent efforts toward cooperative E−H (E = H, Si, and B) bond activation using ruthenium(II) thiolate complexes of type 15 and highlights the broad applicability of this concept to a variety of synthetically useful catalytic transformations.

Scheme 2. Catalytic Hydrogenation Based on Cooperative Dihydrogen Activation at Ir−S and Rh−S Bonds

Scheme 3. Catalytic Hydrogenation Based on Cooperative Dihydrogen Activation at Rh−S and Ru−S Bonds

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COOPERATIVE H−H BOND ACTIVATION IN CATALYSIS Despite the variety of known transition metal−sulfur complexes (cf. Chart 1), only a few have been applied to catalytic hydrogenation reactions. The proof-of-concept was reported in 2002 by Mizobe and Hidai. These authors employed the dihydrogen adduct of heterobimetallic complex 5 as catalyst in the hydrogenation of alkynes such as tert-butyl propiolate (16 → 17; Scheme 2, top) and oct-1-yne (not shown).12 Seino and Mizobe later showed that rhodium thiolate complex 10 promotes the catalytic hydrogenation of imines (18 → 19; Scheme 2, bottom).16 In collaboration with the group of Erker, Ohki and Tatsumi applied rhodium thiolate complex 8 as catalyst in the reduction of benzaldehyde (20 → 21; Scheme 3, top), N-benzylideneaniline (18a → 19a), and cyclohexanone (not shown).20 We recently investigated the reactivity of tethered ruthenium− sulfur complex 15a21 in the catalytic reduction of imines (18 → 19; Scheme 3, bottom).22 While untethered 8 readily decomposes at temperatures higher than −50 °C, tethered congener 15a allowed for performing the hydrogenation at 0 °C. Notably, the group of Ohki and Tatsumi also succeeded in the spectroscopic and crystallographic characterization of the dihydrogen addition product 9·H2, providing unambiguous evidence for M−S cooperativity (Scheme 4). Interestingly, DFT calculations by Tao and Li revealed that the H−H bond activation occurs heterolytically for rhodium complex 8 and homolytically for the iridium congener 9.23 In view of the fact that Noyori-type complexes with polar metal−nitrogen bonds have been intensively studied in catalytic

hydrogenation,3 it is surprising that these have remained the only examples of catalytic reductions enabled by cooperative H−H bond activation at metal−sulfur bonds.

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COOPERATIVE Si−H BOND ACTIVATION IN CATALYSIS

Introduction

In a loose assumption, the silicon atom is nothing but a “fat” hydrogen atom. Although this is clearly an oversimplification, the Si−H bond of hydrosilanes is weaker than the H−H bond of dihydrogen, quantified by a difference in bond dissociation energy of approximately 18 kcal/mol.24 Thus, heterolysis of hydrosilanes is energetically more feasible. Given the hydridic character of hydrosilanes, heterolytic splitting of the Si−H bond should result in the formation of a metal hydride and a donorstabilized silicon cation (Figure 1). In addition, the donor atom of the cooperating ligand is expected to have a profound influence on the stabilization, and consequently on the 1260


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Accounts of Chemical Research Scheme 4. Cooperative Dihydrogen Activation at Rh−S and Ir−S Bonds (Top) and Molecular Structure of Dihydrogen Addition Product 9·H2 (Bottom)a

Scheme 5. Cooperative Si−H Bond Activation at Rh−S and Ir−S Bonds (Top) and Application in the Catalytic Hydrosilylation of Ketones (Bottom)a

a In the hydrosilylation reactions, catalyst 13 was prepared in situ from the corresponding chloride complex and LiB(C6F5)4·2.5Et2O.

Hydrogen atoms, except for the iridium hydride, and the BArF4− counteranion are omitted for clarity. The proton at the sulfur atom could not be localized but was observed as a broad singlet at δ 5.26 ppm in the 1H NMR spectrum. a

two-point binding of the thiolate should prevent ligand dissociation after Si−H bond cleavage. Rationally designed ruthenium complex 15a with a tethered SDmp ligand had already been investigated in the H−H bond activation and had proven to be more stable in the H2 heterolysis.21 When tested in the related Si−H bond activation, 15a readily formed various stable hydrosilane adducts 15·R3SiH at ambient temperature, which were spectroscopically and crystallographically characterized (Scheme 6).28 In addition to the increased stability, the Scheme 6. Cooperative Si−H Bond Activation at a Ru−S Bond (Top) and Molecular Structure of the EtMe2SiH Adduct (Bottom)a

