Carbon-nitrogen bond formation via catalytic alcohol activation
Yan, Tao
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Borrowing hydrogen meets metal-ligand bifunctional catalysis,
an introduction to the thesis
1.1 Introduction
1.1.1 Catalysis: key to a sustainable future
1.1.2 Catalytic carbon-nitrogen bond formation: the focus of this thesis 1.2 Metal-ligand bifunctional catalysis
1.2.1 Background 1.2.2 The Shvo catalyst 1.2.3 The Knölker complex
1.2.4 Recent progress in bifunctional catalysis
1.3 Catalysis based on the borrowing hydrogen strategy
1.3.1 Alkylation of amines with alcohols through borrowing hydrogen 1.3.2 Challenges and recent discoveries
1.4 Conclusion
1.1 Introduction
1.1.1 Catalysis: the key to a sustainable future
Catalysis is the tool to tune the kinetics of a chemical transformation, that allows desired reactions to be conducted selectively, under mild reaction conditions.[1] Catalysis is one of the few areas with a direct influence on our daily life.[2] It contributes directly and indirectly to 35% of the global GDP.[3]
Currently, a variety of chemical transformations still rely on the use of stoichiometric reagents and low atom economy processes.[1,2,4] These include the use of protecting groups or harsh reaction conditions, mainly due to high activation energy of the desired transformations. During these processes, stoichiometric amount of side-products are produced and frequently discarded. Also, the increasing demand for a more environmental benign society and the changing landscape of accessible chemical feedstocks and energy sources, indicate we are facing a transition period of energy and chemical production.[5]
Catalysis, beyond accelerating chemical transformations, allows the use of renewable carbon sources through converting bio-based molecules and CO2 to more valuable chemicals, as well as accessing alternative energy such as converting and storing solar energy in chemicals.[5,6] Consequently, the new advances in catalysis will not only lead to considerable economic benefit, but more importantly, are key to build a sustainable society.[7]
1.1.2 Catalytic carbon-nitrogen bond formation: the focus of this thesis
This thesis discusses an alternative methodology to construct carbon-nitrogen bonds[8], using widely abundant alcohols as substrates instead of the traditional alkyl halides or aldehydes, promoted by metal-ligand bifunctional catalytic systems. As an introduction to this thesis, this chapter gives background to the field of ligand-metal bifunctional catalysis which is dramatically changing the face of chemistry, in particular redox chemistry.[9] The introduction will include catalysis based on the borrowing hydrogen strategy which will be also extensively involved in the following chapters. Further, literature background to the Shvo catalyst[10a] and the Knölker complex[10b] which are important catalysts employed in this thesis will also be discussed.1.2 Metal-ligand bifunctional catalysis
1.2.1 Background
Transition metal catalyzed chemical transformations promote the efficient and environmentally benign synthesis of molecular targets[11]. In conventional transition metal based catalysis, the coordination and further transformation of the substrate is performed at the metal center and the role of the ligand is to keep the metal complex in solution as well as regulate the electronic and steric properties of the transition metal complex[12]. However, there is a class of catalysts, in that the coordination and chemical transformation occurs on both the metal center as well as the ligand. An early example from the Noyori group described the
asymmetric hydrogenation of ketones using a ruthenium diphosphine-diamine complex.[13] The diamine ligand acts as the proton donor that stabilizes the alkoxy before forming an alcohol by reduction of a simple ketone (Scheme 1, intermediate 1). It was proposed that the substrate is in the second coordination sphere of the catalyst complex, not directly coordinated to the metal center[14] (Scheme 1, A). The original halogen contained BINAP-Ru(II) complexes were only found to be active for hydrogenation of functionalized ketones with nitrogen, oxygen or halogen atoms near the carbonyl group (Scheme 1, B)[15]. In the latter case, the additional heteroatom is required to form a metallacycle (Scheme 1, intermediate 2) to stabilize the alkoxy ligand before protonation. The type of catalysis described in Scheme 1A was coined as ‘metal-ligand bifunctional catalysis’ by Noyori in 2001.[9a,16]
Scheme 1: A Asymmetric hydrogenation of acetophenone with a ruthenium BINAP diamine complex; B asymmetric hydrogenation of activated carbonyls with ruthenium BINAP complex.
