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21 June 2019

Bulky-Yet-Flexible Carbene Ligands and Their Use in Palladium Cross-Coupling

,
and
Centre for Sustainable Chemistry, Department of Chemistry, Ghent University, Krijgslaan 281-S3, B-9000 Gent, Belgium
*
Author to whom correspondence should be addressed.
†
Current address: VITO (Flemish Institute for Technological Research), Separation and Conversion Technology, Boeretang 200, B-2400 Mol, Belgium.
Inorganics2019, 7(6), 78;https://doi.org/10.3390/inorganics7060078
This article belongs to the Special Issue Palladium Catalysts: From Design to Applications
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Abstract

In recent years, several classes of new N-heterocyclic carbene (NHC) ligands were developed around the concept of “flexible steric bulk”. The steric hindrance of these ligands brings stability to the active species, while ligand flexibility still allows for the approach of the substrate. In this review, the synthesis of several types of new classes, such as IBiox, cyclic alkyl amino carbenes (CAAC), ITent, and IPr* are discussed, as well as how they move the state-of-the-art in palladium catalyzed cross-coupling forward.

1. Introduction

In recent years, interest on N-heterocyclic carbenes (NHCs) has been growing rapidly due to their excellent stability, diversity and a possible tunability of steric and electronic properties [1,2,3,4,5,6,7,8,9,10,11,12,13]. Fine-tuning the steric hindrance led to major breakthroughs and improvements in catalysis [14,15,16,17]. Bulkier ligands stabilize the active species and disfavor bimolecular decomposition and other routes of deactivation [18,19,20,21]. However, steric bulk disfavors the approach of the substrate, which might diminish catalytic activity. Therefore, a delicate balance between skeletal flexibility and steric bulk is required to enhance catalytic efficacy [18,19,20,21,22].
The focus of this review is on three different classes of bulky ligands: IBiox [23], cyclic alkyl amino carbenes (CAAC) [24] and N,N’-bis(aryl)imidazolylidenes (such as the ITent-, IPr*- and SICyoctNap series) [25,26,27,28,29]. Their synthesis is discussed, as well as their role in palladium-catalyzed cross-coupling reactions.

2. IBiox

Glorius was the first to report on the concept of “flexible steric bulk” with his work on bisoxazoline-derived N-heterocyclic carbene ligands (IBiox) [23,30]. The rigidity of these ligands comes from their tricyclic backbone. These ligands can be easily prepared, starting from their corresponding bisoxazolines [15,23,30] and tuning of steric bulk, flexibility, and chirality can be achieved by judicious substitutions [31,32] (Scheme 1).
Scheme 1. Synthesis of the IBiox ligand series [23,30,32,33,34].
The flexibility of the steric hindrance of these ligands can be demonstrated by the equilibrium between various conformers of IBiox6 (Scheme 2) [30].
Scheme 2. Equilibrium between conformations of IBiox6 (a–c) [30].

2.1. Suzuki–Miyaura Cross-Coupling

The steric flexibility of the IBiox series proved to be an important factor in enabling challenging cross-coupling reactions, such as the formation of ortho-substituted biaryls via a Suzuki–Miyaura coupling [15,23,30]. Screening of a series of IBiox ligands with cycloalkyl substituents showed that the formation of highly hindered tetra-ortho-substituted biaryls requires increased steric hindrance about the NHC ligand [23]. In that context, IBiox12 emerged as the desired catalyst for this reaction (Scheme 3) [23].
Scheme 3. Selected examples of Suzuki–Miyaura cross-coupling using IBiox12 [23].
While this system is produced in situ, the well-defined IBiox12-based dimer [PdCl(µ-Cl)(IBiox12)]2 was subsequently isolated. The latter showed comparable results to its in situ counterpart in cross-coupling reactions [23].

2.2. Alkyl Sonogashira Cross-Coupling

The Sonogashira cross-coupling [14,35,36] of alkynes with organic halides is an efficient method of obtaining functionalized alkynes. However, aromatic halides are typically used. Coupling of non-activated alkyl halides was always elusive due to problematic oxidative addition steps and side-reactions such as β-hydride elimination. The first successful example of this reaction [37] was the palladium-catalyzed coupling of primary alkyl halides with terminal alkynes by Fu and co-workers. Later, the first Sonogashira coupling of secondary bromides with 1-octyne was developed by Glorius and co-workers [38], using the well-defined [PdCl(µ-Cl)(IBiox7)]2 pre-catalyst (Scheme 4).
Scheme 4. Selected examples of Sonogashira coupling using a well-defined Pd-IBiox complex [38].