Figure 1. Comparison of expected Lewis acidities of donor-stabilized silicon cations (Si = Triorganosilyl).

reactivity of the formed silicon electrophile. Compared to harder nitrogen or oxygen donors, the interaction with a sof t sulfur atom should ensure a higher residual Lewis acidity at the silicon atom.25,26 Moreover, a weak stabilization ought to favor reversible coordination, crucial for facilitating silicon cation transfer and turnover in catalytic processes. This assumption was then corroborated by Stradiotto, reporting a catalytic hydrosilylation based on cooperative activation of the Si−H bond in Ph2SiH2 using metal thiolate complexes 13 and 14 (Scheme 5, top).19 While the iridium congener 14 and its hydrosilane adduct 14·Ph2SiH2 were isolated and spectroscopically characterized, rhodium complex 13 and adduct 13·Ph2SiH2 were too reactive for isolation. Conversely, more stable 14 showed only poor catalytic activity in the hydrosilylation of acetophenone-derived ketones 22, but with 13 as catalyst, the free alcohols 23 were obtained with high conversions after hydrolysis (Scheme 5, bottom). It is noteworthy that these complexes did not react with dihydrogen gas (1 atm). Encouraged by these results, our group in collaboration with the group of Ohki and Tatsumi investigated the Si−H bond activation using transition-metal thiolate complexes bearing a bulky SDmp ligand.27 Due to their high reactivity in the related H−H bond activation (vide supra), we initially focused on rhodium and iridium complexes 8 and 9. However, the lability of the monodentate thiolate ligand in these complexes resulted in decomposition in the presence of hydrosilanes, affording complex mixtures of metal hydrides. We then hypothesized that

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Si = Triorganosilyl. Used hydrosilanes are Et3SiH, EtMe2SiH, Me2PhSiH, MePh2SiH, and iPrMePhSiH. Hydrogen atoms, except for the ruthenium hydride, and the BArF4− counteranion are omitted for clarity.

tethered coordination mode also provides a fixed and sterically more accessible ruthenium−sulfur bond, thereby facilitating interaction with external substrates. A detailed NMR spectroscopic analysis as well as quantum-chemical calculations support this activation mode and systematically rule out alternative scenarios, such as oxidative addition or Lewis-base activation via a hypercoordinated silicon atom.28 Based on this concept for the catalytic generation of silicon electrophiles by cooperative Si−H bond activation, various 1261


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Accounts of Chemical Research synthetically useful catalytic transformations have been developed in our laboratory over the last years: (i) dehydrogenative silylation, (ii) hydrosilylation, and (iii) hydrodefluorination reactions.

Scheme 8. Catalytic Generation of Sulfur-Stabilized Silicon Electrophiles for Friedel−Crafts C−H Silylationa

Dehydrogenative Silylation

A well-established method to probe the existence of a reactive silicon electrophile is to trap it with an electron-rich heterocycle. Whereas classical C−H bond activation usually occurs at the C2 position of the indole,29 C−H silylation at the C3 position indicates an electrophilic aromatic substitution (SEAr) mechanism. Indeed, treatment of various N-protected indoles 24 with hydrosilanes such as Me2PhSiH in the presence of catalytic amounts of 15a afforded the corresponding C3silylated indoles 25 as single regioisomers in high yields (Scheme 7).27 Scheme 7. Catalytic Intermolecular Electrophilic C−H Silylation of N-Protected Indoles a

Si = Triorganosilyl. BArF4− counteranion is omitted for clarity.