1.2.2 The Shvo catalyst
In 1984, it was reported by the Shvo group that the reactivity of transfer hydrogenation reactions catalyzed by triruthenium dodecacarbonyl was significantly improved by adding diphenylacetylene (Scheme 2, A).[17] Later on, it was proven that a ruthenium complex bearing cyclopentadienone was formed and played an essential role.[18] The structure of the complex was determined by X ray spectroscopic analysis (Scheme 2, B) by the Shvo group.[10a] The initial synthetic approach to obtain this complex took 2 steps (Scheme 2, C, a) during which Ru3(CO)12 (3) and tetraphenylcyclopentadienone (4) were heated to reflux in benzene, forming [Ph4(4-C4CO)]-Ru(CO)3 (Cat 2)[19]. Subsequently, Cat 2 was refluxed in isopropanol to give Shvo’s catalyst (Cat 1)[10a]. An alternative synthetic
approach to Cat 1 was reported by Bäckvall and coworkers through sequentially treating Cat 2 with an aqueous Na2CO3 in acetone, and an aqueous NH4Cl (Scheme 2, C, b)[20]. Finally, a concise, one-step synthesis of Cat 1 was reported by Casey through heating 3 and 4 in methanol (Scheme 2, C, c)[21].
Scheme 2: A Crystal structure of Cat 1; B approaches of preparation of Cat 1.
Shvo’s catalyst was mainly applied in hydrogen transfer reactions (Scheme 3, A), for example, oxidative coupling of primary alcohols to esters[22a], oxidation of alcohols to ketones[22b] or to form imines with amines[22c], and the hydrogenation of ketones, alkenes[22d] and imines[23]. The complex was also successfully used in the dynamic kinetic resolution of secondary alcohols through the coupling of enzyme catalyzed acetylation of one of the alcohol enantiomers and Cat 1 catalyzed racemization of the remaining alcohols[24]. Recently, several hydrogen auto-transfer reactions were also reported using Shvo’s catalyst, including N-alkylation of amines with aliphatic amines[25a], C-3-alkylation of indoles[25b] and tri-alkylation of ammonium salts with alcohols[25c].
The bench-stable diruthenium complex Cat 1 actually is still a pre-catalyst which is activated by heat and dissociates into two mono-ruthenium complexes, a 16-electron species Cat 2-O, and an 18-16-electron complex Cat 2-H (Scheme 4, A). Cat 2-H was the first reported well-defined metal-ligand bifunctional catalyst. The formed Cat 2-O can then participate in, for example, dehydrogenation of isopropanol to form acetone and Cat 2-H, and subsequent hydrogenation of acetophenone reforms Cat 2-O through intermediate 3 (Scheme 4, B).
The reactivity and application of this complex has been extensively reviewed in 2005[26a], 2009[26b], 2010[26c] and 2011[26d].
Scheme 4: A Activation of Cat 1; B catalytic properties of Cat 2-H and Cat 2-O.
1.2.3 The Knölker complex
One of the first iron complexes used in organic synthesis was Fe(CO)5.[27] Back in the 1950s, the reaction of Fe(CO)5 with alkynes was reported to be a [2+2+1]-cycloaddition[28ab] (Scheme 5, A) resulting in the formation of tricarbonyl(4
-cyclopentadienone)iron complex. The cyclopentadienone ligands obtained through demetallation of the complexes had drawn considerable interest.[28c] However, the reactivity of the complexes was not explored.
Scheme 5: A [2+2+1]-cycloaddition between Fe(CO)5 and 2 alkynes; B synthesis of Cat 3 and Cat 3-H.
Until 1999, Knölker and coworkers reported key reactivity studies of tricarbonyl cyclopentadienone complexes[10b]. It was observed, that especially when treating iron complex Cat 3 that bears two trimethylsilane (TMS) substituents, sequentially with aqueous NaOH in tetrahydrofuran (THF) and H3PO4, the mono-iron hydride complex Cat 3-H (Knölker complex) can be obtained (Scheme 5, B). Cat 3-H was fully characterized also by X ray analysis (Scheme 6, A)[10b].