3. CAAC Series

Bertrand and co-workers recognized the concept of sterically flexible ligands in their development of the novel class of cyclic (alkyl)(amino)carbenes (CAACs) [24,39,40]. These electron-rich ligands are pyrrolidine-based and bear two quaternary carbons. They are prepared from the corresponding imines [24] and after deprotonation of the CAAC·HOTf, under harsh conditions, using LDA (lithium diisopropylamide), addition of palladium allyl chloride dimer leads to the formation of the well-defined [PdCl(η3-allyl)(CAAC)] complex (Scheme 5).
Scheme 5. Synthesis of the CAAC ligand series [24].
The reactivity of these [PdCl(η3-allyl)(CAAC)] complexes was tested in the α-arylation of aryl chlorides with phenylethylketone [24]. It was found that the most sterically demanding ligand (CAACc) was the most effective when using an unsubstituted aryl chloride, whereas the flexibility of CAACb led to higher yields with more sterically hindered aryl chlorides.

4. ITent

Recent studies showed that up to a certain point, there is a proportional relationship between the increase in the NHC steric bulk and the increase in catalytic activity [5,41]. This phenomenon led to the implementation of flexible bulk into the design of one of the most commonly used NHCs: IPr (N,N’-bis-[2,6-(di-iso-propyl)phenyl]imidazol-2-ylidene). This new series is known as ITent (after its tentacular structure) and the synthesis of these IPr-mimicking ligands was described by Nolan and co-workers (Scheme 6a) [42]. Analogues of these ligands with a chloro-substituted backbone were described by Organ and co-workers in their PEPPSI (Pyridine-Enhanced Precatalyst Preparation Stabilization and Initiation) series [43] (Scheme 6b), and more recently, Liu and co-workers described the synthesis of the acenaphthyl-substituted backbone analogue IPentAn (Scheme 6c) [44].
Scheme 6. Synthetic access to ITent ligands (a–c) [42,43,44].
The Nolan group used this ITent series for the synthesis of their “custom-made” pre-catalysts [PdCl(η3-cin)(ITent)] (cin = cinnamyl) and [PdCl(acac)(ITent)] (acac = acetylacetonate) [42], while Organ used them to further optimize the PEPPSI series [16,27].

4.1. Suzuki–Miyaura Cross-Coupling

The synthesis of tetra-ortho-substituted biaryls under mild conditions remains a challenge in the Suzuki–Miyaura reaction. As shown above, the use of ligands with flexible bulk proved to be critical, and the first example using NHC ligands was described by Glorius in 2004 [23]. Organ was able to further optimize the reaction using the ITent series. More specifically, the well-defined PEPPSI-IPent pre-catalyst was used to achieve a wider scope than previous reports [27].
The Nolan group used the well-defined [PdCl(η3-cin)(ITent)] pre-catalyst series to further advance the state-of-the-art [42]. Of particular interest, [PdCl(η3-cin)(IPent)] was able to efficiently catalyze the cross-coupling of highly hindered aryl chlorides with boronic acids at low catalysts loading (Scheme 7).
Scheme 7. Selected examples of the Suzuki–Miyaura coupling using [PdCl(η3-cin)(IPent)] [42].
Organ and co-workers also used the [Pd-PEPPSI-IPent] pre-catalyst for the challenging coupling of allylboronic acid with different aryl halides [45]. In this context, (hetero)aryl halides were successfully coupled with high selectivity for the α-product (Scheme 8).
Scheme 8. Examples of the Suzuki–Miyaura coupling using allylboronic esters and aryl halides [45].
Recently, Liu and co-workers showed that the [Pd-PEPPSI-IPentAn] precatalyst is efficient in the the Suzuki-Miyaura coupling of a wide range of hindered arylboronic acids and hindered aryl chlorides at 80 °C using a 1 mol % Pd loading [44].

4.2. Negishi Cross-Coupling

After the first Negishi coupling with the well-defined [Pd-PEPPSI-IPr] pre-catalyst [46], the Organ group showed that the [Pd-PEPPSI-IPent] pre-catalyst was even more active, especially in the synthesis of tetra-ortho-substituted biaryls [47,48]. The same palladium pre-catalyst also proved to be highly efficient in the coupling of secondary alkylzinc substrates to aryl halides [49]. The flexible steric bulk of the IPent ligand reduced the β-hydride elimination/migratory insertion considerably, limiting the formation of the isomeric by-product. Later, Organ and co-workers improved this selectivity even further by using [Pd-PEPPSI-IPentCl] (Scheme 9) [43].
Scheme 9. Selected examples for the Negishi cross-coupling of secondary alkylzinc substrates [43].
Recently, Organ and co-workers described a silica-supported pre-catalyst, [Pd-PEPPSI-IPent-SiO2], and evaluated its activity in challenging Negishi cross-coupling reactions under flow conditions [50]. The catalyst material was used in a packed-bed reactor at room temperature and small residence times (10 min or less) were enough to obtain high conversions.