Scheme 9. Catalytic Dehydrogenative Si−N Coupling

Electrophilic silicon reagents are frequently used in synthetic organic chemistry for hydrodehalogenation reactions or C−O bond cleavages. Hence, functional-group tolerance is often limited in these strongly Lewis-acidic reaction media. It was, however, shown that, aside from alkyl substituents, a variety of halogen atoms in different positions of the indole core were tolerated. Also, substituents in the C2 position were not detrimental (not shown). Importantly, blocking the C3 position with a methyl group did not lead to the C2-silylated indole, clearly revealing that this regioselectivity is a result of pure electronic control. Therefore, the mechanism is likely to proceed through an electrophilic aromatic substitution (also referred to as a sila-Friedel−Crafts reaction; Scheme 8).30 After cooperative Si−H bond activation across the polar ruthenium− sulfur bond (15 → 15·R3SiH), the resulting sulfur-stabilized silicon cation is transferred to the indole 24, yielding Wheland intermediate 27 (24 → 27) as well as neutral ruthenium hydride 26 (15·R3SiH → 26). Proton abstraction, presumably facilitated by the weakly basic sulfur atom, yields the C3silylated indole 25 (27 → 25) along with dihydrogen adduct 15·H2 (26 → 15·H2). The latter immediately releases dihydrogen (15·H2 → 15), thereby closing the catalytic cycle. An alternative pathway may involve the formation of indoline 28 followed by oxidation to indole 25. Although catalyst 15 is indeed capable of performing indoline-to-indole dehydrogenation at higher temperatures (vide inf ra), this route was ruled out by the use of a deuterated hydrosilane; no deuterium incorporation at C2 was observed. The protective group at the nitrogen atom is necessary, as unprotected indoles undergo dehydrogenative N−Si coupling.31 Treatment of indole 29a with equimolar amounts of Me2PhSiH in the presence of catalyst 15a led to full conversion after 1 h at 60 °C (Scheme 9, top). However, a mixture of the N-silylated indole 24a and the corresponding N-silylated

indoline 30a was obtained. Interestingly, the product distribution was heavily influenced by the reaction time and shifted in favor of indole 24a after prolonged reaction time (12 h). This result demonstrates the capability of complex 15 to catalyze indoline-to-indole dehydrogenation. A variety of indoles and other heterocycles such as pyrrole and carbazole as well as anilines were successfully N-silylated under this reaction protocol (Scheme 9, bottom). Notably, performing the reaction with an excess of hydrosilane at slightly elevated temperature (90 °C) allowed for N−H and C−H silylation (cf. Scheme 7) of indole and pyrrole in one pot (not shown). A few years later, the scope of Friedel−Crafts C−H silylation was extended to the intramolecular C−H silylation of benzenes, that is, the ring closure of ortho-silylated biphenyls 34 to dibenzosiloles 35 (Scheme 10).32 The key to success was the employment of complex 15c bearing an electron-deficient phosphine ligand. Due to the lower nucleophilicity of benzenes compared to indoles, elevated temperatures (140 °C) were required. Notably, a broad range of functional groups was tolerated in this protocol, providing rapid access to directly 1262


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Accounts of Chemical Research Scheme 10. Catalytic Intramolecular Electrophilic C−H Silylation of Benzenes

Scheme 11. Reduction (Left) or Dehydrogenation (Right) in the Reaction of Enolizable Ketones with Hydrosilanes Catalyzed by Ru−S Complex 15

polymerizable dibenzosiloles functionalized at both benzene rings. The combination of intermolecular indole and subsequent intramolecular benzene C−H silylation eventually led to the development of a double sila-Friedel−Crafts protocol that facilitates the catalytic synthesis of indole-fused benzosiloles starting from readily available 2-aryl-substituted indoles and dihydrosilanes (not shown).33 Competing Dehydrogenative Coupling and Hydrosilylation

Scheme 12. Dehydrogenative Coupling of Enolizable Ketones with Hydrosilanes

The activation of carbonyl compounds by Lewis acids is a common technique in organic synthesis. The use of a silicon Lewis acid generally involves the formation of silylcarboxonium ion 36 as the central intermediate (Figure 2). The resulting

Figure 2. Activation of carbonyl compounds by silicon Lewis acids.

LUMO lowering ensures subsequent reaction with nucleophiles such as the hydride in a net hydrosilylation reaction. However, it must be taken into account that coordination of the Lewis acid to the carbonyl group also results in a pKa lowering of the α-proton, potentially facilitating deprotonation in the presence of a base. Consequently, two competing pathways have to be considered in the reaction of enolizable ketones with hydrosilanes , that is, reduction and dehydrogenation (Scheme 11).34 After cooperative Si−H bond activation (15 → 15· R3SiH) and subsequent silyl transfer to the ketone (15·R3SiH + 22 → 36), the resulting ruthenium hydride 26 can act as a reducing agent, transferring its hydride to the silyl carboxonium ion 36 to give silyl ether 37. Alternatively, the weakly basic sulfur atom in 26 may deprotonate 36, thereby affording silyl enol ether 38 along with labile dihydrogen adduct 15·H2 that immediately releases dihydrogen with concomitant regeneration of catalyst 15. To test our hypothesis, various ketones 22 were treated with different hydrosilanes such as Me2PhSiH and EtMe2SiH in the presence of 15a as the catalyst (Scheme 12). Indeed, both possible products 37 and 38 were formed in most cases, but in contrast to Stradiotto’s report (cf. Scheme 5),19 the silyl enol ethers 38 were usually obtained as the major compound. The choice of the hydrosilane as well as steric effects had a profound influence on these product ratios. The best results were obtained with EtMe2SiH. In terms of selectivity, acetophenone-