Scheme 6: A Crystal structure of Cat 3-H; B mechanistic illustration of Cat 3-H catalyzed hydrogenation of acetophenone; C Relative activity for stoichiometric acetophenone reduction in toluene-d8 at 5 °C.
In 2007, the first catalytic reactivity of the Knölker complex (Cat 3-H) was discovered by Casey and Guan[29] (Scheme 6, B). It was established, that the iron hydride complex Cat 3-H, acts as a metal-ligand bifunctional catalyst in the
hydrogenation of ketones. In this case, the non-innocent[30] cyclopentadienone backbone acts as a proton donor, while the metal center bears the hydride. In this way, selective reduction of polar unsaturated bonds such as C=O and C=N[31] becomes possible through an appropriate intermediate (such as intermediate 4). The formed Cat 3-O can be reduced to Cat 3-H with molecular hydrogen (Scheme 6, B). Later, in 2012 Casey and Guan reported that iron hydride complexes Cat 3-H and Cat 5-3-H give comparable activity to the ruthenium hydride complex Cat 4-H in stoichiometric acetophenone reduction. This suggests that these more economical iron catalysts are attractive alternatives to ruthenium catalysts (Scheme 6, C)[32]. The dimerization of Cat 3-H or Cat 4-H for providing binuclear species as Cat 1 has not been detected, suggesting that the bulky TMS groups prevent such formation of the hydride bridged dimeric complexes.
Scheme 7: Tricarbonyl(4-cyclopentadinenone)iron complexes in catalysis: (A) hydrogen transfer reactions and (B) dual catalysis.
After Casey and Guan’s initial discovery on the catalytic behaviour of the Knölker complex, several studies involving this complex have been reported. These included reductive amination[31], hydrogenation of carbonyl compounds and imines in water[33], transfer hydrogenation of carbonyl compounds and imines[34], Oppenauer-type oxidation of alcohols[35] (Scheme 7, A). Furthermore, catalytic systems involving dual catalysis were described, in which the Knölker complex catalyzed hydrogen transfer reactions were coupled with organo-catalysis[36] or enzyme promoted transformations[37] (Scheme 7, B). Moisture and air stable complex Cat 3 is frequently used as a pre-catalyst. One CO ligand in Cat 3 can be easily removed by Me3NO, generating Me3N, CO2 and active species Cat 3-O (Scheme 6A).
Several analogues of the original complex have been reported through steric and electronic modifications[38] of Cat 3 (Scheme 8). Modifications of the proton donor site on the non-innocent ligand were reported by Nakazawa[39] and Guan[40], and chirality was introduced to the metal complex by Wills[41], Berkessel[42] and Gennari[43]. The field has been reviewed by Knölker[44], Guan[45], as well as Quintard and Rodriguez[46] recently.
Scheme 8: Analogues of Knölker’s complex.
1.2.4 Recent progress in metal-ligand bifunctional catalysis
Since Noyori’s ruthenium diphosphine-diamine system was reported in 1995, besides the development of the Shvo and Knölker complexes, considerable progress has been established in this area[47,59]. For example, Morris and coworkers reported well-defined iron complexes bearing PNNP ligands (such as Cat 8, shown in Scheme 9) for the asymmetric transfer hydrogenation of carbonyl compounds and imines. The reactions were completed within minutes in most cases, and the
ee reached 99% when imines were employed as the substrates[48]. Related to these excellent results established by the Morris group, Bullock highlighted the potential of iron-based catalysts to reach reactivity comparable to that obtained with noble metal catalysts[49] (Scheme 9).
Scheme 9: Comparison of bifunctional catalyst systems developed by Noyori and Morris.