4.3. Stille Coupling

In 2010, Organ and co-workers reported a Stille coupling using [Pd-PEPPSI-IPent] (Scheme 10) [51]. While the method can be used for aryls and heteroaryl substrates, at relatively mild temperatures (60–80 °C), high catalyst loadings are needed in all cases (4–8 mol %).
Scheme 10. Examples of the Stille coupling catalyzed by [Pd-PEPPSI-IPent] [51].

4.4. Buchwald-Hartwig Cross-Coupling

After the first report of the use of palladium-NHC systems for the amination reaction by Nolan [52], the concept of ligands with “flexible bulk” advanced the state-of-the-art. Organ reported the use of [Pd-PEPPSI-IPent] [53] under milder conditions than the ones used with the [Pd-PEPPSI-IPr] analogue, allowing for a greater tolerance of functional groups. The IPent derivative was even capable of catalyzing some reactions that were previously unattainable using the IPr-based catalyst [54]. However, it must be noted that high catalyst loadings (4 mol %) were necessary to achieve good results. More recently, the same group successfully achieved the coupling of various amides with aryl and heteroaryl chlorides using [Pd(η3-cin)Cl(DiMeIHeptCl)] as catalyst, assisted by boron-derived Lewis acids [55].
The Nolan group was able to reduce the catalyst loading by using [PdCl(acac)(ITent)] complexes (Scheme 11) [42]. More specifically, the use of the IHept ligand permitted to lower the catalyst loading to 0.1–0.2 mol %. This system allows for the use of otherwise difficult coupling partners, such as electron-poor anilines.
Scheme 11. Examples of the Buchwald-Hartwig coupling catalyzed by [PdCl(acac)(IHept)] [42].
The Organ group investigated the coupling of 2-aminopyridine derivatives using [Pd-PEPPSI-IPentCl] [56]. Such catalysts were successful in the coupling of such aminopyridines with various aryl chlorides, but it must be mentioned that catalyst poisoning occurred due to metal coordination of the aminopyridine.
Recently, Liu and co-workers showed that [Pd-PEPPSI-IPentAn] was an efficient pre-catalyst in the amination of (hetero)aryl chlorides under aerobic conditions [57].

5. N-Naphthyl-Based NHCs

Another class of NHC ligands with naphthyl groups on the nitrogen atoms was synthesized by Dorta and co-workers and used to prepare well-defined palladium pre-catalysts [29,58,59,60]. [PdCl(η3-cin){anti-(2,7)-SICyoctNap}], one of the most important complexes of the series, was the first well-defined complex to promote the coupling of tetra-ortho-substituted biaryls via a Suzuki–Miyaura reaction at room temperature (Scheme 12) [29,60]. It showed that the success of this ligand lied with the NHC being twisted around the metal center with two bulky faces and two less hindered faces.
Scheme 12. Selected examples using [PdCl(η3-cin){anti-(2,7)-SICyoctNap}] [29].

7. Conclusions and Outlook

“Bulky yet flexible” ligands were recently investigated, greatly advancing the state-of-the-art, particularly in reactions involving sterically congested coupling partners. The correlation between flexible steric bulk and catalytic activity is not always straightforward, as is proven by the failure of some extremely large ligands.
Advancements in this field not only benefited palladium chemistry but were also successfully applied to other research areas and related metals such as nickel. Various reports on the design and applications of IPr- and IPr*-based (among other related derivatives) nickel catalysts by Nolan [78,79,80,81], Nakamura [82], Ackermann [83], Sun [84], Cramer [85], Newman [86], and Matsubara [87] pioneered the transfer of the “bulky yet flexible” concept from palladium to nickel-based coupling reactions. The combination of nickel with other “bulky yet flexible” ligands such as CAACs [88,89] and abnormal NHCs [90] also saw some success. Moreover, Montgomery [91], Louie [92] and Johnson [93] used this concept to further investigate and consequently advance the synthesis and catalytic application of styrene-based nickel complexes, thus building upon the work of Belderrain and Nicasio [94].
Future advances, in particular through mechanistic studies, will provide insights into the exact role of electronic and steric parameters of NHC ligands on catalyst activity, thus guiding further catalyst design efforts across the periodic table. Nonetheless, these “bulky yet flexible” NHCs have already pushed the limits of highly hindered cross-coupling reactions, and hold great promise for combined high activity and catalyst recycling for numerous other metal systems.

Funding

Part of this work was funded by a Research Foundation—Flanders (FWO) grant (180327/1S99819N).

Acknowledgments

The authors greatly acknowledge the Research Foundation—Flanders (FWO) for financial support.

Conflicts of Interest

The authors declare no conflict of interest.

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