derived ketones decorated with an ortho-substituent were superior. Purely aliphatic ketones also gave exclusively the silyl enol ether, and unsymmetrically substituted aliphatic ketones selectively formed the silyl enol ether with the less substituted double bond. Ketones with an ethyl group formed the silyl enol ether as a mixture of the Z and E isomers (Z/E = 75:25). However, this ratio was improved to some extent when the reaction was performed at lower temperatures (Z/E = 86:14 at −78 °C). Notably, mechanistic experiments with a deuterated hydrosilane revealed that silyl ether 37 actually forms in a subsequent step through hydrogenation of silyl enol ether 38 by dihydrogen adduct 15·H2 (not shown); deuterium was also incorporated into the β-position of 37. Hence, short reaction times and the use of an open reaction vessel were the key to obtain the synthetically more desirable silyl enol ethers 38. Apparently, intermediate ruthenium hydride 26 (cf. Scheme 11) is a poor reducing agent and preferentially acts as a proton 1263


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lead to products in different oxidation states. Interestingly, treatment of Et3SiH with CO2 (5 bar) in the presence of catalytic amounts of 15a chemoselectively furnished the bis(silyl)acetal 43a, that is, the formaldehyde oxidation state (Scheme 14).36 With this optimized setup, formation of 42a

acceptor, thus favoring dehydrogenative pathways over reductions. This concept was then successfully extended to enolizable imines 18 (Scheme 13).35 Using catalyst 15b, Scheme 13. Dehydrogenative Coupling of Enolizable Imines with Hydrosilanes

Scheme 14. Chemoselective Reduction of CO2 to Bis(silyl)acetal (Top) and Silylated Methanol (Bottom)

(formate oxidation state) and 44a (methanol oxidation state) was not observed. Performing the reaction with EtMe2SiH at a lower catalyst loading (1 mol %) afforded bis(silyl)acetal 43b still in high yield and with good chemoselectivity after maintaining the reaction at 80 °C for 4 days. Notably, additional heating for 7 days at 150 °C shifted the ratio in favor of silylated methanol 44b. Pyridines represent another attractive substrate class that does not contain an acidic proton after silicon cation transfer and, hence, undergo hydrosilylation (Scheme 15).37 After Si−H

synthetically valuable N-silylated enamines 40 were obtained in a highly chemoselective fashion. In these transformations, the reduced product, that is, the N-silylated amine 39 was observed, if at all, only as a minor byproduct. Conversely, the C-silylated N-silylated enamine 41 was formed as a result of a 2-fold dehydrogenative coupling. Optimization studies revealed that steric parameters arising from the interplay of catalyst, substrate, and hydrosilane dramatically influenced the product distribution. The bulky iPr3P ligand in 15b was critical to suppress reduction to the undesired amine 39, and the choice of the protective group at the nitrogen atom was another crucial parameter. While a benzyl group afforded the enamine as a mixture with traces of amine and C-silylated N-silylated enamine, a phenyl group completely suppressed the formation of the amine. Electronic effects did not have a profound influence on the product distribution. However, the conversion as well as the chemoselectivity dropped dramatically using sterically more demanding ortho-substituted imines, now affording the doubly silylated enamine in substantial amounts. To probe the synthetic value of this observation, 2 equiv of hydrosilane was used in an independent experiment to realize the 2-fold dehydrogenative coupling. Indeed, C-silylated Nsilylated enamine 41 was now formed in high yields.