In 2004, Milstein and coworkers reported a new type of ruthenium pincer complex, that operates via the aromatization-rearomatization of a pyridine based heteroaromatic ligand. This complex catalyzed the acceptorless dehydrogenation of alcohols to ketones[50] and esters[51]. Subsequently the acceptorless dehydrogenation and coupling between alcohols and amines to form amides was reported[52] (Scheme 10, A). Comparing to classical metal-ligand bifunctional catalysts that bear N-donors to activate H2 or alcohols and subsequently reducing polar unsaturated bonds, the Milstein-type pincer complex has a C-donor (Scheme 10, B), and is able to activate a wider variety of bonds, including N-H bonds[53], CO2[54], nitriles[55] and O2[56].
Scheme 10: A Milstein pincer complex catalyzed acceptorless dehydrogenative coupling for the synthesis of esters and amides; B activation of H2 by the Milstein pincer complex Cat 9.
Recently, a number of new metal-ligand bifunctional catalysts have been reported[57]. Selected examples are shown in Scheme 11, in which the structures shown are after dihydrogen activation. In these complexes, the p- or π electron on the ligand offers a proton acceptor site that is involved in the heterolytic splitting of dihydrogen which results in the formation of the corresponding metal hydride. Interestingly, Harman and Peters recently reported a nickel complex featuring a borane moiety in the supporting ligand scafford (Scheme 11)[57e]. The property of this complex is more comparable to a heterobimetallic complex (boron mimics a second metal)[58] instead of a Noyori-type bifunctional complex. The catalytic
applications of metal-ligand bifunctional complexes were reviewed by Khusnutdinova and Milstein recently[59].
Scheme 11: Metal-ligand bifunctional complexes.
Starting from the reduction of polar unsaturated bonds, through the activation of diverse bonds, to the recent application in the ‘borrowing hydrogen’ chemistry, metal-ligand bifunctional complexes that operate based on metal-ligand cooperation, have opened the gate to more efficient catalysis.
1.3 Catalysis based on borrowing hydrogen strategy
1.3.1 Alkylation of amines with alcohols through borrowing hydrogen
Selective C-N bond formation is a challenging task for synthetic chemists[8]. The traditional methodologies include reductive amination of carbonyl compounds[60], or nucleophilic substitution of amines with alkyl halides[61]. These methods, however, suffer from either unstable and limited accessible substrates or the formation of stoichiometric amounts of side products as waste. In the chemical industry, alcohols are preferred reaction partners for alkylation of ammonia or various amines, they however require harsher reaction conditions[62].Scheme 12: First examples on transition metal catalyzed alkylation of amines with alcohols by (A) Grigg and (B) Watanabe.
In 1981, Grigg and coworkers reported the first example of alkylation of amines with alcohols under significantly milder conditions using transition metal catalysts[63] (Scheme 12, A). They proposed that the reactivity of the alcohol was improved by the formation of the corresponding carbonyl compound, which subsequently underwent imine formation with the amine reaction partner. Reduction of this imine intermediate resulted in the alkylated amine. At around the same time, Watanabe and coworkers also reported the alkylation of anilines with various alcohols catalyzed by a ruthenium complex[64] (Scheme 12, B).
Following these studies, during the past 3 decades, various catalytic systems have been developed for direct alkylation of amines with alcohols, mostly using ruthenium or iridium based catalysts[65] (Scheme 13). This field has been extensively reviewed[66].
Scheme 13: Alkylation of amines with alcohols catalyzed by ruthenium or iridium based catalytic systems.
To characterize these types of reactions, Williams et al. coined the term ‘borrowing hydrogen’[67] in 2004. During the catalytic cycle, an alcohol is dehydrogenated to the corresponding carbonyl compound, which reacts with the amine to form an imine (Scheme 14, A). The hydrogen delivered from the alcohol is temporarily stored at the metal complex. The imine is reduced in situ to the alkylated amine by the hydrogen stored on the metal complex. Key features are that the process is hydrogen neutral, no other reagents are needed and the only stoichiometric by-product is water. Variations of this reaction have also been reported, for instance, C-C bond formation through alcohol activation[68,69] (Scheme 14, B) and alkane metathesis through dehydrogenation of alkane to alkene, alkene metathesis and hydrogenation of alkene to alkane[70] (Scheme 14, C).