Scheme 15. Key Steps of the 1,4-Selective Hydrosilylation of Pyridines

Hydrosilylation

bond activation (15 → 15·R3SiH), the silicon electrophile is transferred to the nitrogen atom of pyridine 45, forming silylpyridinium ion 46 (45 → 46) and ruthenium hydride 26 (15·R3SiH → 26). Subsequent hydride transfer then affords 1,4-dihydropyridine 47 (46 → 47) under regeneration of catalyst 15 (26 → 15). Different reduction products are conceivable, that is, regioisomeric 1,2-dihydropyridine 48 or

The aforementioned dehydrogenative transformations demonstrate the poor hydride donating ability of ruthenium hydride 26 that preferentially acts as a base. However, the use of nonenolizable carbonyl compounds lacking acidic α-protons steers the reaction toward the reduction path (cf. Scheme 11). We selected carbon dioxide as test substrate. Its hydrosilylation can 1264


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that is, rearomatization to finally yield the C3-silylated pyridine 50. All of these three reaction steps, including the final rearomatization, were shown to be mediated by 15 as the only catalyst. In accordance with the results obtained in the dehydrogenative coupling of enolizable imines (cf. Scheme 13), complex 15b with a bulky iPr3P ligand proved to be the most effective catalyst for this multistep sequence (Scheme 18).38 2-Aryl

over-reduced products (not shown). However, 1,4-selective reduction was observed exclusively. Pyridines 45 with various substitution patterns were treated with equimolar amounts of Me2PhSiH in the presence of catalyst 15a, and those with substituents in the 3- or 4-position afforded the desired 1,4-dihydropyridines usually in very high yields (Scheme 16). 3,5-Disubstituted pyridines also underwent Scheme 16. 1,4-Selective Hydrosilylation of Pyridines

Scheme 18. meta-C−H Silylation of Pyridines

substituted pyridines reacted cleanly and afforded the corresponding silylated pyridines in yields ranging from 24% (for R = 4-NMe2) to 86% (for R = 2,4-F2). Additionally, pyridines decorated with alkyl groups in the 2- and 3-position also underwent the cascade sequence.

the hydrosilylation smoothly. Conversely, pyridines substituted in the 2-position did not react (not shown), presumably due to steric hindrance at the nitrogen atom, preventing the formation of silylpyridinium ion 46. Isoquinolines as well as phenanthridine yielded the 1,2-reduced heterocycles, thereby avoiding breaking the aromaticity of the annulated benzene (not shown).

Hydrodefluorination

A fundamentally different type of reaction can be realized by exploiting the high fluorophilicity of silicon. Treatment of electron-rich CF3-substituted anilines 51a−c with MePh2SiH in the presence of catalytic amounts of 15c bearing an electrondeficient phosphine ligand furnished the corresponding hydrodefluorinated products 52a−c (Scheme 19).39 The required

Cascade Hydrosilylation/Dehydrogenative Silylation

Combining hydrosilylation and dehydrogenative silylation (cf. Scheme 11) eventually led to the development of a cascade reaction for the formal meta-selective electrophilic aromatic substitution of pyridines with silicon electrophiles (45 → 50, Scheme 17).38 NMR spectroscopic analysis confirmed that this sequence begins with hydrosilylation of pyridine 45 to give the 1,4-dihydropyridine 47. This intermediate can also be regarded as an N-silylated enamine and hence can act as a nucleophile in a subsequent dehydrogenative silylation (47 → 49). The resulting C-silylated N-silylated 1,4-dihydropyridine 49 (cf. 41 inScheme 13) was found to undergo a retro-hydrosilylation,

Scheme 19. Hydrodefluorination of CF3-Substituted Anilines

Scheme 17. Electrophilic C−H Silylation of Pyridines Enabled by Temporary Dearomatization

reaction temperatures as well as the achieved conversions were heavily dependent on electronic and steric effects. While parasubstituted 51a was fully converted after stirring for 6 h at room temperature, meta-substituted 51b showed only low conversion even after maintaining the reaction for 72 h at 100 °C, thereby demonstrating the importance of the electron-donating effect of the NMe2 group. Aniline 51c with the NMe2 group in the ortho position exhibits a good electronic but poor steric situation. 1265


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Accounts of Chemical Research Consequently, elevated temperature and a prolonged reaction time were necessary to obtain full conversion. Interestingly, a systematic screening of different ruthenium thiolate complexes 15 with various phosphine ligands showed that, at room temperature, only in situ-prepared 15c promoted the reaction, whereas preformed 15c was completely inactive. However, using half an equivalent (based on the catalyst) of NaOMe as an additive enabled catalytic turnover with preformed 15c. Mechanistic experiments and detailed NMR spectroscopic analyses were performed to understand the influence of the NaOMe additive. It was found that formation of the coordinatively unsaturated complex by chloride abstraction is hampered in case of electron-deficient phosphine ligands (Scheme 20). While ruthenium chloride complexes 53a

Scheme 21. Ru−S Dimer as the Active Species in the Catalytic Hydrodefluorination

Scheme 20. Influence of the Phosphine Ligand on the Formation of Coordinatively Unsaturated Ru−S Complexes

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COOPERATIVE B−H BOND ACTIVATION IN CATALYSIS Having identified tethered ruthenium thiolate complexes 15 as active in both the H−H and Si−H bond activation, our group in collaboration with Ohki and Tatsumi extended this concept to the related cooperative B−H bond activation.40 Treatment of 15a (and 15c; not shown) with various hydroboranes such as (9-BBN)2, Cy2BH, pinBH, and catBH readily afforded the corresponding hydroborane adducts 15a·R2BH, which were NMR spectroscopically characterized (Scheme 22, top). The hydride resonances appeared as clean doublets due to coupling to the 31P nucleus. However, no coupling to the 11B nucleus was observed, indicating full B−H bond cleavage. On the contrary, the molecular structure of the 9-BBN adduct showed a slightly different scenario (Scheme 22, bottom): The B−H bond seems to be still intact, albeit significantly elongated.

and b with electron-rich phosphine ligands readily formed the catalytically active complexes 15a and b at room temperature upon treatment with equimolar amounts of NaBAr F4 , ruthenium chloride complex 53c with an electron-deficient aryl phosphine ligands afforded a chloride-bridged dimer 54c as a result of incomplete chloride abstraction even when treated with 1.5 equiv of NaBArF4. This dimer was found to be the catalytically active species in the in situ protocol. Increasing the temperature results in complete chloride abstraction and finally gives coordinatively unsaturated 15c, which after isolation proved to be catalytically inactive. We hypothesized that the activating role of the remaining ruthenium chloride through dimer formation could also be achieved by hydride bridging (Scheme 21). Hydride-bridged dimer 55c was readily accessible from coordinatively unsaturated 15c by treatment with MePh2SiH (i.e., formation of adduct 15c·R3SiH), followed by addition of half an equivalent of NaOMe. Hence, the catalytic cycle was proposed to start with cooperative Si−H bond activation to yield 15c· R3SiH and ruthenium hydride 26c. Fluoride abstraction then leads to cationic thioether complex 15c·R3CH. Subsequent hydride transfer by 26c is now facilitated by formation of dimer 55c, avoiding unfavorable formation of coordinatively unsaturated complex 15c.

Scheme 22. Cooperative B−H Bond Activation at a Ru−S Bond (Top) and Molecular Structure of the 9-BBN Adduct (Bottom)a

a B = Diorganoboryl. Used hydroboranes are (9-BBN)2, Cy2BH, pinBH, and catBH. Hydrogen atoms, except for the ruthenium hydride, and the BArF4− counteranion are omitted for clarity.

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Accounts of Chemical Research Hence, this structural characterization might be viewed as a snapshot of a σ-bond metathesis that eventually forms the ruthenium hydride and the sulfur-stabilized borenium ion. As in the case of cooperative Si−H bond activation, the formed borenium ion was put to the test in the dehydrogenative borylation of electron-rich indoles (Scheme 23).40

studying chemistry at the Technische Universität Berlin (2012−2014) and obtained his master degree with Martin Oestreich (2014). He is currently a graduate student in the same group and is funded by a fellowship of the Fonds der Chemischen Industrie (2015−2017). C. David F. Königs (born 1985 in Viersen/Germany) studied chemistry at the Westfälische Wilhelms-Universität Münster (2006− 2011) and obtained his diploma degree with Martin Oestreich (2011). He completed his doctoral degree in the same group at the Technische Universität Berlin (2011−2014). During his doctoral studies, he spent six months in the laboratory of Kazuyuki Tatsumi at Nagoya University (Japan). He then pursued postdoctoral training with F. Dean Toste at the University of California, Berkeley, and is now working as a Senior Scientist at the Dow Chemical Company in Freeport, Texas.

Scheme 23. Catalytic Intermolecular Electrophilic C−H Borylation of N-Protected Indoles

Hendrik F. T. Klare (born in 1981 in Ankum/Germany) is Senior Scientist in the group of Martin Oestreich at the Technische Universität Berlin. He earned his diploma degree with Martin Oestreich at the Westfälische Wilhelms-Universität Münster (2007). During his graduate studies, he spent a six month research stay in the group of Kazuyuki Tatsumi at Nagoya University (Japan). After completing his doctoral degree in Münster (2011), he worked together with Gerhard Erker before spending one year with Brian M. Stoltz as a postdoctoral fellow at the California Institute of Technology (2011− 2012).