Scheme 14: A Proposed mechanism of alkylation of amines with alcohols through borrowing hydrogen; B C-C bond formation through alcohol activation; C alkane metathesis through borrowing hydrogen.
Alkylation of amines with alcohols through borrowing hydrogen has been applied in the pharmaceutical industry due to its significant economic benefit compared to traditional methodologies of N-alkylations[66]. For example, Pfizer recently developed a new pathway for synthetizing a GlyT1 inhibitor (9) (Scheme 15)[65h]. Comparing to conventional pathway, the key optimization was a direct amination of alcohol 6 with amine 7 to provide the key intermediate 8 through the borrowing hydrogen strategy. It is a redox-neutral process that avoids the use of stoichiometric amount of oxidant and reductant.
Scheme 15: Conventional and Pfizer’s pathway for the synthesis of a GlyT1 Inhibitor.
1.3.2 Challenges and recent discoveries
The borrowing hydrogen strategy has been recognized as a key concept in catalysis and sustainable chemistry, as it is a highly atom economic process[71]. Since the first examples reported by the groups of Grigg[63] and Watanabe[64], and till recent discoveries[65,66], most reactions were promoted by ruthenium or iridium based catalytic systems. After three decades of discovery, the scientific community realized that the main challenge was to developa non-precious metal based catalyst for promoting this transformation[66d,72]. Iron with its high Earth- abundance[73a], low cost[73b] and toxicity[73c], has been identified as an attractive candidate.
In 2014, our group reported the first example of alkylation of amines with alcohols with the well-defined bifunctional iron complex (Knölker complex)[74] (Scheme 16, A). Subsequently, Wills and coworkers[75a] reported the same transformation with an analogue complex Cat 16. Zhao and coworkers[75d] showed that with the assistance of Lewis acids, the yields of alkylated amines could be significantly improved when secondary alcohols were employed. The synthesis of allylic amines[75c] and pyrroles[76], and -alkylation of ketones with alcohols[75d] were further explored using the same catalytic system. In 2016, Kirchner and coworkers[77] reported a PNP pincer type iron complex Cat 17 catalyzed alkylation of amines with alcohols.
Besides iron, cobalt pincer complexes were successfully applied in the same transformation by the groups of Kempe[78a], Zhang[78b] and Kirchner[78c] (Scheme 16). Also, novel catalytic systems based on manganese pincer complexes were reported by the groups of Beller[79ab] and Sortais[79c] (Scheme 16, B). Several types
of hetero-aromatics were also synthetized employing Co[80] and Mn[81] based catalysts.
Scheme 16: (A) Iron, (B) Cobalt and manganese catalyzed alkylation of anilines with alcohols.
1.4 Conclusion
In the past decades, tremendous progress has been made in the development of metal-ligand bifunctional catalysis and the borrowing hydrogen strategy, mainly with noble metal complexes. Recently, there is a clear interest in moving towards non-precious metals based catalytic systems. In particular, iron based systems are desired due to their lower toxicity, abundance and lower price as more sustainable alternatives to noble metal catalysts.[73] Recent literature suggests that iron catalysis is potentially able to cover the entire range of catalysis for organic synthesis[44,82]. Carefully designed ligands which are capable of stabilizing as well as cooperating with the metal center, are key for promoting desired chemical transformations.
1.5 Outline of the thesis
This thesis describes the development of novel catalytic methods for the selective alkylation of amines with alcohols through the borrowing hydrogen methodology, using metal-ligand bifunctional complexes, in particular the Knölker complex and the Shvo catalyst.
In Chapter 2, the discovery of the first well-defined iron complex catalyzed alkylation of amines with alcohol is described. Chapter 3 describes the application of the discovered method for transformations involving benzyl alcohols in order to obtain benzylamines. In Chapter 4, iron catalyzed pyrrole synthesis is described by N-heterocyclization of amines with unsaturated diols. Chapter 5 describes the direct N-alkylation of unprotected amino acids with alcohols using the Knölker complex and the Shvo catalyst with retention of stereochemistry. Chapter 6 describes the use of the Shvo catalyst in alkylation of amino acids esters with alcohols, without racemization.
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