Treatment of various N-methylindoles 24 with pinBH in the presence of catalyst 15c yielded the desired C3-borylated indoles 56 with complete regiocontrol. In analogy to the catalytic C3-silylation of indoles (cf. Scheme 7), the use of a deuterated hydroborane excluded a pathway involving hydroboration followed by indoline-to-indole dehydrogenation, hence supporting an SEAr mechanism (cf. Scheme 8).

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Martin Oestreich (born in 1971 in Pforzheim/Germany) is Professor of Organic Chemistry at the Technische Universität Berlin. He received his diploma degree with Paul Knochel (Marburg, 1996) and his doctoral degree with Dieter Hoppe (Münster, 1999). After a twoyear postdoctoral stint with Larry E. Overman (Irvine, 1999−2001), he completed his habilitation with Reinhard Brückner (Freiburg, 2001−2005) and was appointed as Professor of Organic Chemistry at the Westfälische Wilhelms-Universität Münster (2006−2011). He also held visiting positions at Cardiff University in Wales (2005) and at The Australian National University in Canberra (2010).

CONCLUSION AND OUTLOOK In this Account, we have summarized catalytic applications enabled by cooperative E−H (E = H, Si, and B) bond activation at metal−sulfur bonds. Although this is still an underrepresented catalyst motif compared to the wellestablished metal−nitrogen bond, our recent contributions to this field demonstrate its high potential, particularly in cooperative Si−H bond activation. This is illustrated by various catalytic applications including dehydrogenative silylation and hydrosilylation as well as hydrodefluorination reactions. Our research in this area is currently directed toward the development of easy-to-handle air-stable ruthenium thiolate catalysts,41 as well as chiral ruthenium−sulfur complexes42,43 for enantioselective hydrosilylation reactions.

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ACKNOWLEDGMENTS

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REFERENCES

L.O. thanks the Fonds der Chemischen Industrie for a predoctoral fellowship (2015−2017), and M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship. We particularly thank Susanne Bähr, Dr. Julia Hermeke, Dr. Alice Lefranc, Dr. Toni T. Metsänen, Dr. Kristine Müther, Dr. Timo Stahl, and Simon Wübbolt for their enthusiasm and commitment. We are also grateful to Professors Ohki and Tatsumi for the fruitful collaboration and hosting stays of H.F.T.K., Dr. Kristine Müther, and C.D.F.K. within the framework of the International Research Training Group Münster-Nagoya (GRK 1143 of the Deutsche Forschungsgemeinschaft).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lukas Omann: 0000-0002-8689-250X C. David F. Königs: 0000-0002-9581-196X Hendrik F. T. Klare: 0000-0003-3748-6609 Martin Oestreich: 0000-0002-1487-9218

(1) For general reviews of metal−ligand cooperativity, see: (a) Khusnutdinova, J. R.; Milstein, D. Metal−Ligand Cooperation. Angew. Chem., Int. Ed. 2015, 54, 12236−12273. (b) Peters, R. Cooperative Catalysis; Wiley-VCH: Weinheim, 2015. (c) Grützmacher, H. Cooperating Ligands in Catalysis. Angew. Chem., Int. Ed. 2008, 47, 1814−1818. (2) For reviews of transition metal FLPs, see: (a) Flynn, S. R.; Wass, D. F. Transition Metal Frustrated Lewis Pairs. ACS Catal. 2013, 3, 2574−2581. (b) Wass, D. F.; Chapman, A. M. Frustrated Lewis Pairs Beyond the Main Group: Transition Metal-Containing Systems. Top. Curr. Chem. 2013, 334, 261−280. (c) Berke, H.; Jiang, Y.; Yang, X.; Jiang, C.; Chakraborty, S.; Landwehr, A. Coexistence of Lewis Acid

Author Contributions †

L.O. and C.D.F.K. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Lukas Omann (born 1988 in Villach/Austria) studied chemistry at the Technische Universität Graz (2008−2012) where he obtained his bachelor degree with Rolf Breinbauer. After a six-month internship at Bayer HealthCare Pharmaceuticals (Berlin/Germany), he continued 1267


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